From mboxrd@z Thu Jan 1 00:00:00 1970 From: Davidlohr Bueso Subject: Re: [PATCH 2/7] locking/rwsem: more aggressive use of optimistic spinning Date: Sun, 03 Aug 2014 21:09:44 -0700 Message-ID: <1407125384.3216.4.camel@buesod1.americas.hpqcorp.net> References: <1407119782-41119-1-git-send-email-Waiman.Long@hp.com> <1407119782-41119-3-git-send-email-Waiman.Long@hp.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1407119782-41119-3-git-send-email-Waiman.Long@hp.com> Sender: linux-kernel-owner@vger.kernel.org To: Waiman Long Cc: Ingo Molnar , Peter Zijlstra , linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, linux-doc@vger.kernel.org, Jason Low , Scott J Norton List-Id: linux-api@vger.kernel.org On Sun, 2014-08-03 at 22:36 -0400, Waiman Long wrote: > The rwsem_can_spin_on_owner() function currently allows optimistic > spinning only if the owner field is defined and is running. That is > too conservative as it will cause some tasks to miss the opportunity > of doing spinning in case the owner hasn't been able to set the owner > field in time or the lock has just become available. > > This patch enables more aggressive use of optimistic spinning by > assuming that the lock is spinnable unless proved otherwise. > > Signed-off-by: Waiman Long > --- > kernel/locking/rwsem-xadd.c | 2 +- > 1 files changed, 1 insertions(+), 1 deletions(-) > > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c > index d058946..dce22b8 100644 > --- a/kernel/locking/rwsem-xadd.c > +++ b/kernel/locking/rwsem-xadd.c > @@ -285,7 +285,7 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem) > static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem) > { > struct task_struct *owner; > - bool on_cpu = false; > + bool on_cpu = true; /* Assume spinnable unless proved not to be */ Nope, unfortunately we need as it fixes some pretty bad regressions when dealing with multiple readers -- as readers do not deal with lock ownership, so another thread can spin for too long in !owner. See commit 37e95624.