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

From	Will Deacon <will.deacon@arm.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()
Date	2015-11-16 17:30 +0100
Message-ID	<qvuPg-1bQ-17@gated-at.bofh.it> (permalink)
References	(5 earlier) <qtID0-6tH-19@gated-at.bofh.it> <qtUkO-5Od-1@gated-at.bofh.it> <qu4Nb-3Zg-1@gated-at.bofh.it> <qvumf-M6-35@gated-at.bofh.it> <qvuvU-151-15@gated-at.bofh.it>
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On Mon, Nov 16, 2015 at 05:04:45PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 16, 2015 at 04:56:58PM +0100, Peter Zijlstra wrote:
> > 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.
> 
> And unless PPC would move to using RCsc locks with a SYNC in
> spin_lock(), I don't think it makes sense to add
> smp_mb__after_unlock_lock() to all tsk->pi_lock instances to fix this.
> As that is far more expensive than flipping the exit path to do
> spin_lock()+spin_unlock().

... or we upgrade spin_unlock_wait to a LOCK operation, which might be
slightly cheaper than spin_lock()+spin_unlock().

Will
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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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