linux-kernel - Re: why are some atomic_t's not volatile, while most are?

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Message-ID: <46B72C58.5030502@redhat.com>
Date:	Mon, 06 Aug 2007 10:12:40 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Jerry Jiang <wjiang@...ilience.com>
CC:	"Robert P. J. Day" <rpjday@...dspring.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: why are some atomic_t's not volatile, while most are?

Jerry Jiang wrote:
> Is there some feedback on this point ?
> 
> Thank you
> ./Jerry
> 
> On Sun, 1 Jul 2007 08:49:37 -0400 (EDT)
> "Robert P. J. Day" <rpjday@...dspring.com> wrote:
> 
>>   prompted by the earlier post on "volatile"s, is there a reason that
>> most atomic_t typedefs use volatile int's, while the rest don't?
>>
>> $ grep "typedef.*struct"  $(find . -name atomic.h)
>> ./include/asm-v850/atomic.h:typedef struct { int counter; } atomic_t;
>> ./include/asm-mips/atomic.h:typedef struct { volatile int counter; } atomic_t;
>> ./include/asm-mips/atomic.h:typedef struct { volatile long counter; } atomic64_t;
>> ...
>>
>>   etc, etc.  just curious.

If your architecture doesn't support SMP, the volatile keyword doesn't do 
anything except add a useless memory fetch.  Also, some SMP architectures (i386, 
x86_64, s390) provide sufficiently strong guarantees about memory access 
ordering that it's not necessary as long as you're using the appropriate 
locked/atomic instructions in the atomic operations.

	-- Chris
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