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

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Message-ID: <20111102152123.GF14726@redhat.com>
Date:	Wed, 2 Nov 2011 17:21:23 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Jason Baron <jbaron@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, mingo@...e.hu
Subject: Re: [PATCH] jump_label_inc may return before the code is patched

On Tue, Oct 18, 2011 at 05:06:25PM -0400, Steven Rostedt wrote:
> 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>
> 
So through what tree this should go to Linus' git?

> -- 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.
> 

--
			Gleb.
--
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