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

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Message-ID: <46BB5960.9040009@redhat.com>
Date:	Thu, 09 Aug 2007 14:13:52 -0400
From:	Chris Snook <csnook@...hat.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	torvalds@...ux-foundation.org, netdev@...r.kernel.org,
	akpm@...ux-foundation.org, ak@...e.de, heiko.carstens@...ibm.com,
	davem@...emloft.net, schwidefsky@...ibm.com, wensong@...ux-vs.org,
	horms@...ge.net.au, wjiang@...ilience.com, cfriesen@...tel.com,
	zlynx@....org, rpjday@...dspring.com, jesper.juhl@...il.com
Subject: Re: [PATCH 1/24] make atomic_read() behave consistently on alpha

Paul E. McKenney wrote:
> On Thu, Aug 09, 2007 at 01:14:35PM -0400, Chris Snook wrote:
>>                                 If you're depending on volatile writes 
>> being visible to other CPUs, you're screwed either way, because the CPU can 
>> hold that data in cache as long as it wants before it writes it to memory.  
>> When this finally does happen, it will happen atomically, which is all that 
>> atomic_set guarantees.  If you need to guarantee that the value is written 
>> to memory at a particular time in your execution sequence, you either have 
>> to read it from memory to force the compiler to store it first (and a 
>> volatile cast in atomic_read will suffice for this) or you have to use 
>> LOCK_PREFIX instructions which will invalidate remote cache lines 
>> containing the same variable.  This patch doesn't change either of these 
>> cases.
> 
> The case that it -can- change is interactions with interrupt handlers.
> And NMI/SMI handlers, for that matter.

You have a point here, but only if you can guarantee that the interrupt handler 
is running on a processor sharing the cache that has the not-yet-written 
volatile value.  That implies a strictly non-SMP architecture.  At the moment, 
none of those have volatile in their declaration of atomic_t, so this patch 
can't break any of them.

	-- Chris
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