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

[Chris Snook <csnook@redhat.com>]

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
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