Re: [PATCH tip/locking/core v9 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt

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From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH tip/locking/core v9 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt
Date	Mon, 09 Nov 2015 18:40:02 +0100
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On Fri, Nov 06, 2015 at 12:47:49PM -0500, Waiman Long wrote:
> On 11/06/2015 09:50 AM, Peter Zijlstra wrote:
> >*urgh*, last time we had:
> >
> >+	if (pv_wait_head_or_steal())
> >+		goto stolen;
> >	while ((val = smp_load_acquire(&lock->val.counter))&  _Q_LOCKED_PENDING_MASK)
> >		cpu_relax();
> >
> >	...
> >
> >+stolen:
> >	while (!(next = READ_ONCE(node->next)))
> >		cpu_relax();
> >
> >	...
> >
> >Now you completely overhaul the native code.. what happened?
> 
> I want to reuse as much of the existing native code as possible instead of
> duplicating that in the PV function. The only difference now is that the PV
> function will acquire that lock.

Right; and while I doubt it hurts the native case (you did benchmark it
I hope), I'm not too keen on the end result code wise.

Maybe just keep the above.

> Semantically, I don't want to call the lock
> acquisition as lock stealing as the queue head is entitled to get the lock
> next. 

Fair enough I suppose, pv_wait_head_or_lock() then?

> I can rename pv_queued_spin_trylock_unfair() to
> pv_queued_spin_steal_lock() to emphasize the fact that this is the routine
> where lock stealing happens.

OK.

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Thread

Re: [PATCH tip/locking/core v9 5/6] locking/pvqspinlock: Allow 1  lock stealing attempt Peter Zijlstra <peterz@infradead.org> - 2015-11-06 16:00 +0100
  Re: [PATCH tip/locking/core v9 5/6] locking/pvqspinlock: Allow 1  lock stealing attempt Waiman Long <waiman.long@hpe.com> - 2015-11-06 18:50 +0100
    Re: [PATCH tip/locking/core v9 5/6] locking/pvqspinlock: Allow 1  lock stealing attempt Peter Zijlstra <peterz@infradead.org> - 2015-11-09 18:40 +0100
      Re: [PATCH tip/locking/core v9 5/6] locking/pvqspinlock: Allow 1  lock stealing attempt Waiman Long <waiman.long@hpe.com> - 2015-11-09 21:00 +0100

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