linux-kernel - Re: [PATCH v3 4/4] futex: Avoid taking hb lock if nothing to wakeup

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Message-ID: <1387496569.8363.12.camel@buesod1.americas.hpqcorp.net>
Date:	Thu, 19 Dec 2013 15:42:49 -0800
From:	Davidlohr Bueso <davidlohr@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>, Jeff Mahoney <jeffm@...e.com>,
	Jason Low <jason.low2@...com>,
	Waiman Long <Waiman.Long@...com>, Tom Vaden <tom.vaden@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v3 4/4] futex: Avoid taking hb lock if nothing to wakeup

On Thu, 2013-12-19 at 15:14 -0800, Linus Torvalds wrote:
> On Thu, Dec 19, 2013 at 10:45 AM, Davidlohr Bueso <davidlohr@...com> wrote:
> >
> > - increment the counter at queue_lock() as we always end up calling
> >   queue_me() which adds the element to the list. Upon any error,
> >   queue_unlock() is called for housekeeping, for which we decrement
> >   to mach the increment done in queue_lock().
> >
> > - decrement the counter at __unqueue_me() to reflect when an element is
> >   removed from the queue for wakeup related purposes.
> 
> I still hate this whole separate counter thing. It seems really annoying.
> 
> If re-ordering things didn't work out, then why can't just the counter
> we *already* have in the spinlock itself work as the counter? Your
> counter update logic seems to basically match when you take the
> spinlock anyway.
> 
> The *testing* side doesn't actually care about how many waiters there
> are, it only cares about whether there are waiters. 

True.

> And it can look at
> the wait-list for that - but you want to close the race between the
> entry actually getting added to the list using this counter. But the
> place you increment the new counter is the same place as you take the
> spinlock, which does that ticket increment. No?

I don't think so. If we rely on this, then we could end up missing
to-be-queued tasks that are in the process of acquiring the lock, so
waiters could sleep forever. So we need a way of acknowledging that a
task is in the process of waiting when concurrently doing wakeups.

Thanks,
Davidlohr

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