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

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Date:	Tue, 14 Jan 2014 21:20:27 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Richard Henderson <rth@...ddle.net>,
	Matt Turner <mattst88@...il.com>,
	Daniel J Blueman <daniel@...ascale.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock()

On Tue, Jan 14, 2014 at 02:09:30PM -0500, Waiman Long wrote:
> I would like to know if the action of writing out a byte (e.g. *byte = 0) is
> atomic in those architectures or is emulated by a compiler-generated
> software read-modify-write.

So on Alpha pre ev56 something like:

  *(volatile u8 *)foo = 0;

_Should_ cause a compile error as the hardware has to do a rmw which is
not compatible with the requirements for volatile -- that said I do not
know if a compiler will actually generate this error.

I can well imagine other load-store archs suffering similar problems,
although I'm not aware of any.
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