Re: [PATCH v3 1/7] locking/pvqspinlock: Unconditional PV kick with _Q_SLOW_VAL

From: Davidlohr Bueso <dave@stgolabs.net>
To: Waiman Long <Waiman.Long@hp.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	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@hp.com>,
	Douglas Hatch <doug.hatch@hp.com>
Subject: Re: [PATCH v3 1/7] locking/pvqspinlock: Unconditional PV kick with _Q_SLOW_VAL
Date: Sun, 26 Jul 2015 18:46:55 -0700	[thread overview]
Message-ID: <1437961615.25997.36.camel@stgolabs.net> (raw)
In-Reply-To: <1437863509.3298.71.camel@stgolabs.net>

On Sat, 2015-07-25 at 15:31 -0700, Davidlohr Bueso wrote:
> On Wed, 2015-07-22 at 16:12 -0400, Waiman Long wrote:
> > The smp_store_release() is not a full barrier. In order to avoid missed
> > wakeup, we may need to add memory barrier around locked and cpu state
> > variables adding to complexity. As the chance of spurious wakeup is very
> > low, it is easier and safer to just do an unconditional kick at unlock
> > time.
> 
> Although I guess if SPIN_THRESHOLD is ever enlarged, the chances of
> spurious wakeups would be greater.
> 
> > Signed-off-by: Waiman Long <Waiman.Long@hp.com>
> 
> Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>

Thinking about this some more, as good practice, could you please add a
comment in the code explicitly mentioning the spurious wakeup side
effect? Perhaps even having something more generic for the entire kernel
might be added/created to Documentation/spurious-wakeups.txt?

Thanks,
Davidlohr