linux-kernel - Re: [PATCH 13/14] x86/ticketlock: add slowpath logic

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Date:	Mon, 17 Jan 2011 20:52:22 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Américo Wang <xiyou.wangcong@...il.com>,
	Eric Dumazet <dada1@...mosbay.com>,
	Jan Beulich <JBeulich@...ell.com>, Avi Kivity <avi@...hat.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	suzuki@...ibm.com
Subject: Re: [PATCH 13/14] x86/ticketlock: add slowpath logic

On Tue, Nov 16, 2010 at 01:08:44PM -0800, Jeremy Fitzhardinge wrote:
> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
> 
> Maintain a flag in both LSBs of the ticket lock which indicates whether
> anyone is in the lock slowpath and may need kicking when the current
> holder unlocks.  The flags are set when the first locker enters
> the slowpath, and cleared when unlocking to an empty queue.
> 
> In the specific implementation of lock_spinning(), make sure to set
> the slowpath flags on the lock just before blocking.  We must do
> this before the last-chance pickup test to prevent a deadlock
> with the unlocker:
> 
> Unlocker			Locker
> 				test for lock pickup
> 					-> fail
> test slowpath + unlock
> 	-> false
> 				set slowpath flags
> 				block
> 
> Whereas this works in any ordering:
> 
> Unlocker			Locker
> 				set slowpath flags
> 				test for lock pickup
> 					-> fail
> 				block
> test slowpath + unlock
> 	-> true, kick

I think this is still racy ..

Unlocker				Locker

				
test slowpath
	-> false
		
				set slowpath flag
				test for lock pickup
					-> fail
				block


unlock

unlock needs to happen first before testing slowpath? I have made that change
for my KVM guest and it seems to be working well with that change .. Will
cleanup and post my patches shortly

- vatsa
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