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Date: Wed, 15 Jun 2016 19:12:50 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Davidlohr Bueso <dave@...olabs.net> Cc: Waiman Long <Waiman.Long@....com>, Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org, x86@...nel.org, linux-alpha@...r.kernel.org, linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org, linux-arch@...r.kernel.org, Jason Low <jason.low2@...com>, Dave Chinner <david@...morbit.com>, Scott J Norton <scott.norton@....com>, Douglas Hatch <doug.hatch@....com> Subject: Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier On Wed, Jun 15, 2016 at 09:56:59AM -0700, Davidlohr Bueso wrote: > On Tue, 14 Jun 2016, Waiman Long wrote: > >+++ b/kernel/locking/osq_lock.c > >@@ -115,7 +115,7 @@ bool osq_lock(struct optimistic_spin_queue *lock) > > * cmpxchg in an attempt to undo our queueing. > > */ > > > >- while (!READ_ONCE(node->locked)) { > >+ while (!smp_load_acquire(&node->locked)) { > > Hmm this being a polling path, that barrier can get pretty expensive and > last I checked it was unnecessary: I think he'll go rely on it later on. In any case, its fairly simple to cure, just add smp_acquire__after_ctrl_dep() at the end. If we bail because need_resched() we don't need the acquire I think.
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