linux-kernel - Re: [PATCH 2/4] locking/qrwlock: Reduce reader/writer to reader lock transfer latency

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Message-ID: <20150708171949.GE6348@arm.com>
Date:	Wed, 8 Jul 2015 18:19:49 +0100
From:	Will Deacon <will.deacon@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Waiman Long <waiman.long@...com>, Ingo Molnar <mingo@...hat.com>,
	Arnd Bergmann <arnd@...db.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH 2/4] locking/qrwlock: Reduce reader/writer to reader lock
 transfer latency

On Wed, Jul 08, 2015 at 10:52:48AM +0100, Peter Zijlstra wrote:
> On Tue, Jul 07, 2015 at 05:29:50PM -0400, Waiman Long wrote:
> > I prefer the current setting as you won't know if the writer has the
> > lock or not when you take a snapshot of the value of the lock. You
> > need the whole time sequence in this case to figure it out and so will
> > be more prone to error.
> 
> I still need to wake up, but I suspect we need to change
> queue_read_{try,}lock() to use cmpxchg/inc_not_zero like things, which
> is fine for ARM, but not so much for x86.
> 
> So I think I agree with Waiman, but am willing to be shown differently.

That's fine; I just wanted to make sure I wasn't going round the twist!

Will
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