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Re: [RFC][PATCH] atomic: Fix atomic_set_release() for 'funny' architectures

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On Fri, Jun 09, 2017 at 01:05:06PM +0200, Peter Zijlstra wrote:

> The spinlock based atomics should be SC, that is, none of them appear to
> place extra barriers in atomic_cmpxchg() or any of the other SC atomic
> primitives and therefore seem to rely on their spinlock implementation
> being SC (I did not fully validate all that).

So I did see that ARC and PARISC have 'superfluous' smp_mb() calls
around their spinlock implementation.

That is, for spinlock semantics you only need one _after_ lock and one
_before_ unlock. But the atomic stuff relies on being SC and thus would
need one before and after both lock and unlock.

Now, afaict PARISC doesn't even have memory barriers (it uses
asm-generic/barrier.h) so that's a bit of a puzzle.

But ARC could probably optimize (if they still care about that hardware)
by pulling out those barriers and putting it in the atomic
implementation.

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[RFC][PATCH]: documentation,atomic: Add a new atomic_t document Peter Zijlstra <peterz@infradead.org> - 2017-06-09 11:30 +0200
  [RFC][PATCH] atomic: Fix atomic_set_release() for 'funny'  architectures Peter Zijlstra <peterz@infradead.org> - 2017-06-09 13:10 +0200
    Re: [RFC][PATCH] atomic: Fix atomic_set_release() for 'funny'  architectures Peter Zijlstra <peterz@infradead.org> - 2017-06-09 13:20 +0200
      Re: [RFC][PATCH] atomic: Fix atomic_set_release() for 'funny'  architectures Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2017-06-09 19:30 +0200
        Re: [RFC][PATCH] atomic: Fix atomic_set_release() for 'funny'  architectures Peter Zijlstra <peterz@infradead.org> - 2017-06-09 21:00 +0200
      Re: [RFC][PATCH] atomic: Fix atomic_set_release() for 'funny'  architectures James Bottomley <James.Bottomley@HansenPartnership.com> - 2017-06-09 21:00 +0200
  Re: [RFC][PATCH]: documentation,atomic: Add a new atomic_t document Will Deacon <will.deacon@arm.com> - 2017-06-09 17:50 +0200
    Re: [RFC][PATCH]: documentation,atomic: Add a new atomic_t document Peter Zijlstra <peterz@infradead.org> - 2017-06-09 21:40 +0200
      Re: [RFC][PATCH]: documentation,atomic: Add a new atomic_t document Boqun Feng <boqun.feng@gmail.com> - 2017-06-11 16:00 +0200
        Re: [RFC][PATCH]: documentation,atomic: Add a new atomic_t document Peter Zijlstra <peterz@infradead.org> - 2017-06-12 17:00 +0200
          Re: [RFC][PATCH]: documentation,atomic: Add a new atomic_t document Boqun Feng <boqun.feng@gmail.com> - 2017-06-13 08:40 +0200
          Re: [RFC][PATCH]: documentation,atomic: Add a new atomic_t document Will Deacon <will.deacon@arm.com> - 2017-06-14 14:40 +0200
  Re: [RFC][PATCH]: documentation,atomic: Add a new atomic_t document Randy Dunlap <rdunlap@infradead.org> - 2017-06-09 20:20 +0200

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