Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()
Date	2015-11-16 17:00 +0100
Message-ID	<qvumf-M6-35@gated-at.bofh.it> (permalink)
References	(3 earlier) <qtAcp-11f-9@gated-at.bofh.it> <qtCxB-2EB-11@gated-at.bofh.it> <qtID0-6tH-19@gated-at.bofh.it> <qtUkO-5Od-1@gated-at.bofh.it> <qu4Nb-3Zg-1@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Thu, Nov 12, 2015 at 10:21:39AM -0800, Linus Torvalds wrote:
> Now, the point of spin_unlock_wait() (and "spin_is_locked()") should
> generally be that you have some external ordering guarantee that
> guarantees that the lock has been taken. For example, for the IPC
> semaphores, we do either one of:
> 
>  (a) get large lock, then - once you hold that lock - wait for each small lock
> 
> or
> 
>  (b) get small lock, then - once you hold that lock - check that the
> largo lock is unlocked
> 
> and that's the case we should really worry about.  The other uses of
> spin_unlock_wait() should have similar "I have other reasons to know
> I've seen that the lock was taken, or will never be taken after this
> because XYZ".

I don't think this is true for the usage in do_exit(), we have no
knowledge on if pi_lock is taken or not. We just want to make sure that
_if_ it were taken, we wait until it is released.

But I'm not sure where task_work_run() sits, at first reading it appears
to also not be true -- there doesn't appear to be a reason we know a
lock to be held.

It does however appear true for the usage in completion_done(), where by
having tested x->done, we know a pi_lock _was_ held.

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Thread

[PATCH 0/4] scheduler ordering bits Peter Zijlstra <peterz@infradead.org> - 2015-11-02 15:00 +0100
  [PATCH 2/4] sched: Document Program-Order guarantees Peter Zijlstra <peterz@infradead.org> - 2015-11-02 15:00 +0100
    Re: [PATCH 2/4] sched: Document Program-Order guarantees Paul Turner <pjt@google.com> - 2015-11-02 21:30 +0100
      Re: [PATCH 2/4] sched: Document Program-Order guarantees Peter Zijlstra <peterz@infradead.org> - 2015-11-02 21:40 +0100
        Re: [PATCH 2/4] sched: Document Program-Order guarantees Paul Turner <pjt@google.com> - 2015-11-02 23:10 +0100
          Re: [PATCH 2/4] sched: Document Program-Order guarantees Peter Zijlstra <peterz@infradead.org> - 2015-11-02 23:20 +0100
      Re: [PATCH 2/4] sched: Document Program-Order guarantees Peter Zijlstra <peterz@infradead.org> - 2015-11-20 11:10 +0100
        Re: [PATCH 2/4] sched: Document Program-Order guarantees Boqun Feng <boqun.feng@gmail.com> - 2015-11-20 15:10 +0100
          Re: [PATCH 2/4] sched: Document Program-Order guarantees Peter Zijlstra <peterz@infradead.org> - 2015-11-20 15:20 +0100
            Re: [PATCH 2/4] sched: Document Program-Order guarantees Boqun Feng <boqun.feng@gmail.com> - 2015-11-20 15:30 +0100
              Re: [PATCH 2/4] sched: Document Program-Order guarantees Peter Zijlstra <peterz@infradead.org> - 2015-11-20 20:50 +0100
  [PATCH 4/4] locking: Introduce smp_cond_acquire() Peter Zijlstra <peterz@infradead.org> - 2015-11-02 15:00 +0100
    Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Peter Zijlstra <peterz@infradead.org> - 2015-11-02 15:00 +0100
      Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Will Deacon <will.deacon@arm.com> - 2015-11-02 18:50 +0100
        Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-03 02:20 +0100
          Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Linus Torvalds <torvalds@linux-foundation.org> - 2015-11-03 02:30 +0100
    Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Will Deacon <will.deacon@arm.com> - 2015-11-02 18:50 +0100
    Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Linus Torvalds <torvalds@linux-foundation.org> - 2015-11-02 19:10 +0100
      Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Will Deacon <will.deacon@arm.com> - 2015-11-02 19:40 +0100
        Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Linus Torvalds <torvalds@linux-foundation.org> - 2015-11-02 20:20 +0100
          Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Will Deacon <will.deacon@arm.com> - 2015-11-02 21:00 +0100
            Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Peter Zijlstra <peterz@infradead.org> - 2015-11-02 21:30 +0100
          Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Peter Zijlstra <peterz@infradead.org> - 2015-11-02 23:00 +0100
          Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-03 03:00 +0100
            Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Linus Torvalds <torvalds@linux-foundation.org> - 2015-11-03 20:50 +0100
              Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-04 05:00 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Linus Torvalds <torvalds@linux-foundation.org> - 2015-11-04 05:50 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-04 14:10 +0100
      Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() David Howells <dhowells@redhat.com> - 2015-11-02 21:40 +0100
        Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Peter Zijlstra <peterz@infradead.org> - 2015-11-02 21:50 +0100
        Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Linus Torvalds <torvalds@linux-foundation.org> - 2015-11-02 22:20 +0100
    Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Oleg Nesterov <oleg@redhat.com> - 2015-11-03 18:10 +0100
      Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Peter Zijlstra <peterz@infradead.org> - 2015-11-03 19:30 +0100
      Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Boqun Feng <boqun.feng@gmail.com> - 2015-11-11 10:50 +0100
        Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Boqun Feng <boqun.feng@gmail.com> - 2015-11-11 11:40 +0100
          Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Oleg Nesterov <oleg@redhat.com> - 2015-11-11 20:00 +0100
          Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-12 15:00 +0100
        Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Peter Zijlstra <peterz@infradead.org> - 2015-11-11 13:20 +0100
          Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Oleg Nesterov <oleg@redhat.com> - 2015-11-11 19:50 +0100
            Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Linus Torvalds <torvalds@linux-foundation.org> - 2015-11-11 22:30 +0100
            Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Boqun Feng <boqun.feng@gmail.com> - 2015-11-12 08:20 +0100
              Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Peter Zijlstra <peterz@infradead.org> - 2015-11-12 11:30 +0100
              Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Oleg Nesterov <oleg@redhat.com> - 2015-11-12 15:10 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Boqun Feng <boqun.feng@gmail.com> - 2015-11-12 15:50 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-12 16:50 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Will Deacon <will.deacon@arm.com> - 2015-11-12 23:00 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-12 15:50 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Peter Zijlstra <peterz@infradead.org> - 2015-11-12 16:00 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-12 16:10 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Peter Zijlstra <peterz@infradead.org> - 2015-11-12 16:10 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-12 16:30 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Will Deacon <will.deacon@arm.com> - 2015-11-12 22:30 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Boqun Feng <boqun.feng@gmail.com> - 2015-11-12 16:20 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Oleg Nesterov <oleg@redhat.com> - 2015-11-12 18:50 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Peter Zijlstra <peterz@infradead.org> - 2015-11-12 19:10 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Oleg Nesterov <oleg@redhat.com> - 2015-11-12 19:40 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-12 20:00 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Will Deacon <will.deacon@arm.com> - 2015-11-12 22:40 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-13 00:50 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Will Deacon <will.deacon@arm.com> - 2015-11-16 15:00 +0100
              Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Linus Torvalds <torvalds@linux-foundation.org> - 2015-11-12 19:30 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Will Deacon <will.deacon@arm.com> - 2015-11-12 23:10 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Peter Zijlstra <peterz@infradead.org> - 2015-11-16 17:00 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Peter Zijlstra <peterz@infradead.org> - 2015-11-16 17:10 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Will Deacon <will.deacon@arm.com> - 2015-11-16 17:30 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-16 17:50 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Will Deacon <will.deacon@arm.com> - 2015-11-16 17:50 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-16 18:20 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Linus Torvalds <torvalds@linux-foundation.org> - 2015-11-16 23:00 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Will Deacon <will.deacon@arm.com> - 2015-11-17 13:00 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-11-17 22:10 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Will Deacon <will.deacon@arm.com> - 2015-11-18 12:30 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Will Deacon <will.deacon@arm.com> - 2015-11-19 19:10 +0100
                Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Peter Zijlstra <peterz@infradead.org> - 2015-11-20 11:20 +0100
  [PATCH 1/4] sched: Better document the try_to_wake_up() barriers Peter Zijlstra <peterz@infradead.org> - 2015-11-02 15:00 +0100
  [PATCH 3/4] sched: Fix a race in try_to_wake_up() vs schedule() Peter Zijlstra <peterz@infradead.org> - 2015-11-02 15:00 +0100

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