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Date: Tue, 4 Oct 2016 20:57:55 -0700 From: Davidlohr Bueso <dave@...olabs.net> To: Peter Zijlstra <peterz@...radead.org> Cc: mingo@...nel.org, tglx@...utronix.de, juri.lelli@....com, rostedt@...dmis.org, xlpang@...hat.com, bigeasy@...utronix.de, linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com, jdesfossez@...icios.com, bristot@...hat.com Subject: Re: [RFC][PATCH 2/4] futex: Use smp_store_release() in mark_wake_futex() 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@...radead.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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