tools/memory-model: Add details about SRCU read-side critical sections

  Add details about SRCU read-side critical sections and how they are
modeled.

Cc: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Jade Alglave <j.alglave@ucl.ac.uk>
Cc: Luc Maranget <luc.maranget@inria.fr>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

---
 .../Documentation/explanation.txt             | 55 ++++++++++++++++++-
 1 file changed, 52 insertions(+), 3 deletions(-)
  

On Mon, Feb 13, 2023 at 01:55:06AM +0000, Joel Fernandes (Google) wrote:
> Add details about SRCU read-side critical sections and how they are
> modeled.
> 
> Cc: Andrea Parri <andrea.parri@amarulasolutions.com>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Jade Alglave <j.alglave@ucl.ac.uk>
> Cc: Luc Maranget <luc.maranget@inria.fr>
> Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
> Suggested-by: Alan Stern <stern@rowland.harvard.edu>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> 
> ---
>  .../Documentation/explanation.txt             | 55 ++++++++++++++++++-
>  1 file changed, 52 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
> index 8e7085238470..5f486d39fe10 100644
> --- a/tools/memory-model/Documentation/explanation.txt
> +++ b/tools/memory-model/Documentation/explanation.txt
> @@ -28,9 +28,10 @@ Explanation of the Linux-Kernel Memory Consistency Model
>    20. THE HAPPENS-BEFORE RELATION: hb
>    21. THE PROPAGATES-BEFORE RELATION: pb
>    22. RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-order, rcu-fence, and rb
> -  23. LOCKING
> -  24. PLAIN ACCESSES AND DATA RACES
> -  25. ODDS AND ENDS
> +  23. SRCU READ-SIDE CRITICAL SECTIONS
> +  24. LOCKING
> +  25. PLAIN ACCESSES AND DATA RACES
> +  26. ODDS AND ENDS
>  
>  
>  
> @@ -1858,6 +1859,54 @@ links having the same SRCU domain with proper nesting); the details
>  are relatively unimportant.
>  
>  
> +SRCU READ-SIDE CRITICAL SECTIONS
> +--------------------------------
> +An SRCU read-side section is modeled with the srcu-rscs relation and
> +is different from rcu-rscs in the following respects:
> +
> +1. SRCU read-side sections are associated with a specific domain and
> +are independent of ones in different domains. Each domain has their
> +own independent grace-periods.
> +
> +2. Partitially overlapping SRCU read-side sections cannot fuse. It is
> +possible that among 2 partitally overlapping readers, the one that
> +starts earlier, starts before a GP started and the later reader starts
> +after the same GP started. These 2 readers are to be treated as
> +different srcu-rscs even for the same SRCU domain.
> +
> +3. The srcu_down_read() and srcu_up_read() primitives permit an SRCU
> +read-side lock to be acquired on one CPU and released another. While
> +this is also true about preemptible RCU, the LKMM does not model
> +preemption.  So unlike SRCU, RCU readers are still modeled and
> +expected to be locked and unlocked on the same CPU in litmus tests.
> +
> +To make it easy to model SRCU readers in LKMM with the above 3
> +properties, an SRCU lock operation is modeled as a load annotated with
> +'srcu-lock' and an SRCU unlock operation is modeled as a store
> +annotated with 'srcu-unlock'. This load and store takes the memory
> +address of an srcu_struct as an input, and the value returned is the
> +SRCU index (value). Thus LKMM creates a data-dependency between them
> +by virtue of the load and store memory accesses before performed on
> +the same srcu_struct:  R[srcu-lock] ->data W[srcu-unlock].
> +This data dependency becomes: R[srcu-lock] ->srcu-rscs W[srcu-unlock].
> +
> +It is also possible that the data loaded from the R[srcu-lock] is
> +stored back into a memory location, and loaded on the same or even
> +another CPU, before doing an unlock.
> +This becomes:
> +  R[srcu-lock] ->data W[once] ->rf R[once] ->data W[srcu-unlock]
> +
> +The model also treats this chaining of ->data and ->rf relations as:
> +  R[srcu-lock] ->srcu-rscs W[srcu-unlock] by the model.
> +
> +Care must be taken that:
> +  R[srcu-lock] ->data W[srcu-unlock] ->rf R[srcu-lock] is not
> +considered as a part of the above ->data and ->rf chain, which happens
> +because of one reader unlocking and another locking right after it.
> +The model excludes these ->rf relations when building the ->srcu-rscs
> +relation.
> +
> +
>  LOCKING
>  -------
>  

I took the liberty of rewriting your text to make it agree better with 
the style used in the rest of the document.  It ended up getting a lot 
bigger, but I think it will be more comprehensible to readers.  Here is 
the result.

Alan


--- usb-devel.orig/tools/memory-model/Documentation/explanation.txt
+++ usb-devel/tools/memory-model/Documentation/explanation.txt
@@ -28,9 +28,10 @@ Explanation of the Linux-Kernel Memory C
   20. THE HAPPENS-BEFORE RELATION: hb
   21. THE PROPAGATES-BEFORE RELATION: pb
   22. RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-order, rcu-fence, and rb
-  23. LOCKING
-  24. PLAIN ACCESSES AND DATA RACES
-  25. ODDS AND ENDS
+  23. SRCU READ-SIDE CRITICAL SECTIONS
+  24. LOCKING
+  25. PLAIN ACCESSES AND DATA RACES
+  26. ODDS AND ENDS
 
 
 
@@ -1848,14 +1849,157 @@ section in P0 both starts before P1's gr
 before it does, and the critical section in P2 both starts after P1's
 grace period does and ends after it does.
 
-Addendum: The LKMM now supports SRCU (Sleepable Read-Copy-Update) in
-addition to normal RCU.  The ideas involved are much the same as
-above, with new relations srcu-gp and srcu-rscsi added to represent
-SRCU grace periods and read-side critical sections.  There is a
-restriction on the srcu-gp and srcu-rscsi links that can appear in an
-rcu-order sequence (the srcu-rscsi links must be paired with srcu-gp
-links having the same SRCU domain with proper nesting); the details
-are relatively unimportant.
+The LKMM supports SRCU (Sleepable Read-Copy-Update) in addition to
+normal RCU.  The ideas involved are much the same as above, with new
+relations srcu-gp and srcu-rscsi added to represent SRCU grace periods
+and read-side critical sections.  However, there are some important
+differences between RCU read-side critical sections and their SRCU
+counterparts, as described in the next section.
+
+
+SRCU READ-SIDE CRITICAL SECTIONS
+--------------------------------
+
+The LKMM models SRCU read-side critical sections with the srcu-rscsi
+relation.  They are different from RCU read-side critical sections in
+the following respects:
+
+1.	Unlike the analogous RCU primitives, synchronize_srcu(),
+	srcu_read_lock(), and srcu_read_unlock() take a pointer to a
+	struct srcu_struct as an argument.  This structure is called
+	an SRCU domain, and calls linked by srcu-rscsi must have the
+	same domain.  Read-side critical sections and grace periods
+	associated with different domains are independent of one
+	another.  The SRCU version of the RCU Guarantee applies only
+	to pairs of critical sections and grace periods having the
+	same domain.
+
+2.	srcu_read_lock() returns a value, called the index, which must
+	be passed to the matching srcu_read_unlock() call.  Unlike
+	rcu_read_lock() and rcu_read_unlock(), an srcu_read_lock()
+	call does not always have to match the next unpaired
+	srcu_read_unlock().  In fact, it is possible for two SRCU
+	read-side critical sections to overlap partially, as in the
+	following example (where s is an srcu_struct and idx1 and idx2
+	are integer variables):
+
+		idx1 = srcu_read_lock(&s);	// Start of first RSCS
+		idx2 = srcu_read_lock(&s);	// Start of second RSCS
+		srcu_read_unlock(&s, idx1);	// End of first RSCS
+		srcu_read_unlock(&s, idx2);	// End of second RSCS
+
+	The matching is determined entirely by the domain pointer and
+	index value.  By contrast, if the calls had been
+	rcu_read_lock() and rcu_read_unlock() then they would have
+	created two nested (fully overlapping) read-side critical
+	sections: an inner one and an outer one.
+
+3.	The srcu_down_read() and srcu_up_read() primitives work
+	exactly like srcu_read_lock() and srcu_read_unlock(), except
+	that matching calls don't have to execute on the same CPU.
+	Since the matching is determined by the domain pointer and
+	index value, these primitives make it possible for an SRCU
+	read-side critical section to start on one CPU and end on
+	another, so to speak.
+
+The LKMM models srcu_read_lock() as a special type of load event
+(which is appropriate, since it takes a memory location as argument
+and returns a value, just like a load does) and srcu_read_unlock() as
+a special type of store event (again appropriate, since it takes as
+arguments a memory location and a value).  These loads and stores are
+annotated as belonging to the "srcu-lock" and "srcu-unlock" event
+classes respectively.
+
+This approach allows the LKMM to tell which unlock matches a
+particular lock, by checking for the presence of a data dependency
+from the load (srcu-lock) to the store (srcu-unlock).  For example,
+given the situation outlined earlier (with statement labels added):
+
+	A: idx1 = srcu_read_lock(&s);
+	B: idx2 = srcu_read_lock(&s);
+	C: srcu_read_unlock(&s, idx1);
+	D: srcu_read_unlock(&s, idx2);
+
+then the LKMM will treat A and B as loads from s yielding the values
+in idx1 and idx2 respectively.  Similarly, it will treat C and D as
+though they stored the values idx1 and idx2 in s.  The end result is
+as if we had written:
+
+	A: idx1 = READ_ONCE(s);
+	B: idx2 = READ_ONCE(s);
+	C: WRITE_ONCE(s, idx1);
+	D: WRITE_ONCE(s, idx2);
+
+(except for the presence of the special srcu-lock and srcu-unlock
+annotations).  You can see at once that we have A ->data C and
+B ->data D.  These dependencies tells the LKMM that C is the
+srcu-unlock event matching srcu-lock event A, and D is the
+srcu-unlock event matching srcu-lock event B.
+
+This approach is admittedly a hack, and it has the potential to lead
+to problems.  For example, in:
+
+	idx1 = srcu_read_lock(&s);
+	srcu_read_unlock(&s, idx1);
+	idx2 = srcu_read_lock(&s);
+	srcu_read_unlock(&s, idx2);
+
+the LKMM will believe that idx2 must have the same value as idx1,
+since it reads from the immediately preceding store of idx1 in s.
+Fortunately this won't matter, assuming that litmus tests never do
+anything with SRCU index values other than pass them to
+srcu_read_unlock() or srcu_up_read() calls.
+
+However, sometimes it is necessary to store an index value in a
+shared variable temporarily.  In fact, this is the only way for
+srcu_down_read() to pass the index it gets to an srcu_up_read() call
+on a different CPU.  In more detail, we might have:
+
+	struct srcu_struct s;
+	int x;
+
+	P0()
+	{
+		int r0;
+
+		A: r0 = srcu_down_read(s);
+		B: WRITE_ONCE(x, r0);
+	}
+
+	P1()
+	{
+		int r1;
+
+		C: r1 = READ_ONCE(x);
+		D: srcu_up_read(s, r1);
+	}
+
+Assuming that P1 executes after P0 and does read the index value
+stored in x, we can write this (using brackets to represent event
+annotations) as:
+
+	A[srcu-lock] ->data B[once] ->rf C[once] ->data D[srcu-unlock].
+
+The LKMM defines a carries-srcu-data relation to express this
+pattern; it permits multiple instances of a
+
+	data ; rf
+
+pair (that is, a data link followed by an rf link) to occur between an
+srcu-lock event and the final data dependency leading to the matching
+srcu-unlock event.  carry-srcu-data has to be careful that none of the
+intermediate store events in this series are instances of srcu-unlock.
+Without this protection, in a sequence like the one above:
+
+	A: idx1 = srcu_read_lock(&s);
+	B: srcu_read_unlock(&s, idx1);
+	C: idx2 = srcu_read_lock(&s);
+	D: srcu_read_unlock(&s, idx2);
+
+it would appear that B was a store to a temporary variable (i.e., s)
+and C was a load from that variable, thereby allowing carry-srcu-data
+to extend a data dependency from A to D and giving the impression
+that D was the srcu-unlock event matching A's srcu-lock.
 
 
 LOCKING
  

On Sun, Feb 19, 2023 at 11:48 AM Alan Stern <stern@rowland.harvard.edu> wrote:
>
> On Mon, Feb 13, 2023 at 01:55:06AM +0000, Joel Fernandes (Google) wrote:
> > Add details about SRCU read-side critical sections and how they are
> > modeled.
> >
> > Cc: Andrea Parri <andrea.parri@amarulasolutions.com>
> > Cc: Boqun Feng <boqun.feng@gmail.com>
> > Cc: Jade Alglave <j.alglave@ucl.ac.uk>
> > Cc: Luc Maranget <luc.maranget@inria.fr>
> > Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Will Deacon <will.deacon@arm.com>
> > Cc: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
> > Suggested-by: Alan Stern <stern@rowland.harvard.edu>
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> >
> > ---
> >  .../Documentation/explanation.txt             | 55 ++++++++++++++++++-
> >  1 file changed, 52 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
> > index 8e7085238470..5f486d39fe10 100644
> > --- a/tools/memory-model/Documentation/explanation.txt
> > +++ b/tools/memory-model/Documentation/explanation.txt
> > @@ -28,9 +28,10 @@ Explanation of the Linux-Kernel Memory Consistency Model
> >    20. THE HAPPENS-BEFORE RELATION: hb
> >    21. THE PROPAGATES-BEFORE RELATION: pb
> >    22. RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-order, rcu-fence, and rb
> > -  23. LOCKING
> > -  24. PLAIN ACCESSES AND DATA RACES
> > -  25. ODDS AND ENDS
> > +  23. SRCU READ-SIDE CRITICAL SECTIONS
> > +  24. LOCKING
> > +  25. PLAIN ACCESSES AND DATA RACES
> > +  26. ODDS AND ENDS
> >
> >
> >
> > @@ -1858,6 +1859,54 @@ links having the same SRCU domain with proper nesting); the details
> >  are relatively unimportant.
> >
> >
> > +SRCU READ-SIDE CRITICAL SECTIONS
> > +--------------------------------
> > +An SRCU read-side section is modeled with the srcu-rscs relation and
> > +is different from rcu-rscs in the following respects:
> > +
> > +1. SRCU read-side sections are associated with a specific domain and
> > +are independent of ones in different domains. Each domain has their
> > +own independent grace-periods.
> > +
> > +2. Partitially overlapping SRCU read-side sections cannot fuse. It is
> > +possible that among 2 partitally overlapping readers, the one that
> > +starts earlier, starts before a GP started and the later reader starts
> > +after the same GP started. These 2 readers are to be treated as
> > +different srcu-rscs even for the same SRCU domain.
> > +
> > +3. The srcu_down_read() and srcu_up_read() primitives permit an SRCU
> > +read-side lock to be acquired on one CPU and released another. While
> > +this is also true about preemptible RCU, the LKMM does not model
> > +preemption.  So unlike SRCU, RCU readers are still modeled and
> > +expected to be locked and unlocked on the same CPU in litmus tests.
> > +
> > +To make it easy to model SRCU readers in LKMM with the above 3
> > +properties, an SRCU lock operation is modeled as a load annotated with
> > +'srcu-lock' and an SRCU unlock operation is modeled as a store
> > +annotated with 'srcu-unlock'. This load and store takes the memory
> > +address of an srcu_struct as an input, and the value returned is the
> > +SRCU index (value). Thus LKMM creates a data-dependency between them
> > +by virtue of the load and store memory accesses before performed on
> > +the same srcu_struct:  R[srcu-lock] ->data W[srcu-unlock].
> > +This data dependency becomes: R[srcu-lock] ->srcu-rscs W[srcu-unlock].
> > +
> > +It is also possible that the data loaded from the R[srcu-lock] is
> > +stored back into a memory location, and loaded on the same or even
> > +another CPU, before doing an unlock.
> > +This becomes:
> > +  R[srcu-lock] ->data W[once] ->rf R[once] ->data W[srcu-unlock]
> > +
> > +The model also treats this chaining of ->data and ->rf relations as:
> > +  R[srcu-lock] ->srcu-rscs W[srcu-unlock] by the model.
> > +
> > +Care must be taken that:
> > +  R[srcu-lock] ->data W[srcu-unlock] ->rf R[srcu-lock] is not
> > +considered as a part of the above ->data and ->rf chain, which happens
> > +because of one reader unlocking and another locking right after it.
> > +The model excludes these ->rf relations when building the ->srcu-rscs
> > +relation.
> > +
> > +
> >  LOCKING
> >  -------
> >
> I took the liberty of rewriting your text to make it agree better with
> the style used in the rest of the document.  It ended up getting a lot
> bigger, but I think it will be more comprehensible to readers.  Here is
> the result.

Great writeup! One comment below:

> Alan
>
>
> --- usb-devel.orig/tools/memory-model/Documentation/explanation.txt
> +++ usb-devel/tools/memory-model/Documentation/explanation.txt
> @@ -28,9 +28,10 @@ Explanation of the Linux-Kernel Memory C
>    20. THE HAPPENS-BEFORE RELATION: hb
>    21. THE PROPAGATES-BEFORE RELATION: pb
>    22. RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-order, rcu-fence, and rb
> -  23. LOCKING
> -  24. PLAIN ACCESSES AND DATA RACES
> -  25. ODDS AND ENDS
> +  23. SRCU READ-SIDE CRITICAL SECTIONS
> +  24. LOCKING
> +  25. PLAIN ACCESSES AND DATA RACES
> +  26. ODDS AND ENDS
>
>
>
> @@ -1848,14 +1849,157 @@ section in P0 both starts before P1's gr
>  before it does, and the critical section in P2 both starts after P1's
>  grace period does and ends after it does.
>
> -Addendum: The LKMM now supports SRCU (Sleepable Read-Copy-Update) in
> -addition to normal RCU.  The ideas involved are much the same as
> -above, with new relations srcu-gp and srcu-rscsi added to represent
> -SRCU grace periods and read-side critical sections.  There is a
> -restriction on the srcu-gp and srcu-rscsi links that can appear in an
> -rcu-order sequence (the srcu-rscsi links must be paired with srcu-gp
> -links having the same SRCU domain with proper nesting); the details
> -are relatively unimportant.
> +The LKMM supports SRCU (Sleepable Read-Copy-Update) in addition to
> +normal RCU.  The ideas involved are much the same as above, with new
> +relations srcu-gp and srcu-rscsi added to represent SRCU grace periods
> +and read-side critical sections.  However, there are some important
> +differences between RCU read-side critical sections and their SRCU
> +counterparts, as described in the next section.
> +
> +
> +SRCU READ-SIDE CRITICAL SECTIONS
> +--------------------------------
> +
> +The LKMM models SRCU read-side critical sections with the srcu-rscsi
> +relation.  They are different from RCU read-side critical sections in
> +the following respects:
> +
> +1.     Unlike the analogous RCU primitives, synchronize_srcu(),
> +       srcu_read_lock(), and srcu_read_unlock() take a pointer to a
> +       struct srcu_struct as an argument.  This structure is called
> +       an SRCU domain, and calls linked by srcu-rscsi must have the
> +       same domain.  Read-side critical sections and grace periods
> +       associated with different domains are independent of one
> +       another.  The SRCU version of the RCU Guarantee applies only
> +       to pairs of critical sections and grace periods having the
> +       same domain.
> +
> +2.     srcu_read_lock() returns a value, called the index, which must
> +       be passed to the matching srcu_read_unlock() call.  Unlike
> +       rcu_read_lock() and rcu_read_unlock(), an srcu_read_lock()
> +       call does not always have to match the next unpaired
> +       srcu_read_unlock().  In fact, it is possible for two SRCU
> +       read-side critical sections to overlap partially, as in the
> +       following example (where s is an srcu_struct and idx1 and idx2
> +       are integer variables):
> +
> +               idx1 = srcu_read_lock(&s);      // Start of first RSCS
> +               idx2 = srcu_read_lock(&s);      // Start of second RSCS
> +               srcu_read_unlock(&s, idx1);     // End of first RSCS
> +               srcu_read_unlock(&s, idx2);     // End of second RSCS
> +
> +       The matching is determined entirely by the domain pointer and
> +       index value.  By contrast, if the calls had been
> +       rcu_read_lock() and rcu_read_unlock() then they would have
> +       created two nested (fully overlapping) read-side critical
> +       sections: an inner one and an outer one.
> +
> +3.     The srcu_down_read() and srcu_up_read() primitives work
> +       exactly like srcu_read_lock() and srcu_read_unlock(), except
> +       that matching calls don't have to execute on the same CPU.
> +       Since the matching is determined by the domain pointer and
> +       index value, these primitives make it possible for an SRCU
> +       read-side critical section to start on one CPU and end on
> +       another, so to speak.
> +
> +The LKMM models srcu_read_lock() as a special type of load event
> +(which is appropriate, since it takes a memory location as argument
> +and returns a value, just like a load does) and srcu_read_unlock() as
> +a special type of store event (again appropriate, since it takes as
> +arguments a memory location and a value).  These loads and stores are
> +annotated as belonging to the "srcu-lock" and "srcu-unlock" event
> +classes respectively.
> +
> +This approach allows the LKMM to tell which unlock matches a
> +particular lock, by checking for the presence of a data dependency
> +from the load (srcu-lock) to the store (srcu-unlock).  For example,
> +given the situation outlined earlier (with statement labels added):
> +
> +       A: idx1 = srcu_read_lock(&s);
> +       B: idx2 = srcu_read_lock(&s);
> +       C: srcu_read_unlock(&s, idx1);
> +       D: srcu_read_unlock(&s, idx2);
> +
> +then the LKMM will treat A and B as loads from s yielding the values
> +in idx1 and idx2 respectively.  Similarly, it will treat C and D as
> +though they stored the values idx1 and idx2 in s.  The end result is
> +as if we had written:
> +
> +       A: idx1 = READ_ONCE(s);
> +       B: idx2 = READ_ONCE(s);
> +       C: WRITE_ONCE(s, idx1);
> +       D: WRITE_ONCE(s, idx2);
> +
> +(except for the presence of the special srcu-lock and srcu-unlock
> +annotations).  You can see at once that we have A ->data C and
> +B ->data D.  These dependencies tells the LKMM that C is the
> +srcu-unlock event matching srcu-lock event A, and D is the
> +srcu-unlock event matching srcu-lock event B.
> +
> +This approach is admittedly a hack, and it has the potential to lead
> +to problems.  For example, in:
> +
> +       idx1 = srcu_read_lock(&s);
> +       srcu_read_unlock(&s, idx1);
> +       idx2 = srcu_read_lock(&s);
> +       srcu_read_unlock(&s, idx2);
> +
> +the LKMM will believe that idx2 must have the same value as idx1,
> +since it reads from the immediately preceding store of idx1 in s.
> +Fortunately this won't matter, assuming that litmus tests never do
> +anything with SRCU index values other than pass them to
> +srcu_read_unlock() or srcu_up_read() calls.
> +
> +However, sometimes it is necessary to store an index value in a
> +shared variable temporarily.  In fact, this is the only way for
> +srcu_down_read() to pass the index it gets to an srcu_up_read() call
> +on a different CPU.  In more detail, we might have:
> +
> +       struct srcu_struct s;
> +       int x;
> +
> +       P0()
> +       {
> +               int r0;
> +
> +               A: r0 = srcu_down_read(s);
> +               B: WRITE_ONCE(x, r0);
> +       }
> +
> +       P1()
> +       {
> +               int r1;
> +
> +               C: r1 = READ_ONCE(x);
> +               D: srcu_up_read(s, r1);
> +       }
> +
> +Assuming that P1 executes after P0 and does read the index value
> +stored in x, we can write this (using brackets to represent event
> +annotations) as:
> +
> +       A[srcu-lock] ->data B[once] ->rf C[once] ->data D[srcu-unlock].
> +
> +The LKMM defines a carries-srcu-data relation to express this
> +pattern; it permits multiple instances of a
> +
> +       data ; rf
> +
> +pair (that is, a data link followed by an rf link) to occur between an
> +srcu-lock event and the final data dependency leading to the matching
> +srcu-unlock event.  carry-srcu-data has to be careful that none of the
> +intermediate store events in this series are instances of srcu-unlock.
> +Without this protection, in a sequence like the one above:
> +
> +       A: idx1 = srcu_read_lock(&s);
> +       B: srcu_read_unlock(&s, idx1);
> +       C: idx2 = srcu_read_lock(&s);
> +       D: srcu_read_unlock(&s, idx2);
> +
> +it would appear that B was a store to a temporary variable (i.e., s)
> +and C was a load from that variable, thereby allowing carry-srcu-data
> +to extend a data dependency from A to D and giving the impression
> +that D was the srcu-unlock event matching A's srcu-lock.

Even though it may be redundant: would it be possible to also mention
(after this paragraph) that this case forms an undesirable "->rf" link
between B and C, which then causes us to link A and D as a result?

A[srcu-lock] ->data B[once] ->rf C[once] ->data D[srcu-unlock].

Just an optional suggestion and I am happy with the change either way:

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Thanks,

 - Joel
  

On Sun, Feb 19, 2023 at 12:11 PM Joel Fernandes <joel@joelfernandes.org> wrote:
>
> On Sun, Feb 19, 2023 at 11:48 AM Alan Stern <stern@rowland.harvard.edu> wrote:
> >
> > On Mon, Feb 13, 2023 at 01:55:06AM +0000, Joel Fernandes (Google) wrote:
[...]
> > +       A: idx1 = srcu_read_lock(&s);
> > +       B: srcu_read_unlock(&s, idx1);
> > +       C: idx2 = srcu_read_lock(&s);
> > +       D: srcu_read_unlock(&s, idx2);
> > +
> > +it would appear that B was a store to a temporary variable (i.e., s)
> > +and C was a load from that variable, thereby allowing carry-srcu-data
> > +to extend a data dependency from A to D and giving the impression
> > +that D was the srcu-unlock event matching A's srcu-lock.
>
> Even though it may be redundant: would it be possible to also mention
> (after this paragraph) that this case forms an undesirable "->rf" link
> between B and C, which then causes us to link A and D as a result?
>
> A[srcu-lock] ->data B[once] ->rf C[once] ->data D[srcu-unlock].

Apologies, I meant here, care must be taken to avoid:

A[srcu-lock] ->data B[srcu-unlock] ->rf C[srcu-lock] ->data D[srcu-unlock].

Thanks,

  - Joel
  

On Sun, Feb 19, 2023 at 12:13:14PM -0500, Joel Fernandes wrote:
> On Sun, Feb 19, 2023 at 12:11 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> > Even though it may be redundant: would it be possible to also mention
> > (after this paragraph) that this case forms an undesirable "->rf" link
> > between B and C, which then causes us to link A and D as a result?
> >
> > A[srcu-lock] ->data B[once] ->rf C[once] ->data D[srcu-unlock].
> 
> Apologies, I meant here, care must be taken to avoid:
> 
> A[srcu-lock] ->data B[srcu-unlock] ->rf C[srcu-lock] ->data D[srcu-unlock].

Revised patch below.  I changed more than just this bit.  Mostly small 
edits to improve readability, but I did add a little additional 
material.

Alan



--- usb-devel.orig/tools/memory-model/Documentation/explanation.txt
+++ usb-devel/tools/memory-model/Documentation/explanation.txt
@@ -28,9 +28,10 @@ Explanation of the Linux-Kernel Memory C
   20. THE HAPPENS-BEFORE RELATION: hb
   21. THE PROPAGATES-BEFORE RELATION: pb
   22. RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-order, rcu-fence, and rb
-  23. LOCKING
-  24. PLAIN ACCESSES AND DATA RACES
-  25. ODDS AND ENDS
+  23. SRCU READ-SIDE CRITICAL SECTIONS
+  24. LOCKING
+  25. PLAIN ACCESSES AND DATA RACES
+  26. ODDS AND ENDS
 
 
 
@@ -1848,14 +1849,169 @@ section in P0 both starts before P1's gr
 before it does, and the critical section in P2 both starts after P1's
 grace period does and ends after it does.
 
-Addendum: The LKMM now supports SRCU (Sleepable Read-Copy-Update) in
-addition to normal RCU.  The ideas involved are much the same as
-above, with new relations srcu-gp and srcu-rscsi added to represent
-SRCU grace periods and read-side critical sections.  There is a
-restriction on the srcu-gp and srcu-rscsi links that can appear in an
-rcu-order sequence (the srcu-rscsi links must be paired with srcu-gp
-links having the same SRCU domain with proper nesting); the details
-are relatively unimportant.
+The LKMM supports SRCU (Sleepable Read-Copy-Update) in addition to
+normal RCU.  The ideas involved are much the same as above, with new
+relations srcu-gp and srcu-rscsi added to represent SRCU grace periods
+and read-side critical sections.  However, there are some important
+differences between RCU read-side critical sections and their SRCU
+counterparts, as described in the next section.
+
+
+SRCU READ-SIDE CRITICAL SECTIONS
+--------------------------------
+
+The LKMM models uses the srcu-rscsi relation to model SRCU read-side
+critical sections.  They are different from RCU read-side critical
+sections in the following respects:
+
+1.	Unlike the analogous RCU primitives, synchronize_srcu(),
+	srcu_read_lock(), and srcu_read_unlock() take a pointer to a
+	struct srcu_struct as an argument.  This structure is called
+	an SRCU domain, and calls linked by srcu-rscsi must have the
+	same domain.  Read-side critical sections and grace periods
+	associated with different domains are independent of one
+	another; the SRCU version of the RCU Guarantee applies only
+	to pairs of critical sections and grace periods having the
+	same domain.
+
+2.	srcu_read_lock() returns a value, called the index, which must
+	be passed to the matching srcu_read_unlock() call.  Unlike
+	rcu_read_lock() and rcu_read_unlock(), an srcu_read_lock()
+	call does not always have to match the next unpaired
+	srcu_read_unlock().  In fact, it is possible for two SRCU
+	read-side critical sections to overlap partially, as in the
+	following example (where s is an srcu_struct and idx1 and idx2
+	are integer variables):
+
+		idx1 = srcu_read_lock(&s);	// Start of first RSCS
+		idx2 = srcu_read_lock(&s);	// Start of second RSCS
+		srcu_read_unlock(&s, idx1);	// End of first RSCS
+		srcu_read_unlock(&s, idx2);	// End of second RSCS
+
+	The matching is determined entirely by the domain pointer and
+	index value.  By contrast, if the calls had been
+	rcu_read_lock() and rcu_read_unlock() then they would have
+	created two nested (fully overlapping) read-side critical
+	sections: an inner one and an outer one.
+
+3.	The srcu_down_read() and srcu_up_read() primitives work
+	exactly like srcu_read_lock() and srcu_read_unlock(), except
+	that matching calls don't have to execute on the same CPU.
+	(The names are meant to be suggestive of operations on
+	semaphores.)  Since the matching is determined by the domain
+	pointer and index value, these primitives make it possible for
+	an SRCU read-side critical section to start on one CPU and end
+	on another, so to speak.
+
+In order to account for these properties of SRCU, the LKMM models
+srcu_read_lock() as a special type of load event (which is
+appropriate, since it takes a memory location as argument and returns
+a value, just as a load does) and srcu_read_unlock() as a special type
+of store event (again appropriate, since it takes as arguments a
+memory location and a value).  These loads and stores are annotated as
+belonging to the "srcu-lock" and "srcu-unlock" event classes
+respectively.
+
+This approach allows the LKMM to tell whether two events are
+associated with the same SRCU domain, simply by checking whether they
+access the same memory location (i.e., they are linked by the loc
+relation).  It also gives a way to tell which unlock matches a
+particular lock, by checking for the presence of a data dependency
+from the load (srcu-lock) to the store (srcu-unlock).  For example,
+given the situation outlined earlier (with statement labels added):
+
+	A: idx1 = srcu_read_lock(&s);
+	B: idx2 = srcu_read_lock(&s);
+	C: srcu_read_unlock(&s, idx1);
+	D: srcu_read_unlock(&s, idx2);
+
+the LKMM will treat A and B as loads from s yielding values saved in
+idx1 and idx2 respectively.  Similarly, it will treat C and D as
+though they stored the values from idx1 and idx2 in s.  The end result
+is much as if we had written:
+
+	A: idx1 = READ_ONCE(s);
+	B: idx2 = READ_ONCE(s);
+	C: WRITE_ONCE(s, idx1);
+	D: WRITE_ONCE(s, idx2);
+
+except for the presence of the special srcu-lock and srcu-unlock
+annotations.  You can see at once that we have A ->data C and
+B ->data D.  These dependencies tell the LKMM that C is the
+srcu-unlock event matching srcu-lock event A, and D is the
+srcu-unlock event matching srcu-lock event B.
+
+This approach is admittedly a hack, and it has the potential to lead
+to problems.  For example, in:
+
+	idx1 = srcu_read_lock(&s);
+	srcu_read_unlock(&s, idx1);
+	idx2 = srcu_read_lock(&s);
+	srcu_read_unlock(&s, idx2);
+
+the LKMM will believe that idx2 must have the same value as idx1,
+since it reads from the immediately preceding store of idx1 in s.
+Fortunately this won't matter, assuming that litmus tests never do
+anything with SRCU index values other than pass them to
+srcu_read_unlock() or srcu_up_read() calls.
+
+However, sometimes it is necessary to store an index value in a
+shared variable temporarily.  In fact, this is the only way for
+srcu_down_read() to pass the index it gets to an srcu_up_read() call
+on a different CPU.  In more detail, we might have soething like:
+
+	struct srcu_struct s;
+	int x;
+
+	P0()
+	{
+		int r0;
+
+		A: r0 = srcu_down_read(&s);
+		B: WRITE_ONCE(x, r0);
+	}
+
+	P1()
+	{
+		int r1;
+
+		C: r1 = READ_ONCE(x);
+		D: srcu_up_read(&s, r1);
+	}
+
+Assuming that P1 executes after P0 and does read the index value
+stored in x, we can write this (using brackets to represent event
+annotations) as:
+
+	A[srcu-lock] ->data B[once] ->rf C[once] ->data D[srcu-unlock].
+
+The LKMM defines a carries-srcu-data relation to express this
+pattern; it permits an arbitrarily long sequence of
+
+	data ; rf
+
+pairs (that is, a data link followed by an rf link) to occur between
+an srcu-lock event and the final data dependency leading to the
+matching srcu-unlock event.  carry-srcu-data is complicated by the
+need to ensure that none of the intermediate store events in this
+sequence are instances of srcu-unlock.  This is necessary because in a
+pattern like the one above:
+
+	A: idx1 = srcu_read_lock(&s);
+	B: srcu_read_unlock(&s, idx1);
+	C: idx2 = srcu_read_lock(&s);
+	D: srcu_read_unlock(&s, idx2);
+
+the LKMM treats B as a store to the variable s and C as a load from
+that variable, creating an undesirable rf link from B to C:
+
+	A ->data B ->rf C ->data D.
+
+This would cause carry-srcu-data to mistakenly extend a data
+dependency from A to D and give the impression that D was the
+srcu-unlock event matching A's srcu-lock.  To avoid such problems,
+carry-srcu-data does not accept sequences in which the ends of any of
+the intermediate ->data links (B above) is an srcu-unlock event.
 
 
 LOCKING
  

> On Feb 20, 2023, at 4:06 PM, Alan Stern <stern@rowland.harvard.edu> wrote:
> 
> On Sun, Feb 19, 2023 at 12:13:14PM -0500, Joel Fernandes wrote:
>>> On Sun, Feb 19, 2023 at 12:11 PM Joel Fernandes <joel@joelfernandes.org> wrote:
>>> Even though it may be redundant: would it be possible to also mention
>>> (after this paragraph) that this case forms an undesirable "->rf" link
>>> between B and C, which then causes us to link A and D as a result?
>>> 
>>> A[srcu-lock] ->data B[once] ->rf C[once] ->data D[srcu-unlock].
>> 
>> Apologies, I meant here, care must be taken to avoid:
>> 
>> A[srcu-lock] ->data B[srcu-unlock] ->rf C[srcu-lock] ->data D[srcu-unlock].
> 
> Revised patch below.  I changed more than just this bit.  Mostly small 
> edits to improve readability, but I did add a little additional 
> material.

Looks good to me. Thanks!

 - Joel


> 
> Alan
> 
> 
> 
> --- usb-devel.orig/tools/memory-model/Documentation/explanation.txt
> +++ usb-devel/tools/memory-model/Documentation/explanation.txt
> @@ -28,9 +28,10 @@ Explanation of the Linux-Kernel Memory C
>   20. THE HAPPENS-BEFORE RELATION: hb
>   21. THE PROPAGATES-BEFORE RELATION: pb
>   22. RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-order, rcu-fence, and rb
> -  23. LOCKING
> -  24. PLAIN ACCESSES AND DATA RACES
> -  25. ODDS AND ENDS
> +  23. SRCU READ-SIDE CRITICAL SECTIONS
> +  24. LOCKING
> +  25. PLAIN ACCESSES AND DATA RACES
> +  26. ODDS AND ENDS
> 
> 
> 
> @@ -1848,14 +1849,169 @@ section in P0 both starts before P1's gr
> before it does, and the critical section in P2 both starts after P1's
> grace period does and ends after it does.
> 
> -Addendum: The LKMM now supports SRCU (Sleepable Read-Copy-Update) in
> -addition to normal RCU.  The ideas involved are much the same as
> -above, with new relations srcu-gp and srcu-rscsi added to represent
> -SRCU grace periods and read-side critical sections.  There is a
> -restriction on the srcu-gp and srcu-rscsi links that can appear in an
> -rcu-order sequence (the srcu-rscsi links must be paired with srcu-gp
> -links having the same SRCU domain with proper nesting); the details
> -are relatively unimportant.
> +The LKMM supports SRCU (Sleepable Read-Copy-Update) in addition to
> +normal RCU.  The ideas involved are much the same as above, with new
> +relations srcu-gp and srcu-rscsi added to represent SRCU grace periods
> +and read-side critical sections.  However, there are some important
> +differences between RCU read-side critical sections and their SRCU
> +counterparts, as described in the next section.
> +
> +
> +SRCU READ-SIDE CRITICAL SECTIONS
> +--------------------------------
> +
> +The LKMM models uses the srcu-rscsi relation to model SRCU read-side
> +critical sections.  They are different from RCU read-side critical
> +sections in the following respects:
> +
> +1.    Unlike the analogous RCU primitives, synchronize_srcu(),
> +    srcu_read_lock(), and srcu_read_unlock() take a pointer to a
> +    struct srcu_struct as an argument.  This structure is called
> +    an SRCU domain, and calls linked by srcu-rscsi must have the
> +    same domain.  Read-side critical sections and grace periods
> +    associated with different domains are independent of one
> +    another; the SRCU version of the RCU Guarantee applies only
> +    to pairs of critical sections and grace periods having the
> +    same domain.
> +
> +2.    srcu_read_lock() returns a value, called the index, which must
> +    be passed to the matching srcu_read_unlock() call.  Unlike
> +    rcu_read_lock() and rcu_read_unlock(), an srcu_read_lock()
> +    call does not always have to match the next unpaired
> +    srcu_read_unlock().  In fact, it is possible for two SRCU
> +    read-side critical sections to overlap partially, as in the
> +    following example (where s is an srcu_struct and idx1 and idx2
> +    are integer variables):
> +
> +        idx1 = srcu_read_lock(&s);    // Start of first RSCS
> +        idx2 = srcu_read_lock(&s);    // Start of second RSCS
> +        srcu_read_unlock(&s, idx1);    // End of first RSCS
> +        srcu_read_unlock(&s, idx2);    // End of second RSCS
> +
> +    The matching is determined entirely by the domain pointer and
> +    index value.  By contrast, if the calls had been
> +    rcu_read_lock() and rcu_read_unlock() then they would have
> +    created two nested (fully overlapping) read-side critical
> +    sections: an inner one and an outer one.
> +
> +3.    The srcu_down_read() and srcu_up_read() primitives work
> +    exactly like srcu_read_lock() and srcu_read_unlock(), except
> +    that matching calls don't have to execute on the same CPU.
> +    (The names are meant to be suggestive of operations on
> +    semaphores.)  Since the matching is determined by the domain
> +    pointer and index value, these primitives make it possible for
> +    an SRCU read-side critical section to start on one CPU and end
> +    on another, so to speak.
> +
> +In order to account for these properties of SRCU, the LKMM models
> +srcu_read_lock() as a special type of load event (which is
> +appropriate, since it takes a memory location as argument and returns
> +a value, just as a load does) and srcu_read_unlock() as a special type
> +of store event (again appropriate, since it takes as arguments a
> +memory location and a value).  These loads and stores are annotated as
> +belonging to the "srcu-lock" and "srcu-unlock" event classes
> +respectively.
> +
> +This approach allows the LKMM to tell whether two events are
> +associated with the same SRCU domain, simply by checking whether they
> +access the same memory location (i.e., they are linked by the loc
> +relation).  It also gives a way to tell which unlock matches a
> +particular lock, by checking for the presence of a data dependency
> +from the load (srcu-lock) to the store (srcu-unlock).  For example,
> +given the situation outlined earlier (with statement labels added):
> +
> +    A: idx1 = srcu_read_lock(&s);
> +    B: idx2 = srcu_read_lock(&s);
> +    C: srcu_read_unlock(&s, idx1);
> +    D: srcu_read_unlock(&s, idx2);
> +
> +the LKMM will treat A and B as loads from s yielding values saved in
> +idx1 and idx2 respectively.  Similarly, it will treat C and D as
> +though they stored the values from idx1 and idx2 in s.  The end result
> +is much as if we had written:
> +
> +    A: idx1 = READ_ONCE(s);
> +    B: idx2 = READ_ONCE(s);
> +    C: WRITE_ONCE(s, idx1);
> +    D: WRITE_ONCE(s, idx2);
> +
> +except for the presence of the special srcu-lock and srcu-unlock
> +annotations.  You can see at once that we have A ->data C and
> +B ->data D.  These dependencies tell the LKMM that C is the
> +srcu-unlock event matching srcu-lock event A, and D is the
> +srcu-unlock event matching srcu-lock event B.
> +
> +This approach is admittedly a hack, and it has the potential to lead
> +to problems.  For example, in:
> +
> +    idx1 = srcu_read_lock(&s);
> +    srcu_read_unlock(&s, idx1);
> +    idx2 = srcu_read_lock(&s);
> +    srcu_read_unlock(&s, idx2);
> +
> +the LKMM will believe that idx2 must have the same value as idx1,
> +since it reads from the immediately preceding store of idx1 in s.
> +Fortunately this won't matter, assuming that litmus tests never do
> +anything with SRCU index values other than pass them to
> +srcu_read_unlock() or srcu_up_read() calls.
> +
> +However, sometimes it is necessary to store an index value in a
> +shared variable temporarily.  In fact, this is the only way for
> +srcu_down_read() to pass the index it gets to an srcu_up_read() call
> +on a different CPU.  In more detail, we might have soething like:
> +
> +    struct srcu_struct s;
> +    int x;
> +
> +    P0()
> +    {
> +        int r0;
> +
> +        A: r0 = srcu_down_read(&s);
> +        B: WRITE_ONCE(x, r0);
> +    }
> +
> +    P1()
> +    {
> +        int r1;
> +
> +        C: r1 = READ_ONCE(x);
> +        D: srcu_up_read(&s, r1);
> +    }
> +
> +Assuming that P1 executes after P0 and does read the index value
> +stored in x, we can write this (using brackets to represent event
> +annotations) as:
> +
> +    A[srcu-lock] ->data B[once] ->rf C[once] ->data D[srcu-unlock].
> +
> +The LKMM defines a carries-srcu-data relation to express this
> +pattern; it permits an arbitrarily long sequence of
> +
> +    data ; rf
> +
> +pairs (that is, a data link followed by an rf link) to occur between
> +an srcu-lock event and the final data dependency leading to the
> +matching srcu-unlock event.  carry-srcu-data is complicated by the
> +need to ensure that none of the intermediate store events in this
> +sequence are instances of srcu-unlock.  This is necessary because in a
> +pattern like the one above:
> +
> +    A: idx1 = srcu_read_lock(&s);
> +    B: srcu_read_unlock(&s, idx1);
> +    C: idx2 = srcu_read_lock(&s);
> +    D: srcu_read_unlock(&s, idx2);
> +
> +the LKMM treats B as a store to the variable s and C as a load from
> +that variable, creating an undesirable rf link from B to C:
> +
> +    A ->data B ->rf C ->data D.
> +
> +This would cause carry-srcu-data to mistakenly extend a data
> +dependency from A to D and give the impression that D was the
> +srcu-unlock event matching A's srcu-lock.  To avoid such problems,
> +carry-srcu-data does not accept sequences in which the ends of any of
> +the intermediate ->data links (B above) is an srcu-unlock event.
> 
> 
> LOCKING
>
  

On Mon, Feb 20, 2023 at 04:06:13PM -0500, Alan Stern wrote:
> On Sun, Feb 19, 2023 at 12:13:14PM -0500, Joel Fernandes wrote:
> > On Sun, Feb 19, 2023 at 12:11 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> > > Even though it may be redundant: would it be possible to also mention
> > > (after this paragraph) that this case forms an undesirable "->rf" link
> > > between B and C, which then causes us to link A and D as a result?
> > >
> > > A[srcu-lock] ->data B[once] ->rf C[once] ->data D[srcu-unlock].
> > 
> > Apologies, I meant here, care must be taken to avoid:
> > 
> > A[srcu-lock] ->data B[srcu-unlock] ->rf C[srcu-lock] ->data D[srcu-unlock].
> 
> Revised patch below.  I changed more than just this bit.  Mostly small 
> edits to improve readability, but I did add a little additional 
> material.

Looks good to me, thank you!

Would you like to send a formal patch, or are you thinking in terms
of making other changes first?

							Thanx, Paul

> Alan
> 
> 
> 
> --- usb-devel.orig/tools/memory-model/Documentation/explanation.txt
> +++ usb-devel/tools/memory-model/Documentation/explanation.txt
> @@ -28,9 +28,10 @@ Explanation of the Linux-Kernel Memory C
>    20. THE HAPPENS-BEFORE RELATION: hb
>    21. THE PROPAGATES-BEFORE RELATION: pb
>    22. RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-order, rcu-fence, and rb
> -  23. LOCKING
> -  24. PLAIN ACCESSES AND DATA RACES
> -  25. ODDS AND ENDS
> +  23. SRCU READ-SIDE CRITICAL SECTIONS
> +  24. LOCKING
> +  25. PLAIN ACCESSES AND DATA RACES
> +  26. ODDS AND ENDS
>  
>  
>  
> @@ -1848,14 +1849,169 @@ section in P0 both starts before P1's gr
>  before it does, and the critical section in P2 both starts after P1's
>  grace period does and ends after it does.
>  
> -Addendum: The LKMM now supports SRCU (Sleepable Read-Copy-Update) in
> -addition to normal RCU.  The ideas involved are much the same as
> -above, with new relations srcu-gp and srcu-rscsi added to represent
> -SRCU grace periods and read-side critical sections.  There is a
> -restriction on the srcu-gp and srcu-rscsi links that can appear in an
> -rcu-order sequence (the srcu-rscsi links must be paired with srcu-gp
> -links having the same SRCU domain with proper nesting); the details
> -are relatively unimportant.
> +The LKMM supports SRCU (Sleepable Read-Copy-Update) in addition to
> +normal RCU.  The ideas involved are much the same as above, with new
> +relations srcu-gp and srcu-rscsi added to represent SRCU grace periods
> +and read-side critical sections.  However, there are some important
> +differences between RCU read-side critical sections and their SRCU
> +counterparts, as described in the next section.
> +
> +
> +SRCU READ-SIDE CRITICAL SECTIONS
> +--------------------------------
> +
> +The LKMM models uses the srcu-rscsi relation to model SRCU read-side
> +critical sections.  They are different from RCU read-side critical
> +sections in the following respects:
> +
> +1.	Unlike the analogous RCU primitives, synchronize_srcu(),
> +	srcu_read_lock(), and srcu_read_unlock() take a pointer to a
> +	struct srcu_struct as an argument.  This structure is called
> +	an SRCU domain, and calls linked by srcu-rscsi must have the
> +	same domain.  Read-side critical sections and grace periods
> +	associated with different domains are independent of one
> +	another; the SRCU version of the RCU Guarantee applies only
> +	to pairs of critical sections and grace periods having the
> +	same domain.
> +
> +2.	srcu_read_lock() returns a value, called the index, which must
> +	be passed to the matching srcu_read_unlock() call.  Unlike
> +	rcu_read_lock() and rcu_read_unlock(), an srcu_read_lock()
> +	call does not always have to match the next unpaired
> +	srcu_read_unlock().  In fact, it is possible for two SRCU
> +	read-side critical sections to overlap partially, as in the
> +	following example (where s is an srcu_struct and idx1 and idx2
> +	are integer variables):
> +
> +		idx1 = srcu_read_lock(&s);	// Start of first RSCS
> +		idx2 = srcu_read_lock(&s);	// Start of second RSCS
> +		srcu_read_unlock(&s, idx1);	// End of first RSCS
> +		srcu_read_unlock(&s, idx2);	// End of second RSCS
> +
> +	The matching is determined entirely by the domain pointer and
> +	index value.  By contrast, if the calls had been
> +	rcu_read_lock() and rcu_read_unlock() then they would have
> +	created two nested (fully overlapping) read-side critical
> +	sections: an inner one and an outer one.
> +
> +3.	The srcu_down_read() and srcu_up_read() primitives work
> +	exactly like srcu_read_lock() and srcu_read_unlock(), except
> +	that matching calls don't have to execute on the same CPU.
> +	(The names are meant to be suggestive of operations on
> +	semaphores.)  Since the matching is determined by the domain
> +	pointer and index value, these primitives make it possible for
> +	an SRCU read-side critical section to start on one CPU and end
> +	on another, so to speak.
> +
> +In order to account for these properties of SRCU, the LKMM models
> +srcu_read_lock() as a special type of load event (which is
> +appropriate, since it takes a memory location as argument and returns
> +a value, just as a load does) and srcu_read_unlock() as a special type
> +of store event (again appropriate, since it takes as arguments a
> +memory location and a value).  These loads and stores are annotated as
> +belonging to the "srcu-lock" and "srcu-unlock" event classes
> +respectively.
> +
> +This approach allows the LKMM to tell whether two events are
> +associated with the same SRCU domain, simply by checking whether they
> +access the same memory location (i.e., they are linked by the loc
> +relation).  It also gives a way to tell which unlock matches a
> +particular lock, by checking for the presence of a data dependency
> +from the load (srcu-lock) to the store (srcu-unlock).  For example,
> +given the situation outlined earlier (with statement labels added):
> +
> +	A: idx1 = srcu_read_lock(&s);
> +	B: idx2 = srcu_read_lock(&s);
> +	C: srcu_read_unlock(&s, idx1);
> +	D: srcu_read_unlock(&s, idx2);
> +
> +the LKMM will treat A and B as loads from s yielding values saved in
> +idx1 and idx2 respectively.  Similarly, it will treat C and D as
> +though they stored the values from idx1 and idx2 in s.  The end result
> +is much as if we had written:
> +
> +	A: idx1 = READ_ONCE(s);
> +	B: idx2 = READ_ONCE(s);
> +	C: WRITE_ONCE(s, idx1);
> +	D: WRITE_ONCE(s, idx2);
> +
> +except for the presence of the special srcu-lock and srcu-unlock
> +annotations.  You can see at once that we have A ->data C and
> +B ->data D.  These dependencies tell the LKMM that C is the
> +srcu-unlock event matching srcu-lock event A, and D is the
> +srcu-unlock event matching srcu-lock event B.
> +
> +This approach is admittedly a hack, and it has the potential to lead
> +to problems.  For example, in:
> +
> +	idx1 = srcu_read_lock(&s);
> +	srcu_read_unlock(&s, idx1);
> +	idx2 = srcu_read_lock(&s);
> +	srcu_read_unlock(&s, idx2);
> +
> +the LKMM will believe that idx2 must have the same value as idx1,
> +since it reads from the immediately preceding store of idx1 in s.
> +Fortunately this won't matter, assuming that litmus tests never do
> +anything with SRCU index values other than pass them to
> +srcu_read_unlock() or srcu_up_read() calls.
> +
> +However, sometimes it is necessary to store an index value in a
> +shared variable temporarily.  In fact, this is the only way for
> +srcu_down_read() to pass the index it gets to an srcu_up_read() call
> +on a different CPU.  In more detail, we might have soething like:
> +
> +	struct srcu_struct s;
> +	int x;
> +
> +	P0()
> +	{
> +		int r0;
> +
> +		A: r0 = srcu_down_read(&s);
> +		B: WRITE_ONCE(x, r0);
> +	}
> +
> +	P1()
> +	{
> +		int r1;
> +
> +		C: r1 = READ_ONCE(x);
> +		D: srcu_up_read(&s, r1);
> +	}
> +
> +Assuming that P1 executes after P0 and does read the index value
> +stored in x, we can write this (using brackets to represent event
> +annotations) as:
> +
> +	A[srcu-lock] ->data B[once] ->rf C[once] ->data D[srcu-unlock].
> +
> +The LKMM defines a carries-srcu-data relation to express this
> +pattern; it permits an arbitrarily long sequence of
> +
> +	data ; rf
> +
> +pairs (that is, a data link followed by an rf link) to occur between
> +an srcu-lock event and the final data dependency leading to the
> +matching srcu-unlock event.  carry-srcu-data is complicated by the
> +need to ensure that none of the intermediate store events in this
> +sequence are instances of srcu-unlock.  This is necessary because in a
> +pattern like the one above:
> +
> +	A: idx1 = srcu_read_lock(&s);
> +	B: srcu_read_unlock(&s, idx1);
> +	C: idx2 = srcu_read_lock(&s);
> +	D: srcu_read_unlock(&s, idx2);
> +
> +the LKMM treats B as a store to the variable s and C as a load from
> +that variable, creating an undesirable rf link from B to C:
> +
> +	A ->data B ->rf C ->data D.
> +
> +This would cause carry-srcu-data to mistakenly extend a data
> +dependency from A to D and give the impression that D was the
> +srcu-unlock event matching A's srcu-lock.  To avoid such problems,
> +carry-srcu-data does not accept sequences in which the ends of any of
> +the intermediate ->data links (B above) is an srcu-unlock event.
>  
>  
>  LOCKING
>
  

On Wed, Feb 22, 2023 at 11:50:51AM -0800, Paul E. McKenney wrote:
> On Mon, Feb 20, 2023 at 04:06:13PM -0500, Alan Stern wrote:
> > On Sun, Feb 19, 2023 at 12:13:14PM -0500, Joel Fernandes wrote:
> > > On Sun, Feb 19, 2023 at 12:11 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> > > > Even though it may be redundant: would it be possible to also mention
> > > > (after this paragraph) that this case forms an undesirable "->rf" link
> > > > between B and C, which then causes us to link A and D as a result?
> > > >
> > > > A[srcu-lock] ->data B[once] ->rf C[once] ->data D[srcu-unlock].
> > > 
> > > Apologies, I meant here, care must be taken to avoid:
> > > 
> > > A[srcu-lock] ->data B[srcu-unlock] ->rf C[srcu-lock] ->data D[srcu-unlock].
> > 
> > Revised patch below.  I changed more than just this bit.  Mostly small 
> > edits to improve readability, but I did add a little additional 
> > material.
> 
> Looks good to me, thank you!
> 
> Would you like to send a formal patch, or are you thinking in terms
> of making other changes first?

I'll send a formal patch when I find time to write an appropriate 
Changelog description.

I also have in mind making other changes along the lines Joel suggested, 
but they will be separate patches.

Alan
  

On Wed, Feb 22, 2023 at 03:32:12PM -0500, Alan Stern wrote:
> On Wed, Feb 22, 2023 at 11:50:51AM -0800, Paul E. McKenney wrote:
> > On Mon, Feb 20, 2023 at 04:06:13PM -0500, Alan Stern wrote:
> > > On Sun, Feb 19, 2023 at 12:13:14PM -0500, Joel Fernandes wrote:
> > > > On Sun, Feb 19, 2023 at 12:11 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> > > > > Even though it may be redundant: would it be possible to also mention
> > > > > (after this paragraph) that this case forms an undesirable "->rf" link
> > > > > between B and C, which then causes us to link A and D as a result?
> > > > >
> > > > > A[srcu-lock] ->data B[once] ->rf C[once] ->data D[srcu-unlock].
> > > > 
> > > > Apologies, I meant here, care must be taken to avoid:
> > > > 
> > > > A[srcu-lock] ->data B[srcu-unlock] ->rf C[srcu-lock] ->data D[srcu-unlock].
> > > 
> > > Revised patch below.  I changed more than just this bit.  Mostly small 
> > > edits to improve readability, but I did add a little additional 
> > > material.
> > 
> > Looks good to me, thank you!
> > 
> > Would you like to send a formal patch, or are you thinking in terms
> > of making other changes first?
> 
> I'll send a formal patch when I find time to write an appropriate 
> Changelog description.
> 
> I also have in mind making other changes along the lines Joel suggested, 
> but they will be separate patches.

Sounds good!

							Thanx, Paul