From: Will Deacon <will.deacon@arm.com>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: Waiman Long <waiman.long@hpe.com>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
linux-kernel@vger.kernel.org, x86@kernel.org,
linux-alpha@vger.kernel.org, linux-ia64@vger.kernel.org,
linux-s390@vger.kernel.org, linux-arch@vger.kernel.org,
Davidlohr Bueso <dave@stgolabs.net>,
Jason Low <jason.low2@hp.com>, Dave Chinner <david@fromorbit.com>,
Scott J Norton <scott.norton@hpe.com>,
Douglas Hatch <doug.hatch@hpe.com>
Subject: Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier
Date: Thu, 16 Jun 2016 11:16:28 +0100 [thread overview]
Message-ID: <20160616101627.GA5827@arm.com> (raw)
In-Reply-To: <20160616021951.GA16918@insomnia>
Hi guys,
On Thu, Jun 16, 2016 at 10:19:51AM +0800, Boqun Feng wrote:
> On Wed, Jun 15, 2016 at 03:01:19PM -0400, Waiman Long wrote:
> > On 06/15/2016 04:04 AM, Boqun Feng wrote:
> > > On Tue, Jun 14, 2016 at 06:48:04PM -0400, Waiman Long wrote:
> > > > @@ -198,7 +198,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
> > > > * Second most likely case.
> > > > */
> > > > node = this_cpu_ptr(&osq_node);
> > > > - next = xchg(&node->next, NULL);
> > > > + next = xchg_release(&node->next, NULL);
> > > > if (next) {
> > > > WRITE_ONCE(next->locked, 1);
> > > So we still use WRITE_ONCE() rather than smp_store_release() here?
> > >
> > > Though, IIUC, This is fine for all the archs but ARM64, because there
> > > will always be a xchg_release()/xchg() before the WRITE_ONCE(), which
> > > carries a necessary barrier to upgrade WRITE_ONCE() to a RELEASE.
> > >
> > > Not sure whether it's a problem on ARM64, but I think we certainly need
> > > to add some comments here, if we count on this trick.
> > >
> > > Am I missing something or misunderstanding you here?
> > >
> > The change on the unlock side is more for documentation purpose than is
> > actually needed. As you had said, the xchg() call has provided the necessary
> > memory barrier. Using the _release variant, however, may have some
>
> But I'm afraid the barrier doesn't remain if we replace xchg() with
> xchg_release() on ARM64v8, IIUC, xchg_release() is just a ldxr+stlxr
> loop with no barrier on ARM64v8. This means the following code:
>
> CPU 0 CPU 1 (next)
> ======================== ==================
> WRITE_ONCE(x, 1); r1 = smp_load_acquire(next->locked, 1);
> xchg_release(&node->next, NULL); r2 = READ_ONCE(x);
> WRITE_ONCE(next->locked, 1);
>
> could result in (r1 == 1 && r2 == 0) on ARM64v8, IIUC.
Yes, of course. Why is that unexpected? You could just as easily make
the xchg_release an smp_store_release and this would still be permitted,
that's the whole point of acquire/release -- they're semi-permeable
barriers that allow accesses outside of the critical section to leak in,
but not the other way around.
It's worth noting that you've omitted the control dependency from
xchg_release to the subsequent write in your litmus tests, but I don't
think that actually changes anything here.
Will
next prev parent reply other threads:[~2016-06-16 10:16 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-14 22:48 [RFC PATCH-tip v2 0/6] locking/rwsem: Enable reader optimistic spinning Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier Waiman Long
2016-06-15 8:04 ` Boqun Feng
2016-06-15 17:18 ` Peter Zijlstra
2016-06-15 19:01 ` Waiman Long
2016-06-16 2:19 ` Boqun Feng
2016-06-16 10:16 ` Will Deacon [this message]
2016-06-16 21:35 ` Waiman Long
2016-06-17 0:48 ` Boqun Feng
2016-06-17 15:26 ` Waiman Long
2016-06-17 15:45 ` Will Deacon
2016-06-17 18:17 ` Waiman Long
2016-06-18 8:46 ` Boqun Feng
2016-06-20 7:59 ` Will Deacon
2016-06-15 16:56 ` Davidlohr Bueso
2016-06-15 17:12 ` Peter Zijlstra
2016-06-15 18:27 ` Davidlohr Bueso
2016-06-15 18:40 ` Peter Zijlstra
2016-06-15 18:56 ` Davidlohr Bueso
2016-06-17 1:11 ` Davidlohr Bueso
2016-06-17 14:28 ` Waiman Long
2016-06-17 16:29 ` Davidlohr Bueso
2016-06-17 16:46 ` Davidlohr Bueso
2016-06-15 19:08 ` Waiman Long
2016-06-15 20:04 ` Waiman Long
2016-06-15 21:59 ` Peter Zijlstra
2016-06-14 22:48 ` [RFC PATCH-tip v2 2/6] locking/rwsem: Stop active read lock ASAP Waiman Long
2016-06-15 17:22 ` Peter Zijlstra
2016-06-15 19:17 ` Waiman Long
2016-06-16 2:14 ` Davidlohr Bueso
2016-06-16 21:25 ` Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 3/6] locking/rwsem: Enable count-based spinning on reader Waiman Long
2016-06-15 17:38 ` Peter Zijlstra
2016-06-15 19:28 ` Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 4/6] locking/rwsem: move down rwsem_down_read_failed function Waiman Long
2016-06-15 17:40 ` Peter Zijlstra
2016-06-15 19:21 ` Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 5/6] locking/rwsem: Change RWSEM_WAITING_BIAS for better disambiguation Waiman Long
2016-06-15 17:43 ` Peter Zijlstra
2016-06-15 19:31 ` Waiman Long
2016-06-15 21:57 ` Peter Zijlstra
2016-06-15 17:45 ` Peter Zijlstra
2016-06-15 19:35 ` Waiman Long
2016-06-14 22:48 ` [RFC PATCH-tip v2 6/6] locking/rwsem: Enable spinning readers Waiman Long
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