linux-kernel - Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

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Date:	Thu, 16 Aug 2007 03:30:44 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	heiko.carstens@...ibm.com, horms@...ge.net.au,
	linux-kernel@...r.kernel.org, rpjday@...dspring.com, ak@...e.de,
	netdev@...r.kernel.org, cfriesen@...tel.com,
	akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
	jesper.juhl@...il.com, linux-arch@...r.kernel.org, zlynx@....org,
	satyam@...radead.org, clameter@....com, schwidefsky@...ibm.com,
	Chris Snook <csnook@...hat.com>,
	Herbert Xu <herbert.xu@...hat.com>, davem@...emloft.net,
	wensong@...ux-vs.org, wjiang@...ilience.com
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

>>> Part of the motivation here is to fix heisenbugs.  If I knew where 
>>> they
>>
>> By the same token we should probably disable optimisations
>> altogether since that too can create heisenbugs.
>
> Precisely the point -- use of volatile (whether in casts or on asms)
> in these cases are intended to disable those optimizations likely to
> result in heisenbugs.

The only thing volatile on an asm does is create a side effect
on the asm statement; in effect, it tells the compiler "do not
remove this asm even if you don't need any of its outputs".

It's not disabling optimisation likely to result in bugs,
heisen- or otherwise; _not_ putting the volatile on an asm
that needs it simply _is_ a bug :-)


Segher

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