linux-kernel - Re: [PATCH] "volatile considered harmful", take 3

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Message-ID: <464551D5.2050709@zytor.com>
Date:	Fri, 11 May 2007 22:34:13 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Satyam Sharma <satyam.sharma@...il.com>
CC:	Jonathan Corbet <corbet@....net>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, Johannes Stezenbach <js@...uxtv.org>,
	Jesper Juhl <jesper.juhl@...il.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Heikki Orsila <shdl@...alwe.fi>,
	jimmy bahuleyan <knight.camelot@...il.com>,
	Stefan Richter <stefanr@...6.in-berlin.de>
Subject: Re: [PATCH] "volatile considered harmful", take 3

Satyam Sharma wrote:
> 
>> +  - Pointers to data structures in coherent memory which might be
>> modified
>> +    by I/O devices can, sometimes, legitimately be volatile.  A ring
>> buffer
>> +    used by a network adapter, where that adapter changes pointers to
>> +    indicate which descriptors have been processed, is an example of
>> this
>> +    type of situation.
> 
> is a legitimate use case for volatile is still not clear to me (I
> agree with Alan's
> comment in a previous thread that this seems to be a case where a memory
> barrier would be applicable^Wbetter, actually). I could be wrong here, so
> would be nice if Peter explains why volatile is legitimate here.
> 
> Otherwise, it's fine with me.
> 

I don't see why Alan's way is necessarily better; it should work but is
more heavy-handed as it's disabling *all* optimization such as loop
invariants across the barrier.

	-hpa
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