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Re: sem_lock() vs qspinlocks

From Davidlohr Bueso <dave@stgolabs.net>
Newsgroups linux.kernel
Subject Re: sem_lock() vs qspinlocks
Date 2016-05-20 18:30 +0200
Message-ID <rAVtf-7Ld-17@gated-at.bofh.it> (permalink)
References <rALkd-1uD-9@gated-at.bofh.it> <rARfX-51X-9@gated-at.bofh.it>
Organization linux.* mail to news gateway

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On Fri, 20 May 2016, Peter Zijlstra wrote:

>The problem is that the clear_pending_set_locked() is an unordered
>store, therefore this store can be delayed until no later than
>spin_unlock() (which orders against it due to the address dependency).
>
>This opens numerous races; for example:
>
>	ipc_lock_object(&sma->sem_perm);
>	sem_wait_array(sma);
>
>				false   ->	spin_is_locked(&sma->sem_perm.lock)
>
>is entirely possible, because sem_wait_array() consists of pure reads,
>so the store can pass all that, even on x86.

I had pondered at the unordered stores in clear_pending_set_locked() for arm,
for example, but I _certainly_ missed this for x86 inside the ACQUIRE region.

Thanks,
Davidlohr

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Thread

sem_lock() vs qspinlocks Davidlohr Bueso <dave@stgolabs.net> - 2016-05-20 07:40 +0200
  Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 10:00 +0200
    Re: sem_lock() vs qspinlocks Davidlohr Bueso <dave@stgolabs.net> - 2016-05-20 17:10 +0200
      Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 17:10 +0200
        Re: sem_lock() vs qspinlocks Davidlohr Bueso <dave@stgolabs.net> - 2016-05-20 17:30 +0200
        Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 17:30 +0200
      Re: sem_lock() vs qspinlocks Waiman Long <waiman.long@hpe.com> - 2016-05-20 22:50 +0200
        Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 23:00 +0200
          Re: sem_lock() vs qspinlocks Davidlohr Bueso <dave@stgolabs.net> - 2016-05-21 03:00 +0200
            Re: sem_lock() vs qspinlocks Waiman Long <waiman.long@hpe.com> - 2016-05-21 06:10 +0200
              Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-21 09:50 +0200
  Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 10:00 +0200
  Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 10:20 +0200
    Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 10:20 +0200
      Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 11:40 +0200
  Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 10:40 +0200
    Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 11:10 +0200
      Re: sem_lock() vs qspinlocks Ingo Molnar <mingo@kernel.org> - 2016-05-20 12:10 +0200
        Re: sem_lock() vs qspinlocks Mel Gorman <mgorman@techsingularity.net> - 2016-05-20 12:50 +0200
  Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 14:00 +0200
    Re: sem_lock() vs qspinlocks Boqun Feng <boqun.feng@gmail.com> - 2016-05-20 16:10 +0200
      Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 17:30 +0200
        Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 18:10 +0200
          Re: sem_lock() vs qspinlocks Linus Torvalds <torvalds@linux-foundation.org> - 2016-05-20 19:10 +0200
            Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 23:10 +0200
              Re: sem_lock() vs qspinlocks Linus Torvalds <torvalds@linux-foundation.org> - 2016-05-20 23:50 +0200
                Re: sem_lock() vs qspinlocks Davidlohr Bueso <dave@stgolabs.net> - 2016-05-21 02:50 +0200
                Re: sem_lock() vs qspinlocks Linus Torvalds <torvalds@linux-foundation.org> - 2016-05-21 04:40 +0200
                Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-21 09:40 +0200
                Re: sem_lock() vs qspinlocks Manfred Spraul <manfred@colorfullife.com> - 2016-05-21 15:50 +0200
                Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-24 13:00 +0200
                Re: sem_lock() vs qspinlocks Davidlohr Bueso <dave@stgolabs.net> - 2016-05-21 19:20 +0200
            Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-23 14:30 +0200
              Re: sem_lock() vs qspinlocks Linus Torvalds <torvalds@linux-foundation.org> - 2016-05-23 20:00 +0200
                Re: sem_lock() vs qspinlocks Boqun Feng <boqun.feng@gmail.com> - 2016-05-25 08:40 +0200
          Re: sem_lock() vs qspinlocks Manfred Spraul <manfred@colorfullife.com> - 2016-05-22 10:50 +0200
            Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-22 11:40 +0200
    Re: sem_lock() vs qspinlocks Davidlohr Bueso <dave@stgolabs.net> - 2016-05-20 18:30 +0200
    Re: sem_lock() vs qspinlocks Waiman Long <waiman.long@hpe.com> - 2016-05-20 22:50 +0200
      Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 23:00 +0200

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