linux-kernel - Re: [PATCH] jump_label_inc may return before the code is patched

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Date:	Tue, 18 Oct 2011 17:06:25 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Jason Baron <jbaron@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] jump_label_inc may return before the code is patched

On Tue, 2011-10-18 at 19:55 +0200, Gleb Natapov wrote:
> If cpu A calls jump_label_inc() just after atomic_add_return() is
> called by cpu B, atomic_inc_not_zero() will return value greater then
> zero and jump_label_inc() will return to a caller before jump_label_update()
> finishes its job on cpu B.

OK, I see what you are saying. There's a race here that jump_label_inc
may return before jump labels are actually activated. I have no issue
with this change. This guarantees that jump labels will be active on
return of jump_label_inc().

I'm assuming that jump_label_update() does memory barries, as it does
modify code, and would be itself a synchronization point.

Acked-by: Steven Rostedt <rostedt@...dmis.org>

-- Steve

> 
> Signed-off-by: Gleb Natapov <gleb@...hat.com>
> diff --git a/kernel/jump_label.c b/kernel/jump_label.c
> index a8ce450..e6f1f24 100644
> --- a/kernel/jump_label.c
> +++ b/kernel/jump_label.c
> @@ -66,8 +66,9 @@ void jump_label_inc(struct jump_label_key *key)
>  		return;
>  
>  	jump_label_lock();
> -	if (atomic_add_return(1, &key->enabled) == 1)
> +	if (atomic_read(&key->enabled) == 0)
>  		jump_label_update(key, JUMP_LABEL_ENABLE);
> +	atomic_inc(&key->enabled);
>  	jump_label_unlock();
>  }
>  
> --
> 			Gleb.


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