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From: Segher Boessenkool <segher@kernel.crashing.org>
Subject: Re: [PATCH 1/24] make atomic_read() behave consistently on alpha
Date: Thu, 9 Aug 2007 21:19:20 +0200
To: Chris Snook <csnook@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
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>>>> The only safe way to get atomic accesses is to write
>>>> assembler code.  Are there any downsides to that?  I don't
>>>> see any.
>>>
>>> The assumption that aligned word reads and writes are atomic, and 
>>> that words are aligned unless explicitly packed otherwise, is 
>>> endemic in the kernel.  No sane compiler violates this assumption.  
>>> It's true that we're not portable to insane compilers after this 
>>> patch, but we never were in the first place.
>> You didn't answer my question: are there any downsides to using
>> explicit coded-in-assembler accesses for atomic accesses?  You
>> can handwave all you want that it should "just work" with
>> volatile accesses, but volatility != atomicity, volatile in C
>> is really badly defined, GCC never officially gave stronger
>> guarantees, and we have a bugzilla full of PRs to show what a
>> minefield it is.
>> So, why not use the well-defined alternative?
>
> Because we don't need to,

You don't need to use volatile objects, or accesses through
valatile-cast pointers, either.

> and it hurts performance.

Please show how it does this -- one load is one load either way,
and one store is one store.


Segher

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