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

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Message-ID: <52DE9D43.1020305@hp.com>
Date:	Tue, 21 Jan 2014 11:16:03 -0500
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
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 01/21/2014 10:41 AM, Peter Zijlstra wrote:
> 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.

I don't see any problem with my current logic. If a writer has the write 
lock, the writer byte has to have a value of 0xff. So atomically 
subtracting 0xff from it will guarantee that the writer byte will become 
zero, which is the same as assigning a zero value to that byte. The only 
difference is that an atomic subtract instruction will need to be used 
instead of a simple byte assignment.

Please let me know if there is any flaw in my logic.

-Longman
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