Re: [Xenomai-core] [PATCH] POSIX: Fix race when setting claimed bit

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Jan Kiszka wrote:
> Patch 2 of my fast lock series actually also contained an attempt to fix
> a race I spotted in the code that atomically sets the claimed bit. I
> forgot about this fact and even, worse, I only replace the original race
> with another one.
> 
> So here comes a new attempt to fix the issue that the lock owner and/or
> the claimed bit can change between trylock and the cmpxchg under nklock.
> Please have a look and cross-check the logic.
> 
> The patch applies on top of vanilla SVN, so my series has to be rebased
> and the fix has to be ported to native as well - where we just found it
> in the field.

Ok. Got it.

> 
> Jan
> 
> ---
>  ksrc/skins/posix/mutex.h |   11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> Index: b/ksrc/skins/posix/mutex.h
> ===================================================================
> --- a/ksrc/skins/posix/mutex.h
> +++ b/ksrc/skins/posix/mutex.h
> @@ -134,17 +134,18 @@ static inline int pse51_mutex_timedlock_
>  	/* Set bit 0, so that mutex_unlock will know that the mutex is claimed.
>  	   Hold the nklock, for mutual exclusion with slow mutex_unlock. */
>  	xnlock_get_irqsave(&nklock, s);
> +	owner = xnarch_atomic_intptr_get(mutex->owner);

Bad, this makes two atomic ops. So, I would rather propose:

>  	while(!test_claimed(owner)) {
- while(!test_claimed(owner)) {
+ do {
> -		old = xnarch_atomic_intptr_cmpxchg(mutex->owner,
> -						   owner, set_claimed(owner, 1));
> -		if (likely(old == owner))
> -			break;
> -		if (old == NULL) {
> +		if (owner == NULL) {
>  			/* Owner called fast mutex_unlock
>  			   (on another cpu) */
>  			xnlock_put_irqrestore(&nklock, s);
>  			goto retry_lock;
>  		}
> +		old = xnarch_atomic_intptr_cmpxchg(mutex->owner,
> +						   owner, set_claimed(owner, 1));
> +		if (likely(old == owner))
> +			break;
>  		owner = old;
-  	}
+       } while (!test_claimed(owner))
>  
> 
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-- 
                                                 Gilles.