[lkml]   [2006]   [Oct]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH] splice : two smp_mb() can be omitted
On Tue, Oct 31 2006, Nick Piggin wrote:
> Eric Dumazet wrote:
> >This patch deletes two calls to smp_mb() that were done after
> >mutex_unlock() that contains an implicit memory barrier.
> Uh, there is nothing that says mutex_unlock or any unlock
> functions contain an implicit smp_mb(). What is given is that the
> lock and unlock obey aquire and release memory ordering,
> respectively.
> a = x;
> xxx_unlock
> b = y;
> In this situation, the load of y can be executed before that of x.
> And some architectures will even do so (i386 can, because the
> unlock is an unprefixed store; ia64 can, because it uses a release
> barrier in the unlock).
> Whenever you rely on orderings of things *outside* locks (even
> partially outside), you do need to be very careful about barriers
> and can't rely on locks to do the right thing for you.

Good point, we should not make any assumptions on the way the
architecture implements the mutexes.

Jens Axboe

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2006-10-31 10:51    [from the cache]
©2003-2014 Jasper Spaans. hosted at Digital Ocean