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

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Message-ID: <CAEdQ38GOSr+yk2=HApW3c0GM0MMysCvW6Fkd5UNMjS1ynrcZDg@mail.gmail.com>
Date:	Tue, 14 Jan 2014 09:08:41 -0800
From:	Matt Turner <mattst88@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Daniel J Blueman <daniel@...ascale.com>,
	Waiman Long <waiman.long@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Richard Henderson <rth@...ddle.net>,
	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 3:03 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Jan 14, 2014 at 10:28:23AM +0800, Daniel J Blueman wrote:
>> >Peter,
>> >
>> >I found out that the build failure was caused by the fact that the
>> >__native_word() macro (used internally by compiletime_assert_atomic())
>> >allows only a size of 4 or 8 for x86-64. The data type that I used is a
>> >byte. Is there a reason why byte and short are not considered native?
>>
>> It seems likely it was implemented like that since there was no existing
>> need; long can be relied on as the largest native type, so this should
>> suffice and works here:
>
> There's Alphas that cannot actually atomically adres a byte; I do not
> konw if Linux cares about them, but if it does, we cannot in fact rely
> on this in generic primitives like this.

That's right, and thanks for the heads-up. Alpha can only address 4
and 8 bytes atomically. (LDL_L, LDQ_L, STL_C, STQ_C).

The Byte-Word extension in EV56 doesn't add new atomics, so in fact no
Alphas can address < 4 bytes atomically.
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