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Subject: Re: [PATCH 13/14] x86/ticketlock: add slowpath logic
From: Peter Zijlstra <peterz@infradead.org>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Nick Piggin <npiggin@kernel.dk>,
        Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
        =?ISO-8859-1?Q?Am=E9rico?= Wang <xiyou.wangcong@gmail.com>,
        Eric Dumazet <dada1@cosmosbay.com>, Jan Beulich <JBeulich@novell.com>,
        Avi Kivity <avi@redhat.com>, Xen-devel <xen-devel@lists.xensource.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>,
        Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
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Date: Wed, 17 Nov 2010 13:21:43 +0100
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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 :-)