linux-kernel - Re: [PATCH v2 2/5] locking/pvqspinlock: Fix missed PV wakeup problem

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Date:	Fri, 15 Jul 2016 10:47:32 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
	Boqun Feng <boqun.feng@...il.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v2 2/5] locking/pvqspinlock: Fix missed PV wakeup problem

So the reason I never get around to this is because the patch stinks.

It simply doesn't make sense... Remember, the harder you make a reviewer
work the less likely the review will be done.

Present things in clear concise language and draw a picture.

On Tue, May 31, 2016 at 12:53:48PM -0400, Waiman Long wrote:
> Currently, calling pv_hash() and setting _Q_SLOW_VAL is only
> done once for any pv_node. It is either in pv_kick_node() or in
> pv_wait_head_or_lock().

So far so good....

> Because of lock stealing, a pv_kick'ed node is
> not guaranteed to get the lock before the spinning threshold expires
> and has to call pv_wait() again. As a result, the new lock holder
> won't see _Q_SLOW_VAL and so won't wake up the sleeping vCPU.

*brain melts* what!? pv_kick'ed node reads like pv_kick_node() and that
doesn't make any kind of sense.

I'm thinking you're trying to say this:

CPU0			CPU1			CPU2

__pv_queued_spin_unlock_slowpath()
  ...
  smp_store_release(&l->locked, 0);
			__pv_queued_spin_lock_slowpath()
			  ...
			  pv_queued_spin_steal_lock()
			    cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0

						pv_wait_head_or_lock()

  pv_kick(node->cpu);  ---------------------->	  pv_wait(&l->locked, _Q_SLOW_VAL);

			__pv_queued_spin_unlock()
			  cmpxchg(&l->locked, _Q_LOCKED_VAL, 0) == _Q_LOCKED_VAL

						  for () {
						    trylock_clear_pending();
						    cpu_relax();
						  }

						  pv_wait(&l->locked, _Q_SLOW_VAL);

Which is indeed 'bad', but not fatal, note that the later pv_wait() will
not in fact go wait, since l->locked will _not_ be _Q_SLOW_VAL.

Is this indeed the 3 CPU scenario you tried to describe in a scant 4
lines of text, or is there more to it?