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Re: futex atomic vs ordering constraints

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On Wed, 2015-08-26 at 20:16 +0200, Peter Zijlstra wrote:
> Of course, if anything else prior to futex_atomic_op_inuser() implies an
> (RCsc) RELEASE or stronger the primitive can do without providing
> anything itself.
> 
> This turns out to be the case, a successful get_futex_key() implies a
> full memory barrier; recent: 1d0dcb3ad9d3 ("futex: Implement lockless
> wakeups").

Hmm while it is certainly true that get_futex_key() implies a full
barrier, I don't see why you're referring to the recent wake_q stuff;
where the futex "wakeup" is done much after futex_atomic_op_inuser. Yes,
that too implies a barrier, but not wrt get_futex_key() -- which
fundamentally relies on get_futex_key_refs().

> 
> And since get_futex_key() is fundamental to doing _anything_ with a
> futex, I think its semi-sane to rely on this.

Right, and it wouldn't be the first thing that relies on get_futex_key()
implying a full barrier.

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Thread

futex atomic vs ordering constraints Peter Zijlstra <peterz@infradead.org> - 2015-08-26 20:20 +0200
  Re: futex atomic vs ordering constraints Davidlohr Bueso <dave@stgolabs.net> - 2015-08-29 03:40 +0200
    Re: futex atomic vs ordering constraints Peter Zijlstra <peterz@infradead.org> - 2015-09-01 18:40 +0200
  Re: futex atomic vs ordering constraints Will Deacon <will.deacon@arm.com> - 2015-09-01 18:40 +0200
    Re: futex atomic vs ordering constraints Peter Zijlstra <peterz@infradead.org> - 2015-09-01 18:50 +0200
      Re: futex atomic vs ordering constraints Will Deacon <will.deacon@arm.com> - 2015-09-01 18:50 +0200
      Re: futex atomic vs ordering constraints Thomas Gleixner <tglx@linutronix.de> - 2015-09-01 21:10 +0200
        Re: futex atomic vs ordering constraints Peter Zijlstra <peterz@infradead.org> - 2015-09-02 15:00 +0200
          Re: futex atomic vs ordering constraints Chris Metcalf <cmetcalf@ezchip.com> - 2015-09-02 18:20 +0200
            Re: futex atomic vs ordering constraints Peter Zijlstra <peterz@infradead.org> - 2015-09-02 19:10 +0200
              Re: futex atomic vs ordering constraints Chris Metcalf <cmetcalf@ezchip.com> - 2015-09-02 19:30 +0200
              Re: futex atomic vs ordering constraints Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-02 23:20 +0200
                Re: futex atomic vs ordering constraints Peter Zijlstra <peterz@infradead.org> - 2015-09-05 20:00 +0200

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