linux-kernel - Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write

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Message-ID: <20140121154113.GB31570@twins.programming.kicks-ass.net>
Date:	Tue, 21 Jan 2014 16:41:13 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	paulmck@...ux.vnet.ibm.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Matt Turner <mattst88@...il.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Daniel J Blueman <daniel@...ascale.com>,
	Richard Henderson <rth@...ddle.net>
Subject: Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock()

On Tue, Jan 21, 2014 at 10:02:06AM -0500, Waiman Long wrote:
> My latest v9 series of qrwlock patch will automatically adapt to the lack of
> atomic byte access by using an atomic integer instruction instead. So the
> new series should work for pre-EV56 Alpha, it is just a bit less efficient
> in this case.

See my other email; I don't think you can do that without also changing
the implementation of the queue_read_{try}lock() functions.

Without those changes you can have transient values in your 'read-count'
part of the word and a full word write will wreck things.
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