From: Boqun Feng <boqun.feng@gmail.com>
To: Waiman Long <waiman.long@hpe.com>
Cc: 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>,
Will Deacon <will.deacon@arm.com>
Subject: Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier
Date: Thu, 16 Jun 2016 10:19:51 +0800 [thread overview]
Message-ID: <20160616021951.GA16918@insomnia> (raw)
In-Reply-To: <5761A5FF.5070703@hpe.com>
[-- Attachment #1: Type: text/plain, Size: 3794 bytes --]
On Wed, Jun 15, 2016 at 03:01:19PM -0400, Waiman Long wrote:
> On 06/15/2016 04:04 AM, Boqun Feng wrote:
> > Hi Waiman,
> >
> > On Tue, Jun 14, 2016 at 06:48:04PM -0400, Waiman Long wrote:
> > > The osq_lock() and osq_unlock() function may not provide the necessary
> > > acquire and release barrier in some cases. This patch makes sure
> > > that the proper barriers are provided when osq_lock() is successful
> > > or when osq_unlock() is called.
> > >
> > > Signed-off-by: Waiman Long<Waiman.Long@hpe.com>
> > > ---
> > > kernel/locking/osq_lock.c | 4 ++--
> > > 1 files changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> > > index 05a3785..7dd4ee5 100644
> > > --- a/kernel/locking/osq_lock.c
> > > +++ 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)) {
> > > /*
> > > * If we need to reschedule bail... so we can block.
> > > */
> > > @@ -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?
> >
> > Regards,
> > Boqun
>
> 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.
I translated it into a litmus test:
AArch64 stlxr+str
""
{
0:X4=x; 0:X5=node; node=next;
1:X4=x; 1:X5=next;
}
P0 | P1 ;
MOV W0,#1 | LDAR W1,[X5];
STR W0,[X4] | LDR W2,[X4] ;
MOV X0,#0 | ;
LDXR X2,[X5] | ;
STLXR W1,X0,[X5]| ;
CBNZ W1, fail | ;
MOV W0, #1 | ;
STR W0,[X2] | ;
fail: | ;
exists
(0:X0 = 1 /\ 1:X1 = 1 /\ 1:X2 = 0)
and herd said "Sometimes".
But I may miss something here or make a mistake in the translation. So
add Will in Cc list ;-)
> performance benefit in some architectures.
>
> BTW, osq_lock/osq_unlock aren't general purpose locking primitives. So there
> is some leeways on how fancy we want on the lock and unlock sides.
>
Understood, I think it's fine if we rely on something subtle here, but
I just want to make we won't be bitten by some corner cases.
Regards,
Boqun
> Cheers,
> Longman
[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 473 bytes --]
next prev parent reply other threads:[~2016-06-16 2:16 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 [this message]
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
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20160616021951.GA16918@insomnia \
--to=boqun.feng@gmail.com \
--cc=dave@stgolabs.net \
--cc=david@fromorbit.com \
--cc=doug.hatch@hpe.com \
--cc=jason.low2@hp.com \
--cc=linux-alpha@vger.kernel.org \
--cc=linux-arch@vger.kernel.org \
--cc=linux-ia64@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-s390@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=peterz@infradead.org \
--cc=scott.norton@hpe.com \
--cc=waiman.long@hpe.com \
--cc=will.deacon@arm.com \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).