On Tue, Oct 04, 2016 at 08:57:55PM -0700, Davidlohr Bueso wrote: > 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) <pet...@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;
Less obvious but would work I suppose, I didn't look too hard at the ordering requirements on __unqueue_futex(), but an early morning glance (without tea) doesn't show any obvious problems with that.
