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

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Message-ID: <BLU0-SMTP35EF654A11660330F0AFA196D00@phx.gbl>
Date:	Mon, 14 Feb 2011 18:03:01 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Matt Fleming <matt@...sole-pimps.org>
CC:	David Miller <davem@...emloft.net>, rostedt@...dmis.org,
	peterz@...radead.org, will.newton@...il.com, jbaron@...hat.com,
	hpa@...or.com, mingo@...e.hu, tglx@...utronix.de,
	andi@...stfloor.org, roland@...hat.com, rth@...hat.com,
	masami.hiramatsu.pt@...achi.com, fweisbec@...il.com,
	avi@...hat.com, sam@...nborg.org, ddaney@...iumnetworks.com,
	michael@...erman.id.au, linux-kernel@...r.kernel.org,
	vapier@...too.org, cmetcalf@...era.com, dhowells@...hat.com,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	benh@...nel.crashing.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 0/2] jump label: 2.6.38 updates

* Matt Fleming (matt@...sole-pimps.org) wrote:
> On Mon, 14 Feb 2011 13:46:00 -0800 (PST)
> David Miller <davem@...emloft.net> wrote:
> 
> > From: Steven Rostedt <rostedt@...dmis.org>
> > 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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