linux-kernel - Re: [PATCH] jump_label: use atomic_try_cmpxchg in static_key_slow_inc

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Message-ID: <20221122161446.28907755@gandalf.local.home>
Date:   Tue, 22 Nov 2022 16:14:46 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Jason Baron <jbaron@...mai.com>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH] jump_label: use atomic_try_cmpxchg in
 static_key_slow_inc_cpuslocked

On Wed, 19 Oct 2022 16:08:50 +0200
Uros Bizjak <ubizjak@...il.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@...radead.org>
> Cc: Josh Poimboeuf <jpoimboe@...nel.org>
> Cc: Jason Baron <jbaron@...mai.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ard Biesheuvel <ardb@...nel.org>
> Signed-off-by: Uros Bizjak <ubizjak@...il.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) {