linux-kernel - Re: [PATCH v4 1/7] locking/pvqspinlock: Unconditional PV kick with _Q_SLOW

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Date:	Mon, 3 Aug 2015 21:56:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Waiman Long <Waiman.Long@...com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v4 1/7] locking/pvqspinlock: Unconditional PV kick with
 _Q_SLOW_VAL

On Mon, Aug 03, 2015 at 12:09:42PM -0700, Davidlohr Bueso wrote:
> On Mon, 2015-08-03 at 20:37 +0200, Peter Zijlstra wrote:
> > OK, so there's no 'fix'? The patch claims we can loose a wakeup and I
> > just don't see how that is true.
> 
> Taking another look, I think you could hit something like this:
> 
> CPU0 (lock):					CPU1 (unlock):
>   pv_wait_head					  __pv_queued_spin_unlock
> 						    <load ->state> [bogus ->state != halted]

I don't think this can happen, see below, IF you take the slow path, you
_must_ see halted.

>     <spin>					    smp_store_release(&l->locked, 0);
> 						    
>     WRITE_ONCE(pn->state, vcpu_halted);             
>     pv_wait(&l->locked, _Q_SLOW_VAL);		    if (->state == vcpu_halted)
> 					              pv_kick(node->cpu); <-- missing wakeup, never called
> 
> So basically you can miss a wakeup if node->state load is done while the
> locking thread is spinning and hasn't gotten a chance to update the
> state to halted. That would also imply that it occurs right when the
> threshold limit is about to be reached.

	pv_wait_head()			__pv_queued_spin_unlock()

	[S] node->state = halted
	[S] hash(lock, node)
	    MB
	[S] ->locked = SLOW
	    MB
					[L] ->locked == SLOW
					    RMB
					[L] node = unhash(lock)
					[L] node->state == halted
					    RELEASE
					[S] ->locked = 0

					   kick(node->cpu)
	   CLI
	[L] ->locked

If we don't see SLOW, nothing to be done. If we do, we _must_ then also
see ->state == halted and will kick.

And note that the load of node->state _cannot_ be pushed up further, it
depends on the load of node, which in turn depends on the load of
->locked.

So I'm still not seeing it. You cannot miss a kick.
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