From: Will Deacon <will.deacon@arm.com>
To: Waiman Long <waiman.long@hpe.com>
Cc: Boqun Feng <boqun.feng@gmail.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: Fri, 17 Jun 2016 16:45:36 +0100 [thread overview]
Message-ID: <20160617154536.GB1284@arm.com> (raw)
In-Reply-To: <576416B1.6020006@hpe.com>
On Fri, Jun 17, 2016 at 11:26:41AM -0400, Waiman Long wrote:
> On 06/16/2016 08:48 PM, Boqun Feng wrote:
> >On Thu, Jun 16, 2016 at 05:35:54PM -0400, Waiman Long wrote:
> >>If you look into the actual code:
> >>
> >> next = xchg_release(&node->next, NULL);
> >> if (next) {
> >> WRITE_ONCE(next->locked, 1);
> >> return;
> >> }
> >>
> >>There is a control dependency that WRITE_ONCE() won't happen until
> >But a control dependency only orders LOAD->STORE pairs, right? And here
> >the control dependency orders the LOAD part of xchg_release() and the
> >WRITE_ONCE().
> >
> >Along with the fact that RELEASE only orders the STORE part of xchg with
> >the memory operations preceding the STORE part, so for the following
> >code:
> >
> > WRTIE_ONCE(x,1);
> > next = xchg_release(&node->next, NULL);
> > if (next)
> > WRITE_ONCE(next->locked, 1);
> >
> >such a reordering is allowed to happen on ARM64v8
> >
> > next = ldxr [&node->next] // LOAD part of xchg_release()
> >
> > if (next)
> > WRITE_ONCE(next->locked, 1);
> >
> > WRITE_ONCE(x,1);
> > stlxr NULL [&node->next] // STORE part of xchg_releae()
> >
> >Am I missing your point here?
>
> My understanding of the release barrier is that both prior LOADs and STOREs
> can't move after the barrier. If WRITE_ONCE(x, 1) can move to below as shown
> above, it is not a real release barrier and we may need to change the
> barrier code.
You seem to be missing the point.
{READ,WRITE}_ONCE accesses appearing in program order after a release
are not externally ordered with respect to the release unless they
access the same location.
This is illustrated by Boqun's example, which shows two WRITE_ONCE
accesses being reordered before a store-release forming the write
component of an xchg_release. In both cases, WRITE_ONCE(x, 1) remains
ordered before the store-release.
Will
next prev parent reply other threads:[~2016-06-17 15:45 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
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 [this message]
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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