Messages in this thread | | | Date | Mon, 14 Feb 2011 18:03:01 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH 0/2] jump label: 2.6.38 updates |
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* Matt Fleming (matt@console-pimps.org) wrote: > On Mon, 14 Feb 2011 13:46:00 -0800 (PST) > David Miller <davem@davemloft.net> wrote: > > > From: Steven Rostedt <rostedt@goodmis.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(¤t->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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