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

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Date:	Mon, 3 Aug 2015 20:37:45 +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 11:22:09AM -0700, Davidlohr Bueso wrote:
> On Sun, 2015-08-02 at 00:29 +0200, Peter Zijlstra wrote:
> > That's just gibberish, even in the same cacheline stuff can get
> > reordered.
> 
> true dat
> 
> > 
> >                                                         So either we insert
> > > +	 * memory barrier here and in the corresponding pv_wait_head()
> > > +	 * function or we do an unconditional kick which is what is done here.
> > 
> > why, why why ? You've added words, but you've not actually described
> > what the problem is you're trying to fix.
> > 
> > AFAICT the only thing we really care about here is that the load in
> > question happens _after_ we observe SLOW, and that is still true.
> > 
> > The order against the unlock is irrelevant.
> > 
> > So we set ->state before we hash and before we set SLOW. Given that
> > we've seen SLOW, we must therefore also see ->state.
> > 
> > If ->state == halted, this means the CPU in question is blocked and the
> > pv_node will not get re-used -- if it does get re-used, it wasn't
> > blocked and we don't care either.
> 
> Right, if it does get re-used, we were burning SPIN_THRESHOLD and racing
> only wastes a few spins, afaict. In fact this is explicitly stated:
> 
> 		/*
> 		 * The unlocker should have freed the lock before kicking the
> 		 * CPU. So if the lock is still not free, it is a spurious
> 		 * wakeup and so the vCPU should wait again after spinning for
> 		 * a while.
> 		 */
> 
> The thing I like about this patch is that it simplifies the
> pv_kick/pv_wait flow, not having to depend on minutia like ->state
> checking. But the condition about spurious wakeups is already there, so
> really nothing changes.

OK, so there's no 'fix'? The patch claims we can loose a wakeup and I
just don't see how that is true.
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