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

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Message-ID: <CA+55aFwj_zWyUb8w3nqoWbmRXcHSsuYD2BrJ+aD8x7A0Sg6ZXw@mail.gmail.com>
Date:	Fri, 20 Dec 2013 11:54:24 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Davidlohr Bueso <davidlohr@...com>
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 Fri, Dec 20, 2013 at 11:30 AM, Davidlohr Bueso <davidlohr@...com> wrote:
>
> So we'd need the barrier right after the ticket increment (ie: the xadd
> TICKET_LOCK_INC in x86), and cannot rely on the barrier after the lock
> is taken as we could miss waiters that are just spinning trying to take
> it. Is this implicitly guaranteed across all archs?

Not necessarily. But I don't see why threads spinning on it would be
special? If you spin on things, you've already updated the head
counter, so even spinners *are* visible, even if they haven't actually
gotten the lock yet.

                 Linus
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