linux-kernel - Re: [PATCH 1/1] x86, ticketlock: spin_unlock

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Message-ID: <CA+55aFz6a99n_bGGi0CrOeWA3KRVdAJtPiF_3iP1aVx_wE+mMA@mail.gmail.com>
Date:	Mon, 1 Dec 2014 13:49:35 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <Waiman.Long@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] x86, ticketlock: spin_unlock_wait() can livelock

On Mon, Dec 1, 2014 at 1:34 PM, Oleg Nesterov <oleg@...hat.com> wrote:
>
> But we do not need to wait for arch_spin_is_locked() == F. If it is
> locked we only need to wait until the current owner drops this lock.
> So we could simply spin until old_head != lock->tickets.head in this
> case, but .head can overflow and thus we can't check "unlocked" only
> once before the main loop.

Ack. Looks like a good idea. Can you point to the report that you
think this might be the cause of?

                    Linus
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