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Re: sem_lock() vs qspinlocks

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On Sat, 21 May 2016, Peter Zijlstra wrote:

>On Fri, May 20, 2016 at 05:48:39PM -0700, Davidlohr Bueso wrote:
>> On Fri, 20 May 2016, Linus Torvalds wrote:
>>
>>
>> >Oh, I definitely agree on the stable part, and yes, the "splt things
>> >up" model should come later if people agree that it's a good thing.
>>
>> The backporting part is quite nice, yes, but ultimately I think I prefer
>> Linus' suggestion making things explicit, as opposed to consulting the spinlock
>> implying barriers. I also hate to have an smp_mb() (particularly for spin_is_locked)
>> given that we are not optimizing for the common case (regular mutual excl).
>
>I'm confused; we _are_ optimizing for the common case. spin_is_locked()
>is very unlikely to be used. And arguably should be used less in favour
>of lockdep_assert_held().

Indeed we are.

But by 'common case' I was really thinking about spin_is_locked() vs spin_wait_unlock().
The former being the more common of the two, and the one which mostly will _not_ be used
for lock correctness purposes, hence it doesn't need that new smp_mb. Hence allowing users
to explicitly set the ordering needs (ie spin_lock_synchronize()) seems like the better
long term alternative. otoh, with your approach all such bugs are automatically fixed :)

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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