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

[Waiman Long]

On 11/09/2015 12:29 PM, Peter Zijlstra wrote:
> 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.

I can jump over the smp_load_acquire() for PV instead of adding an 
additional if block. For the native code, the only thing that was added 
was an additional masking of val with _Q_TAIL_MASK which I don't think 
will make too much of a difference.
> > 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 am fine with that name.

> > 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.

Cheers,
Longman
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