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

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Date:	Tue, 14 Jan 2014 10:25:16 -0500
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Daniel J Blueman <daniel@...ascale.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>, rth@...ddle.net,
	ink@...assic.park.msu.ru, mattst88@...il.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock()

On 01/14/2014 06:03 AM, Peter Zijlstra 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.

Thank for the explanation.

Can we allow architectural override of __native_word() macro? Like

#ifdef __arch_native_word
#define __native_word(t)    __arch_native_word(t)
#else
#define __native_word(t)    (sizeof(t) == sizeof(int) || sizeof(t) == 
sizeof(long))
#endif

In this way, we can allow x86 to support byte-based atomic type while 
restricting the generic macro to int and long only. I will also modify 
the code to use cmpxchg() when byte is not an atomic type.

-Longman


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