netdev - Re: [PATCH 1/24] make atomic_read() behave consistently on alpha

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Message-ID: <46BB656D.6090408@redhat.com>
Date:	Thu, 09 Aug 2007 15:05:17 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Segher Boessenkool <segher@...nel.crashing.org>
CC:	wjiang@...ilience.com, cfriesen@...tel.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, zlynx@....org,
	rpjday@...dspring.com, schwidefsky@...ibm.com, davem@...emloft.net
Subject: Re: [PATCH 1/24] make atomic_read() behave consistently on alpha

Segher Boessenkool wrote:
>>> 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, and it hurts performance.
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