Date: Mon, 9 Nov 2015 18:29:45 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Scott J Norton <scott.norton@hpe.com>,
        Douglas Hatch <doug.hatch@hpe.com>,
        Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH tip/locking/core v9 5/6] locking/pvqspinlock: Allow 1
 lock stealing attempt
Message-ID: <20151109172945.GM17308@twins.programming.kicks-ass.net>
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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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