Message-ID: <46BB656D.6090408@redhat.com>
Date: Thu, 09 Aug 2007 15:05:17 -0400
From: Chris Snook <csnook@redhat.com>
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To: Segher Boessenkool <segher@kernel.crashing.org>
CC: wjiang@resilience.com, cfriesen@nortel.com, wensong@linux-vs.org,
       heiko.carstens@de.ibm.com, linux-kernel@vger.kernel.org, ak@suse.de,
       netdev@vger.kernel.org, paulmck@linux.vnet.ibm.com, horms@verge.net.au,
       akpm@linux-foundation.org, linux-arch@vger.kernel.org,
       jesper.juhl@gmail.com, torvalds@linux-foundation.org, zlynx@acm.org,
       rpjday@mindspring.com, schwidefsky@de.ibm.com, davem@davemloft.net
Subject: Re: [PATCH 1/24] make atomic_read() behave consistently on alpha
References: <20070809132442.GA13042@shell.boston.redhat.com> <20070809143255.GA8424@linux.vnet.ibm.com> <46BB2A5A.5090006@redhat.com> <20070809150445.GB8424@linux.vnet.ibm.com> <46BB31A6.4080507@redhat.com> <3bfabd7472d6f019aa1880b14013f7a1@kernel.crashing.org> <46BB3ECF.2070100@redhat.com> <1abc2c621d6b62b3ac9f489d4d18806a@kernel.crashing.org>
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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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