Re: [PATCH 0/2] jump label: 2.6.38 updates

[Mathieu Desnoyers]

* Matt Fleming (matt@xxxxxxxxxxxxxxxxx) wrote:
> On Mon, 14 Feb 2011 13:46:00 -0800 (PST)
> David Miller <davem@xxxxxxxxxxxxx> wrote:
> 
> > From: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Date: Mon, 14 Feb 2011 16:39:36 -0500
> > 
> > > Thus it is not about global, as global is updated by normal means
> > > and will update the caches. atomic_t is updated via the ll/sc that
> > > ignores the cache and causes all this to break down. IOW... broken
> > > hardware ;)
> > 
> > I don't see how cache coherency can possibly work if the hardware
> > behaves this way.
> 
> Cache coherency is still maintained provided writes/reads both go
> through the cache ;-)
> 
> The problem is that for read-modify-write operations the arbitration
> logic that decides who "wins" and is allowed to actually perform the
> write, assuming two or more CPUs are competing for a single memory
> address, is not implemented in the cache controller, I think. I'm not a
> hardware engineer and I never understood how the arbitration logic
> worked but I'm guessing that's the reason that the ll/sc instructions
> bypass the cache.
> 
> Which is why the atomic_t functions worked out really well for that
> arch, such that any accesses to an atomic_t * had to go through the
> wrapper functions.

If this is true, then we have bugs in lots of xchg/cmpxchg users (which
do not reside in atomic.h), e.g.:

fs/fs_struct.c:
int current_umask(void)
{
        return current->fs->umask;
}
EXPORT_SYMBOL(current_umask);

kernel/sys.c:
SYSCALL_DEFINE1(umask, int, mask)
{
        mask = xchg(&current->fs->umask, mask & S_IRWXUGO);
        return mask;
}

The solution to this would be to force all xchg/cmpxchg users to swap to
atomic.h variables, which would force the ll semantic on read. But I'd
really like to see where this is documented first -- or which PowerPC
engineer we should talk to.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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