diff mbox series

jump_label: use atomic_try_cmpxchg in static_key_slow_inc_cpuslocked

Message ID 20221019140850.3395-1-ubizjak@gmail.com
State New
Headers
Series jump_label: use atomic_try_cmpxchg in static_key_slow_inc_cpuslocked |

Commit Message

Uros Bizjak Oct. 19, 2022, 2:08 p.m. UTC 
  Use atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old
in static_key_slow_inc_cpuslocked.  x86 CMPXCHG instruction returns success
in ZF flag, so this change saves a compare after cmpxchg (and related move
instruction in front of cmpxchg).

Also, atomic_try_cmpxchg implicitly assigns old *ptr value to "old" when
cmpxchg fails, enabling further code simplifications.

No functional change intended.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
 kernel/jump_label.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

Comments

Steven Rostedt Nov. 22, 2022, 9:14 p.m. UTC | #1 
On Wed, 19 Oct 2022 16:08:50 +0200
Uros Bizjak <ubizjak@gmail.com> wrote:

> Use atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old
> in static_key_slow_inc_cpuslocked.  x86 CMPXCHG instruction returns success
> in ZF flag, so this change saves a compare after cmpxchg (and related move
> instruction in front of cmpxchg).
> 
> Also, atomic_try_cmpxchg implicitly assigns old *ptr value to "old" when
> cmpxchg fails, enabling further code simplifications.
> 
> No functional change intended.
> 
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Josh Poimboeuf <jpoimboe@kernel.org>
> Cc: Jason Baron <jbaron@akamai.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> ---
>  kernel/jump_label.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/jump_label.c b/kernel/jump_label.c
> index 714ac4c3b556..4d6c6f5f60db 100644
> --- a/kernel/jump_label.c
> +++ b/kernel/jump_label.c
> @@ -115,8 +115,6 @@ EXPORT_SYMBOL_GPL(static_key_count);
>  
>  void static_key_slow_inc_cpuslocked(struct static_key *key)
>  {
> -	int v, v1;
> -
>  	STATIC_KEY_CHECK_USE(key);
>  	lockdep_assert_cpus_held();
>  
> @@ -132,11 +130,9 @@ void static_key_slow_inc_cpuslocked(struct static_key *key)
>  	 * so it counts as "enabled" in jump_label_update().  Note that
>  	 * atomic_inc_unless_negative() checks >= 0, so roll our own.
>  	 */
> -	for (v = atomic_read(&key->enabled); v > 0; v = v1) {
> -		v1 = atomic_cmpxchg(&key->enabled, v, v + 1);
> -		if (likely(v1 == v))
> +	for (int v = atomic_read(&key->enabled); v > 0; )

Although it's permitted by the compiler, the kernel style is to not add
declarations in conditionals.

Please keep the "int v;" at the beginning.


> +		if (likely(atomic_try_cmpxchg(&key->enabled, &v, v + 1)))

I'm fine with this change.

-- Steve


>  			return;
> -	}
>  
>  	jump_label_lock();
>  	if (atomic_read(&key->enabled) == 0) {
Peter Zijlstra Nov. 23, 2022, 9:08 a.m. UTC | #2 
On Tue, Nov 22, 2022 at 04:14:46PM -0500, Steven Rostedt wrote:

> > +	for (int v = atomic_read(&key->enabled); v > 0; )
> 
> Although it's permitted by the compiler, the kernel style is to not add
> declarations in conditionals.

I'm thinking the whole motivation for upping to C99 was exactly so that
we could start using this pattern.
Mark Rutland Nov. 23, 2022, 11:28 a.m. UTC | #3 
On Wed, Nov 23, 2022 at 10:08:59AM +0100, Peter Zijlstra wrote:
> On Tue, Nov 22, 2022 at 04:14:46PM -0500, Steven Rostedt wrote:
> 
> > > +	for (int v = atomic_read(&key->enabled); v > 0; )
> > 
> > Although it's permitted by the compiler, the kernel style is to not add
> > declarations in conditionals.
> 
> I'm thinking the whole motivation for upping to C99 was exactly so that
> we could start using this pattern.

That was one reason, yes. Marco and I wanted to be able to use C99-style
declarations in for loops to make it easier/possible to build macros with
locally-scoped control variables.

I personally prefer using C99-style declarations in for loops, but I don't have
a strong feeling that we *must* do so.

Thanks,
Mark.
Steven Rostedt Nov. 23, 2022, 1:26 p.m. UTC | #4 
On Wed, 23 Nov 2022 10:08:59 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, Nov 22, 2022 at 04:14:46PM -0500, Steven Rostedt wrote:
> 
> > > +	for (int v = atomic_read(&key->enabled); v > 0; )  
> > 
> > Although it's permitted by the compiler, the kernel style is to not add
> > declarations in conditionals.  
> 
> I'm thinking the whole motivation for upping to C99 was exactly so that
> we could start using this pattern.

OK, if you are fine with it then sure. I personally like seeing all
variables declared in one place. Maybe because I've been trained that way,
and I can easily be confused when I see a variable somewhere and don't see
it in the beginning declarations.

-- Steve
diff mbox series

Patch

diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 714ac4c3b556..4d6c6f5f60db 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -115,8 +115,6 @@  EXPORT_SYMBOL_GPL(static_key_count);
 
 void static_key_slow_inc_cpuslocked(struct static_key *key)
 {
-	int v, v1;
-
 	STATIC_KEY_CHECK_USE(key);
 	lockdep_assert_cpus_held();
 
@@ -132,11 +130,9 @@  void static_key_slow_inc_cpuslocked(struct static_key *key)
 	 * so it counts as "enabled" in jump_label_update().  Note that
 	 * atomic_inc_unless_negative() checks >= 0, so roll our own.
 	 */
-	for (v = atomic_read(&key->enabled); v > 0; v = v1) {
-		v1 = atomic_cmpxchg(&key->enabled, v, v + 1);
-		if (likely(v1 == v))
+	for (int v = atomic_read(&key->enabled); v > 0; )
+		if (likely(atomic_try_cmpxchg(&key->enabled, &v, v + 1)))
 			return;
-	}
 
 	jump_label_lock();
 	if (atomic_read(&key->enabled) == 0) {