Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs
Date	2017-08-11 11:10 +0200
Message-ID	<ude78-1CC-17@gated-at.bofh.it> (permalink)
References	(2 earlier) <ucVHb-65t-1@gated-at.bofh.it> <ucWae-6pd-25@gated-at.bofh.it> <ucYlH-806-3@gated-at.bofh.it> <ucYvo-83r-25@gated-at.bofh.it> <ud0dQ-Jr-15@gated-at.bofh.it>
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On Thu, Aug 10, 2017 at 02:18:30PM -0400, Waiman Long wrote:
> On 08/10/2017 12:22 PM, Waiman Long wrote:
> > On 08/10/2017 12:15 PM, Peter Zijlstra wrote:

> >> Might as well do an explicit:
> >>
> >> 	smp_mb__before_atomic()
> >> 	cmpxchg_relaxed()
> >> 	smp_mb__after_atomic()
> >>
> >> I suppose and not introduce new primitives.
> 
> I think we don't need smp_mb__after_atomic(). The read has to be fully
> ordered, but the write part may not need it as the control dependency of
> the old value should guard against incorrect action. Right?

You'd think that, but IIRC there was something funny about using the SC
return flag for control dependencies. Will?

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Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to  improve performance on some archs Peter Zijlstra <peterz@infradead.org> - 2017-08-10 14:00 +0200
  Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve  performance on some archs Waiman Long <longman@redhat.com> - 2017-08-10 15:30 +0200
    Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve  performance on some archs Waiman Long <longman@redhat.com> - 2017-08-10 16:00 +0200
      Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to  improve performance on some archs Peter Zijlstra <peterz@infradead.org> - 2017-08-10 18:20 +0200
        Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve  performance on some archs Waiman Long <longman@redhat.com> - 2017-08-10 18:30 +0200
          Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve  performance on some archs Waiman Long <longman@redhat.com> - 2017-08-10 20:20 +0200
            Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to  improve performance on some archs Peter Zijlstra <peterz@infradead.org> - 2017-08-11 11:10 +0200

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