Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1423249

Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier

From Peter Zijlstra <peterz@infradead.org>
Newsgroups linux.kernel
Subject Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier
Date 2016-06-15 19:20 +0200
Message-ID <rKmDU-5r2-17@gated-at.bofh.it> (permalink)
References <rK5jH-2zl-5@gated-at.bofh.it> <rK5jH-2zl-3@gated-at.bofh.it> <rKe3E-8rI-37@gated-at.bofh.it>
Organization linux.* mail to news gateway

Show all headers | View raw

On Wed, Jun 15, 2016 at 04:04:46PM +0800, 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. On PPC for example, you'll use lwsync() but will that not
attach to the store to &node->next instead?

Still leaving that store and the WRITE_ONCE() unordered.

Also I don't see the control dependency between xchg-load and WRITE_ONCE
helping anything to order the two stores.


So yeah, subtle if not broken, definitely needs more explanation.

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

[RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier Waiman Long <Waiman.Long@hpe.com> - 2016-06-15 00:50 +0200
  Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Boqun Feng <boqun.feng@gmail.com> - 2016-06-15 10:10 +0200
    Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Peter Zijlstra <peterz@infradead.org> - 2016-06-15 19:20 +0200
    Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Boqun Feng <boqun.feng@gmail.com> - 2016-06-16 04:20 +0200
      Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Will Deacon <will.deacon@arm.com> - 2016-06-16 12:20 +0200
      Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Boqun Feng <boqun.feng@gmail.com> - 2016-06-17 02:50 +0200
        Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Will Deacon <will.deacon@arm.com> - 2016-06-17 17:50 +0200
          Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Boqun Feng <boqun.feng@gmail.com> - 2016-06-18 10:50 +0200
            Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Will Deacon <will.deacon@arm.com> - 2016-06-20 10:10 +0200
  Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Davidlohr Bueso <dave@stgolabs.net> - 2016-06-15 19:00 +0200
    Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Peter Zijlstra <peterz@infradead.org> - 2016-06-15 19:20 +0200
      Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Davidlohr Bueso <dave@stgolabs.net> - 2016-06-15 20:30 +0200
        Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Peter Zijlstra <peterz@infradead.org> - 2016-06-15 20:50 +0200
          Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Davidlohr Bueso <dave@stgolabs.net> - 2016-06-15 21:00 +0200
          Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Davidlohr Bueso <dave@stgolabs.net> - 2016-06-17 03:20 +0200
            Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Davidlohr Bueso <dave@stgolabs.net> - 2016-06-17 18:30 +0200
              Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Davidlohr Bueso <dave@stgolabs.net> - 2016-06-17 18:50 +0200
      Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper  acquire/release barrier Peter Zijlstra <peterz@infradead.org> - 2016-06-16 00:00 +0200

csiph-web