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

From Davidlohr Bueso <dave@stgolabs.net>
Newsgroups linux.kernel
Subject Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier
Date 2016-06-15 21:00 +0200
Message-ID <rKocF-6cT-7@gated-at.bofh.it> (permalink)
References (1 earlier) <rK5jH-2zl-3@gated-at.bofh.it> <rKmkx-52w-7@gated-at.bofh.it> <rKmDU-5r2-19@gated-at.bofh.it> <rKnJE-62T-15@gated-at.bofh.it> <rKo30-69o-19@gated-at.bofh.it>
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On Wed, 15 Jun 2016, Peter Zijlstra wrote:

>On Wed, Jun 15, 2016 at 11:27:24AM -0700, Davidlohr Bueso wrote:
>> On Wed, 15 Jun 2016, Peter Zijlstra wrote:
>>
>> >In any case, its fairly simple to cure, just add
>> >smp_acquire__after_ctrl_dep() at the end. If we bail because
>> >need_resched() we don't need the acquire I think.
>>
>> I was just considering this for your smp_cond_acquire/smp_cond_load_acquire
>
>Right, so that need_resched break makes that a bit awkward. Not to
>mention the cpu_relaxed() vs cpu_relaxed_lowlatency() difference.

Oh sure, I was merely refering to the ordering semantics, not the calls
themselves -- although at some point, as archs begin to port locking/core
optimizations, we _will_ need the variants for dealing with '_lowlatency'.

>
>> rework, so yeah I guess an smp_acquire__after_ctrl_dep would be a nice
>> compromise.
>>
>> However, I was always under the impression that races with node->locked were
>> rather harmless (as indicated in the mentioned commit) -- which is why ->locked
>> are simple load/stores, with the exception of the unqueueing -- but yeah, that's
>> not even paired.
>
>Yeah, see a few patches further in this series, where he guards a
>variables with the osq_lock.

*sigh*

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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

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