[1/2,v2] cleanup: Add cond_guard() to conditional guards

  Add cond_guard() macro to conditional guards.

cond_guard() is a guard to be used with the conditional variants of locks,
like down_read_trylock() or mutex_lock_interruptible().

It takes a statement (or more statements in a block) that is passed to its
second argument. That statement (or block) is executed if waiting for a
lock is interrupted or if a _trylock() fails in case of contention.

Usage example:

	cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);

Consistently with the other guards, locks are unlocked at the exit of the
scope where cond_guard() is called.

Cc: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Dan Williams <dan.j.williams@intel.com>
Suggested-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Fabio M. De Francesco <fabio.maria.de.francesco@linux.intel.com>
---
 include/linux/cleanup.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
  

On 2/5/24 7:26 AM, Fabio M. De Francesco wrote:
> Add cond_guard() macro to conditional guards.
> 
> cond_guard() is a guard to be used with the conditional variants of locks,
> like down_read_trylock() or mutex_lock_interruptible().
> 
> It takes a statement (or more statements in a block) that is passed to its
> second argument. That statement (or block) is executed if waiting for a
> lock is interrupted or if a _trylock() fails in case of contention.
> 
> Usage example:
> 
> 	cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> 
> Consistently with the other guards, locks are unlocked at the exit of the
> scope where cond_guard() is called.
> 
> Cc: Peter Zijlstra <peterz@infradead.org>
> Suggested-by: Dan Williams <dan.j.williams@intel.com>
> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Fabio M. De Francesco <fabio.maria.de.francesco@linux.intel.com>

Reviewed-by: Dave Jiang <dave.jiang@intel.com>

> ---
>  include/linux/cleanup.h | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> index c2d09bc4f976..88af56600325 100644
> --- a/include/linux/cleanup.h
> +++ b/include/linux/cleanup.h
> @@ -134,6 +134,16 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
>   *	an anonymous instance of the (guard) class, not recommended for
>   *	conditional locks.
>   *
> + * cond_guard(name, fail, args...):
> + *	a guard to be used with the conditional variants of locks, like
> + *	down_read_trylock() or mutex_lock_interruptible. 'fail' are one or more
> + *	statements that are executed when waiting for a lock is interrupted or
> + *	when a _trylock() fails in case of contention.
> + *
> + *	Example:
> + *
> + *		cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> + *
>   * scoped_guard (name, args...) { }:
>   *	similar to CLASS(name, scope)(args), except the variable (with the
>   *	explicit name 'scope') is declard in a for-loop such that its scope is
> @@ -165,6 +175,10 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
>  
>  #define __guard_ptr(_name) class_##_name##_lock_ptr
>  
> +#define cond_guard(_name, _fail, args...) \
> +	CLASS(_name, scope)(args); \
> +	if (!__guard_ptr(_name)(&scope)) _fail
> +
>  #define scoped_guard(_name, args...)					\
>  	for (CLASS(_name, scope)(args),					\
>  	     *done = NULL; __guard_ptr(_name)(&scope) && !done; done = (void *)1)
  

Fabio M. De Francesco wrote:
> Add cond_guard() macro to conditional guards.
> 
> cond_guard() is a guard to be used with the conditional variants of locks,
> like down_read_trylock() or mutex_lock_interruptible().
> 
> It takes a statement (or more statements in a block) that is passed to its
> second argument. That statement (or block) is executed if waiting for a
> lock is interrupted or if a _trylock() fails in case of contention.
> 
> Usage example:
> 
> 	cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> 
> Consistently with the other guards, locks are unlocked at the exit of the
> scope where cond_guard() is called.
> 
> Cc: Peter Zijlstra <peterz@infradead.org>
> Suggested-by: Dan Williams <dan.j.williams@intel.com>
> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Fabio M. De Francesco <fabio.maria.de.francesco@linux.intel.com>
> ---
>  include/linux/cleanup.h | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> index c2d09bc4f976..88af56600325 100644
> --- a/include/linux/cleanup.h
> +++ b/include/linux/cleanup.h
> @@ -134,6 +134,16 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
>   *	an anonymous instance of the (guard) class, not recommended for
>   *	conditional locks.
>   *
> + * cond_guard(name, fail, args...):
> + *	a guard to be used with the conditional variants of locks, like
> + *	down_read_trylock() or mutex_lock_interruptible. 'fail' are one or more
> + *	statements that are executed when waiting for a lock is interrupted or
> + *	when a _trylock() fails in case of contention.
> + *
> + *	Example:
> + *
> + *		cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);

That _fail argument likely needs to be a statement expression for the
multi-statement case.

> + *
>   * scoped_guard (name, args...) { }:
>   *	similar to CLASS(name, scope)(args), except the variable (with the
>   *	explicit name 'scope') is declard in a for-loop such that its scope is
> @@ -165,6 +175,10 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
>  
>  #define __guard_ptr(_name) class_##_name##_lock_ptr
>  
> +#define cond_guard(_name, _fail, args...) \
> +	CLASS(_name, scope)(args); \
> +	if (!__guard_ptr(_name)(&scope)) _fail

No, as I stated before this is broken for usages of:

    if () cond_guard() else if ()

The 'else' in the definition is critical, this builds for me (untested):

diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
index 88af56600325..665407498781 100644
--- a/include/linux/cleanup.h
+++ b/include/linux/cleanup.h
@@ -142,7 +142,7 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
  *
  *	Example:
  *
- *		cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
+ *		cond_guard(rwsem_read_try, ({ printk(...); return 0; }), &semaphore);
  *
  * scoped_guard (name, args...) { }:
  *	similar to CLASS(name, scope)(args), except the variable (with the
@@ -177,7 +177,8 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
 
 #define cond_guard(_name, _fail, args...) \
 	CLASS(_name, scope)(args); \
-	if (!__guard_ptr(_name)(&scope)) _fail
+	if (!__guard_ptr(_name)(&scope)) _fail; \
+	else
 
 #define scoped_guard(_name, args...)					\
 	for (CLASS(_name, scope)(args),					\
  

Dan Williams wrote:
> Fabio M. De Francesco wrote:
> > Add cond_guard() macro to conditional guards.
> > 
> > cond_guard() is a guard to be used with the conditional variants of locks,
> > like down_read_trylock() or mutex_lock_interruptible().
> > 
> > It takes a statement (or more statements in a block) that is passed to its
> > second argument. That statement (or block) is executed if waiting for a
> > lock is interrupted or if a _trylock() fails in case of contention.
> > 
> > Usage example:
> > 
> > 	cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> > 
> > Consistently with the other guards, locks are unlocked at the exit of the
> > scope where cond_guard() is called.
> > 
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Suggested-by: Dan Williams <dan.j.williams@intel.com>
> > Suggested-by: Ira Weiny <ira.weiny@intel.com>
> > Signed-off-by: Fabio M. De Francesco <fabio.maria.de.francesco@linux.intel.com>
> > ---
> >  include/linux/cleanup.h | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> > 
> > diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> > index c2d09bc4f976..88af56600325 100644
> > --- a/include/linux/cleanup.h
> > +++ b/include/linux/cleanup.h
> > @@ -134,6 +134,16 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
> >   *	an anonymous instance of the (guard) class, not recommended for
> >   *	conditional locks.
> >   *
> > + * cond_guard(name, fail, args...):
> > + *	a guard to be used with the conditional variants of locks, like
> > + *	down_read_trylock() or mutex_lock_interruptible. 'fail' are one or more
> > + *	statements that are executed when waiting for a lock is interrupted or
> > + *	when a _trylock() fails in case of contention.
> > + *
> > + *	Example:
> > + *
> > + *		cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> 
> That _fail argument likely needs to be a statement expression for the
> multi-statement case.

You mean ({ ... }) as discussed here?

https://lore.kernel.org/all/65c1578c76def_37447929456@iweiny-mobl.notmuch/

> 
> > + *
> >   * scoped_guard (name, args...) { }:
> >   *	similar to CLASS(name, scope)(args), except the variable (with the
> >   *	explicit name 'scope') is declard in a for-loop such that its scope is
> > @@ -165,6 +175,10 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
> >  
> >  #define __guard_ptr(_name) class_##_name##_lock_ptr
> >  
> > +#define cond_guard(_name, _fail, args...) \
> > +	CLASS(_name, scope)(args); \
> > +	if (!__guard_ptr(_name)(&scope)) _fail
> 
> No, as I stated before this is broken for usages of:
> 
>     if () cond_guard() else if ()
> 
> The 'else' in the definition is critical, this builds for me (untested):

I did not test Fabios work directly but I don't understand this example.
It seems like your suggestion does nothing useful.  The cond_guard()
becomes a single statement like...

	if ()
		cond_guard();
	else ...

.. And can't protect anything.  NOTE From my understanding of
cond_guard() as defined, the ';' must be used as part of cond_guard() and
should complete the internal macro 'if' statement.

I think this would work:

	if () {
		cond_guard();
		... do locked stuff ...
	} else ...

> 
> diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> index 88af56600325..665407498781 100644
> --- a/include/linux/cleanup.h
> +++ b/include/linux/cleanup.h
> @@ -142,7 +142,7 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
>   *
>   *	Example:
>   *
> - *		cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> + *		cond_guard(rwsem_read_try, ({ printk(...); return 0; }), &semaphore);
>   *
>   * scoped_guard (name, args...) { }:
>   *	similar to CLASS(name, scope)(args), except the variable (with the
> @@ -177,7 +177,8 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
>  
>  #define cond_guard(_name, _fail, args...) \
>  	CLASS(_name, scope)(args); \
> -	if (!__guard_ptr(_name)(&scope)) _fail
> +	if (!__guard_ptr(_name)(&scope)) _fail; \

Building on what I found for scoped_cond_guard() this should be

> +	if (!__guard_ptr(_name)(&scope)) { _fail; }

And drop the else.  The else needs to clearly be part of an outside if in
your example.

Ira

> +	else
>  
>  #define scoped_guard(_name, args...)					\
>  	for (CLASS(_name, scope)(args),					\
  

Ira Weiny wrote:
> Dan Williams wrote:
> > Fabio M. De Francesco wrote:
> > > Add cond_guard() macro to conditional guards.
> > > 
> > > cond_guard() is a guard to be used with the conditional variants of locks,
> > > like down_read_trylock() or mutex_lock_interruptible().
> > > 
> > > It takes a statement (or more statements in a block) that is passed to its
> > > second argument. That statement (or block) is executed if waiting for a
> > > lock is interrupted or if a _trylock() fails in case of contention.
> > > 
> > > Usage example:
> > > 
> > > 	cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> > > 
> > > Consistently with the other guards, locks are unlocked at the exit of the
> > > scope where cond_guard() is called.
> > > 
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > Suggested-by: Dan Williams <dan.j.williams@intel.com>
> > > Suggested-by: Ira Weiny <ira.weiny@intel.com>
> > > Signed-off-by: Fabio M. De Francesco <fabio.maria.de.francesco@linux.intel.com>
> > > ---
> > >  include/linux/cleanup.h | 14 ++++++++++++++
> > >  1 file changed, 14 insertions(+)
> > > 
> > > diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> > > index c2d09bc4f976..88af56600325 100644
> > > --- a/include/linux/cleanup.h
> > > +++ b/include/linux/cleanup.h
> > > @@ -134,6 +134,16 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
> > >   *	an anonymous instance of the (guard) class, not recommended for
> > >   *	conditional locks.
> > >   *
> > > + * cond_guard(name, fail, args...):
> > > + *	a guard to be used with the conditional variants of locks, like
> > > + *	down_read_trylock() or mutex_lock_interruptible. 'fail' are one or more
> > > + *	statements that are executed when waiting for a lock is interrupted or
> > > + *	when a _trylock() fails in case of contention.
> > > + *
> > > + *	Example:
> > > + *
> > > + *		cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> > 
> > That _fail argument likely needs to be a statement expression for the
> > multi-statement case.
> 
> You mean ({ ... }) as discussed here?
> 
> https://lore.kernel.org/all/65c1578c76def_37447929456@iweiny-mobl.notmuch/

Yes.

> > > + *
> > >   * scoped_guard (name, args...) { }:
> > >   *	similar to CLASS(name, scope)(args), except the variable (with the
> > >   *	explicit name 'scope') is declard in a for-loop such that its scope is
> > > @@ -165,6 +175,10 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
> > >  
> > >  #define __guard_ptr(_name) class_##_name##_lock_ptr
> > >  
> > > +#define cond_guard(_name, _fail, args...) \
> > > +	CLASS(_name, scope)(args); \
> > > +	if (!__guard_ptr(_name)(&scope)) _fail
> > 
> > No, as I stated before this is broken for usages of:
> > 
> >     if () cond_guard() else if ()
> > 
> > The 'else' in the definition is critical, this builds for me (untested):
> 
> I did not test Fabios work directly but I don't understand this example.
> It seems like your suggestion does nothing useful.  The cond_guard()
> becomes a single statement like...
> 
> 	if ()
> 		cond_guard();
> 	else ...
> 
> ... And can't protect anything.

A sequence to acquire and drop a lock is sometimes a barrier semantic.
Is it typical, no, is it possible, yes. I otherwise do not understand
the need to include the subtle side effect.

> cond_guard() as defined, the ';' must be used as part of cond_guard() and
> should complete the internal macro 'if' statement.
> 
> I think this would work:
> 
> 	if () {
> 		cond_guard();
> 		... do locked stuff ...
> 	} else ...
> 
> > 
> > diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> > index 88af56600325..665407498781 100644
> > --- a/include/linux/cleanup.h
> > +++ b/include/linux/cleanup.h
> > @@ -142,7 +142,7 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
> >   *
> >   *	Example:
> >   *
> > - *		cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> > + *		cond_guard(rwsem_read_try, ({ printk(...); return 0; }), &semaphore);
> >   *
> >   * scoped_guard (name, args...) { }:
> >   *	similar to CLASS(name, scope)(args), except the variable (with the
> > @@ -177,7 +177,8 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
> >  
> >  #define cond_guard(_name, _fail, args...) \
> >  	CLASS(_name, scope)(args); \
> > -	if (!__guard_ptr(_name)(&scope)) _fail
> > +	if (!__guard_ptr(_name)(&scope)) _fail; \
> 
> Building on what I found for scoped_cond_guard() this should be
> 
> > +	if (!__guard_ptr(_name)(&scope)) { _fail; }

That's still a dangling if () statement.

> 
> And drop the else.  The else needs to clearly be part of an outside if in
> your example.

Please just rely on a statement-expression for the odd multi-statement _fail
use case and include the else in the definition to remove any room for
confusion.
  

Dan Williams wrote:
> Ira Weiny wrote:
> > Dan Williams wrote:
> > > Fabio M. De Francesco wrote:
> > > > Add cond_guard() macro to conditional guards.
> > > > 
> > > > cond_guard() is a guard to be used with the conditional variants of locks,
> > > > like down_read_trylock() or mutex_lock_interruptible().
> > > > 
> > > > It takes a statement (or more statements in a block) that is passed to its
> > > > second argument. That statement (or block) is executed if waiting for a
> > > > lock is interrupted or if a _trylock() fails in case of contention.
> > > > 
> > > > Usage example:
> > > > 
> > > > 	cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> > > > 
> > > > Consistently with the other guards, locks are unlocked at the exit of the
> > > > scope where cond_guard() is called.
> > > > 
> > > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > > Suggested-by: Dan Williams <dan.j.williams@intel.com>
> > > > Suggested-by: Ira Weiny <ira.weiny@intel.com>
> > > > Signed-off-by: Fabio M. De Francesco <fabio.maria.de.francesco@linux.intel.com>
> > > > ---
> > > >  include/linux/cleanup.h | 14 ++++++++++++++
> > > >  1 file changed, 14 insertions(+)
> > > > 
> > > > diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> > > > index c2d09bc4f976..88af56600325 100644
> > > > --- a/include/linux/cleanup.h
> > > > +++ b/include/linux/cleanup.h
> > > > @@ -134,6 +134,16 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
> > > >   *	an anonymous instance of the (guard) class, not recommended for
> > > >   *	conditional locks.
> > > >   *
> > > > + * cond_guard(name, fail, args...):
> > > > + *	a guard to be used with the conditional variants of locks, like
> > > > + *	down_read_trylock() or mutex_lock_interruptible. 'fail' are one or more
> > > > + *	statements that are executed when waiting for a lock is interrupted or
> > > > + *	when a _trylock() fails in case of contention.
> > > > + *
> > > > + *	Example:
> > > > + *
> > > > + *		cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> > > 
> > > That _fail argument likely needs to be a statement expression for the
> > > multi-statement case.
> > 
> > You mean ({ ... }) as discussed here?
> > 
> > https://lore.kernel.org/all/65c1578c76def_37447929456@iweiny-mobl.notmuch/
> 
> Yes.
> 
> > > > + *
> > > >   * scoped_guard (name, args...) { }:
> > > >   *	similar to CLASS(name, scope)(args), except the variable (with the
> > > >   *	explicit name 'scope') is declard in a for-loop such that its scope is
> > > > @@ -165,6 +175,10 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
> > > >  
> > > >  #define __guard_ptr(_name) class_##_name##_lock_ptr
> > > >  
> > > > +#define cond_guard(_name, _fail, args...) \
> > > > +	CLASS(_name, scope)(args); \
> > > > +	if (!__guard_ptr(_name)(&scope)) _fail
> > > 
> > > No, as I stated before this is broken for usages of:
> > > 
> > >     if () cond_guard() else if ()
> > > 
> > > The 'else' in the definition is critical, this builds for me (untested):
> > 
> > I did not test Fabios work directly but I don't understand this example.
> > It seems like your suggestion does nothing useful.  The cond_guard()
> > becomes a single statement like...
> > 
> > 	if ()
> > 		cond_guard();
> > 	else ...
> > 
> > ... And can't protect anything.
> 
> A sequence to acquire and drop a lock is sometimes a barrier semantic.
> Is it typical, no, is it possible, yes. I otherwise do not understand
> the need to include the subtle side effect.

I was not trying to include a subtle side effect.  I was thinking that the
else block would be the only block covered by the lock.  I've looked at
the preprocessor output again and I now see what you are saying.  Also I
see I was thinking incorrectly.  The else will be an empty statement and
the rest of the outer block will be covered by the lock...

Sorry for not seeing this before.

> > cond_guard() as defined, the ';' must be used as part of cond_guard() and
> > should complete the internal macro 'if' statement.
> > 
> > I think this would work:
> > 
> > 	if () {
> > 		cond_guard();
> > 		... do locked stuff ...
> > 	} else ...
> > 
> > > 
> > > diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> > > index 88af56600325..665407498781 100644
> > > --- a/include/linux/cleanup.h
> > > +++ b/include/linux/cleanup.h
> > > @@ -142,7 +142,7 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
> > >   *
> > >   *	Example:
> > >   *
> > > - *		cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> > > + *		cond_guard(rwsem_read_try, ({ printk(...); return 0; }), &semaphore);
> > >   *
> > >   * scoped_guard (name, args...) { }:
> > >   *	similar to CLASS(name, scope)(args), except the variable (with the
> > > @@ -177,7 +177,8 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
> > >  
> > >  #define cond_guard(_name, _fail, args...) \
> > >  	CLASS(_name, scope)(args); \
> > > -	if (!__guard_ptr(_name)(&scope)) _fail
> > > +	if (!__guard_ptr(_name)(&scope)) _fail; \
> > 
> > Building on what I found for scoped_cond_guard() this should be
> > 
> > > +	if (!__guard_ptr(_name)(&scope)) { _fail; }
> 
> That's still a dangling if () statement.
> 
> > 
> > And drop the else.  The else needs to clearly be part of an outside if in
> > your example.
> 
> Please just rely on a statement-expression for the odd multi-statement _fail
> use case and include the else in the definition to remove any room for
> confusion.

Yea ok I see it now,
Ira