linux-kernel - Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

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Message-ID: <1385176773.5402.14.camel@buesod1.americas.hpqcorp.net>
Date:	Fri, 22 Nov 2013 19:19:33 -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@...nel.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>, jeffm@...e.com,
	"Norton, Scott J" <scott.norton@...com>, tom.vaden@...com,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Waiman Long <Waiman.Long@...com>, Jason Low <jason.low2@...com>
Subject: Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

On Fri, 2013-11-22 at 17:25 -0800, Linus Torvalds wrote:
> On Fri, Nov 22, 2013 at 4:56 PM, Davidlohr Bueso <davidlohr@...com> wrote:
> > In futex_wake() there is clearly no point in taking the hb->lock if
> > we know beforehand that there are no tasks to be woken. This comes
> > at the smaller cost of doing some atomic operations to keep track of
> > the list's size.
> 
> Hmm. Why? Afaik, you only care about "empty or not". And if you don't
> need the serialization from locking, then afaik you can just do a
> "plist_head_empty()" without holding the lock.

I remember this being the original approach, but after noticing some
strange behavior we quickly decided it wasn't the path. And sure enough,
I just double checked and tried the patch without atomic ops and can see
things being off: one of the futextest performance cases is stuck
blocked on a futex and I couldn't reboot the machine either -- nothing
apparent in dmesg, just not 100% functional. The thing is, we can only
avoid taking the lock only if nobody else is trying to add itself to the
list.

Thanks,
Davidlohr

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