Messages in this thread |  | | Date | Tue, 4 Oct 2016 20:57:55 -0700 | From | Davidlohr Bueso <> | Subject | Re: [RFC][PATCH 2/4] futex: Use smp_store_release() in mark_wake_futex() |
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On Mon, 03 Oct 2016, Peter Zijlstra wrote:
>Since the futex_q can dissapear the instruction after assigning NULL, >this really should be a RELEASE barrier. That stops loads from hitting >dead memory too. > >Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> >--- > kernel/futex.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > >--- a/kernel/futex.c >+++ b/kernel/futex.c >@@ -1288,8 +1288,7 @@ static void mark_wake_futex(struct wake_ > * memory barrier is required here to prevent the following > * store to lock_ptr from getting ahead of the plist_del. > */ >- smp_wmb(); >- q->lock_ptr = NULL; >+ smp_store_release(&q->lock_ptr, NULL); > }
Hmm, what if we relied on the implicit barrier in the wake_q_add() above and got rid of the smp_wmb altogether? We'd obviously have to move up __unqueue_futex(), but all we care about is the publishing store to lock_ptr being the last operation, or at least the plist_del, such that the wakeup order is respected; ie:
__unqueue_futex(q); wake_q_add(wake_q, p); q->lock_ptr = NULL;
Thanks, Davidlohr
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