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

From: Chris Snook <[email-masked]>
Date: Thu, 09 Aug 2007 14:13:52 -0400

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