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

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Date:	Tue, 14 Jan 2014 14:09:30 -0500
From:	Waiman Long <waiman.long@...com>
To:	Richard Henderson <rth@...ddle.net>
CC:	Matt Turner <mattst88@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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 01/14/2014 01:01 PM, Richard Henderson wrote:
> On 01/14/2014 09:08 AM, Matt Turner wrote:
>> 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.
>>
> Emulated with aligned 4 byte atomics, and masking.  The same is true for arm,
> ppc, mips which, depending on cpu, also lack<  4 byte atomics.
>

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.

-Longman
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