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

From: Chris Snook <[email-masked]>
Date: Thu, 09 Aug 2007 15:24:40 -0400

Paul E. McKenney wrote:
> On Thu, Aug 09, 2007 at 02:13:52PM -0400, Chris Snook wrote:
>> 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.
> 
> This can also happen when using per-CPU variables.  And there are a
> number of per-CPU variables that are either atomic themselves or are
> structures containing atomic fields.

Accessing per-CPU variables in this fashion reliably already requires a suitable 
smp/non-smp read/write memory barrier.  I maintain that if we break anything 
with this change, it was really already broken, if less obviously.  Can you give 
a real or synthetic example of legitimate code that could break?

	-- Chris