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Re: [PATCH 1/4] locking/rwsem: Avoid stale ->count for rwsem_down_write_failed()

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On Sun, 08 May 2016, Peter Hurley wrote:

>On 05/08/2016 09:56 PM, Davidlohr Bueso wrote:
>> The field is obviously updated w.o the lock and needs a READ_ONCE
>> while waiting for lock holder(s) to go away, just like we do with
>> all other ->count accesses.
>
>This isn't actually fixing a bug because it's passed through
>several full barriers which will force reloading from sem->count.

Yes.

>
>I think the patch is ok if you want it just for consistency anyway,
>but please change $subject and changelog.

Yeah, I wasn't actually concerned about a specific bug, it was more
just for documentation and consistency. This code has been like this
for ever, but it would still be good to have the READ_ONCE. It is
slightly suboptimal to use, but I do not see any real impact either.

Thanks,
Davidlohr

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[PATCH -tip 0/4] locking/rwsem (xadd): Reader waiter optimizations Davidlohr Bueso <dave@stgolabs.net> - 2016-05-09 07:00 +0200
  [PATCH 4/4] locking/rwsem: Rework zeroing reader waiter->task Davidlohr Bueso <dave@stgolabs.net> - 2016-05-09 07:10 +0200
    Re: [PATCH 4/4] locking/rwsem: Rework zeroing reader waiter->task Peter Zijlstra <peterz@infradead.org> - 2016-05-09 10:00 +0200
  [PATCH 3/4] locking/rwsem: Enable lockless waiter wakeup(s) Davidlohr Bueso <dave@stgolabs.net> - 2016-05-09 07:10 +0200
    Re: [PATCH 3/4] locking/rwsem: Enable lockless waiter wakeup(s) Peter Zijlstra <peterz@infradead.org> - 2016-05-09 09:50 +0200
  [PATCH 2/4] locking/rwsem: Drop superfluous waiter refcount Davidlohr Bueso <dave@stgolabs.net> - 2016-05-09 07:10 +0200
    Re: [PATCH 2/4] locking/rwsem: Drop superfluous waiter refcount Peter Zijlstra <peterz@infradead.org> - 2016-05-09 09:40 +0200
      Re: [PATCH 2/4] locking/rwsem: Drop superfluous waiter refcount Davidlohr Bueso <dave@stgolabs.net> - 2016-05-09 18:00 +0200
        Re: [PATCH 2/4] locking/rwsem: Drop superfluous waiter refcount Peter Zijlstra <peterz@infradead.org> - 2016-05-09 18:20 +0200
          Re: [PATCH 2/4] locking/rwsem: Drop superfluous waiter refcount Davidlohr Bueso <dave@stgolabs.net> - 2016-05-09 21:00 +0200
            Re: [PATCH 2/4] locking/rwsem: Drop superfluous waiter refcount Peter Zijlstra <peterz@infradead.org> - 2016-05-09 21:10 +0200
  [PATCH 1/4] locking/rwsem: Avoid stale ->count for rwsem_down_write_failed() Davidlohr Bueso <dave@stgolabs.net> - 2016-05-09 07:10 +0200
    Re: [PATCH 1/4] locking/rwsem: Avoid stale ->count for  rwsem_down_write_failed() Peter Hurley <peter@hurleysoftware.com> - 2016-05-09 07:40 +0200
      Re: [PATCH 1/4] locking/rwsem: Avoid stale ->count for  rwsem_down_write_failed() Peter Zijlstra <peterz@infradead.org> - 2016-05-09 09:20 +0200
      Re: [PATCH 1/4] locking/rwsem: Avoid stale ->count for  rwsem_down_write_failed() Davidlohr Bueso <dave@stgolabs.net> - 2016-05-09 16:40 +0200
  Re: [PATCH -tip 0/4] locking/rwsem (xadd): Reader waiter optimizations Waiman Long <waiman.long@hpe.com> - 2016-05-10 03:40 +0200

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