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[Xen-devel] Re: [PATCH 13/14] x86/ticketlock: add slowpath logic

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: [Xen-devel] Re: [PATCH 13/14] x86/ticketlock: add slowpath logic
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Wed, 17 Nov 2010 13:21:43 +0100
Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>, Nick Piggin <npiggin@xxxxxxxxx>, Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>, Eric Dumazet <dada1@xxxxxxxxxxxxx>, Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>, Avi Kivity <avi@xxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Américo Wang <xiyou.wangcong@xxxxxxxxx>, Linux Virtualization <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx>
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On Tue, 2010-11-16 at 13:08 -0800, Jeremy Fitzhardinge wrote:
> 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.

So here you say you set both LSBs in order to keep head == tail working,
but the code seems to suggest you only use the tail LSB.

I think I see why using only one LSB is sufficient, but some consistency
would be nice :-)

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