svn commit: r357989 - head/sys/sys

[Konstantin Belousov]

On Sun, Feb 16, 2020 at 03:14:55AM +0000, Mateusz Guzik wrote:
> Author: mjg
> Date: Sun Feb 16 03:14:55 2020
> New Revision: 357989
> URL: https://svnweb.freebsd.org/changeset/base/357989
> 
> Log:
>   refcount: add missing release fence to refcount_release_if_gt
I think you should update comment in refcount_release_last() to point
to if_gt() as well.  And probably annotate this fence with a pointer
to acquire in release_last().

>   
>   The CPU succeeding in releasing the not last reference can still have pending
>   stores to the object protected by the affected counter. This opens a time
>   window where another CPU can release the last reference and free the object,
>   resulting in use-after-free. On top of that this prevents the compiler from
>   generating more accesses to the object regardless of how atomic_fcmpset_rel_int
>   is implemented (of course as long as it provides the release semantic).
>   
>   Reviewed by:	markj
> 
> Modified:
>   head/sys/sys/refcount.h
> 
> Modified: head/sys/sys/refcount.h
> ==============================================================================
> --- head/sys/sys/refcount.h	Sun Feb 16 01:07:19 2020	(r357988)
> +++ head/sys/sys/refcount.h	Sun Feb 16 03:14:55 2020	(r357989)
> @@ -198,7 +198,7 @@ refcount_release_if_gt(volatile u_int *count, u_int n)
>  			return (false);
>  		if (__predict_false(REFCOUNT_SATURATED(old)))
>  			return (true);
> -		if (atomic_fcmpset_int(count, &old, old - 1))
> +		if (atomic_fcmpset_rel_int(count, &old, old - 1))
>  			return (true);
>  	}
>  }