linux-kernel - Re: [PATCH 11/13] x86/ticketlock: only do kick after doing unlock

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Message-ID: <1314974968.1861.3.camel@twins>
Date:	Fri, 02 Sep 2011 16:49:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>, Andi Kleen <andi@...stfloor.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 11/13] x86/ticketlock: only do kick after doing unlock

On Thu, 2011-09-01 at 17:55 -0700, Jeremy Fitzhardinge wrote:
> From: Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
> 
> We must release the lock before checking to see if the lock is in
> slowpath or else there's a potential race where the lock enters the
> slow path after the unlocker has checked the slowpath flag, but before
> it has actually unlocked.
> 
> Signed-off-by: Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com> 

Wouldn't it be much better to fold it back so that this bug never
happens when you bisect?
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