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

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Message-ID: <20131220085426.GZ21999@twins.programming.kicks-ass.net>
Date:	Fri, 20 Dec 2013 09:54:26 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Davidlohr Bueso <davidlohr@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	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, Dec 19, 2013 at 07:13:09PM -0800, Linus Torvalds wrote:
> I think we might have to order the two reads with an smp_rmb - making
> sure that we check the lock state first - but I think it should be
> otherwise pretty solid.

> Yeah, I said "spin_contended()" myself initially, but it needs to be
> "spin_is_locked()". It's hopefully unlikely to ever actually be
> contended (which in ticket lock terms is "head is _more_ than one away
> from the tail").


Right, which would give us something like:

  LOCK m		is_locked m
  list_{add,del}	RMB
  UNLOCK m		list_empty

Where the rmb matches the release/unlock. This guarantees that when we
see the unlock we must also see the list modification.


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