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Message-Id: <06d22e947a5357cd53adb070225bc7c1@kernel.crashing.org>
Date: Thu, 9 Aug 2007 20:51:31 +0200
From: Segher Boessenkool <segher@...nel.crashing.org>
To: Chris Snook <csnook@...hat.com>
Cc: wjiang@...ilience.com, rpjday@...dspring.com, wensong@...ux-vs.org,
heiko.carstens@...ibm.com, linux-kernel@...r.kernel.org,
ak@...e.de, netdev@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
horms@...ge.net.au, akpm@...ux-foundation.org,
linux-arch@...r.kernel.org, jesper.juhl@...il.com,
torvalds@...ux-foundation.org, schwidefsky@...ibm.com,
davem@...emloft.net, cfriesen@...tel.com, zlynx@....org
Subject: Re: [PATCH 1/24] make atomic_read() behave consistently on alpha
>> The compiler is within its rights to read a 32-bit quantity 16 bits at
>> at time, even on a 32-bit machine. I would be glad to help pummel any
>> compiler writer that pulls such a dirty trick, but the C standard
>> really
>> does permit this.
>
> Yes, but we don't write code for these compilers. There are countless
> pieces of kernel code which would break in this condition, and there
> doesn't seem to be any interest in fixing this.
"Other things are broken too". Great argument :-)
> Sequence points enforce read-after-write ordering, not
> write-after-write.
Sequence points order *all* side effects; sequence points exist in the
domain of the abstract sequential model of the C language only. The
compiler translates that to machine code that is equivalent to that C
code under the "as-if" rule; but this is still in that abstract model,
which doesn't include things such as SMP, visibility by I/O devices,
store queues, etc. etc.
> We flush writes with reads for MMIO because of this effect as well as
> the CPU/bus effects.
You cannot flush all MMIO writes with reads; this is a PCI-specific
thing. And even then, you need more than just the read itself: you
have to make sure the read completed and returned data.
>> In short, please retain atomic_set()'s volatility, especially on those
>> architectures that declared the atomic_t's counter to be volatile.
>
> Like i386 and x86_64? These used to have volatile in the atomic_t
> declaration. We removed it, and the sky did not fall.
And this proves what? Lots of stuff "works" by accident.
Segher
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