On Sun, Jun 13, 2010 at 10:46:35PM -0400, Mathieu Desnoyers wrote:
> * Mathieu Desnoyers ([email protected]) wrote:
> > * Paul E. McKenney ([email protected]) wrote:
> > > On Sun, Jun 13, 2010 at 05:36:05PM -0400, Mathieu Desnoyers wrote:
> > > > * Paul E. McKenney ([email protected]) wrote:
> > > > > On Sun, Jun 13, 2010 at 05:20:34PM -0400, Mathieu Desnoyers wrote:
> > > > > > * Paul E. McKenney ([email protected]) wrote:
> > > > > > > On Thu, Jun 10, 2010 at 09:46:31PM -0400, Mathieu Desnoyers wrote:
> > > > > >
> > > > > > [...]
> > > > > >
> > > > > > (will reply to the rest in the individual patches)
> > > > > >
> > > > > > > > Can we trust __sync_lock_test_and_set/__sync_add_and_fetch
> > > > > > > > given that
> > > > > > > > __sync_synchronize is broken ?
> > > > > > >
> > > > > > > I don't know yet. If it turns out that we cannot, then I will
> > > > > > > use some
> > > > > > > form of global locking. But the __sync_lock_test_and_set() do at
> > > > > > > least
> > > > > > > generate instructions, unlike __sync_synchronize(). ;-)
> > > > > >
> > > > > > I'm concerned about the fact that their synchronization primitives
> > > > > > might have
> > > > > > the assembly all with, except for the memory barriers.
> > > > >
> > > > > OK. How about if I used a hashed array of locks, indexed by a hash of
> > > > > the cacheline number of the access in question? Then the "unknown"
> > > > > case
> > > > > doesn't depend at all on the __sync_ primitives.
> > > >
> > > > I'm afraid this won't work with signal handlers. :-/ It would be a
> > > > shame to have
> > > > to make these primitives non-signal-safe just for the sake of the
> > > > "unknown"
> > > > arch. Or maybe it could make sense to disable signals around these,
> > > > even though
> > > > it will be really slow ?
> > >
> > > Hmmm...
> > >
> > > Maybe the right thing to do is to make the build fail in the "unknown"
> > > case. By the time we make this safe, the performance will be pretty
> > > bad!
> > >
> > > My thought is to simply not have the two "unknown" include files, so
> > > that configuration dies when trying to copy them over. Perhaps better
> > > yet, have the two "unknown" include files have nothing but a #error
> > > statement. Seem like a reasonable approach?
> >
> > Yep, I like the #error approach. It will provide a meaningful error message.
>
> Hrm. OK, I think I got a compromise for you so we can have a working "unknown"
> architecture.
>
> smp_mb(): double-fake-mutex (stack-local)
> Note: we depend on having correct memory barriers in the pthread
> mutex
> implementation. Hopefully they got, at least, that right for ARM. ;)
>
> uatomic_xchg():
>
> smp_mb();
> ret = __sync_lock_test_and_set(addr, v);
> smp_mb();
> ret;
>
> uatomic_cmpxchg():
>
> smp_mb();
> ret = __sync_val_compare_and_swap(addr, old, _new)
> smp_mb();
> ret;
>
> uatomic_add_return():
>
> smp_mb();
> ret = __sync_add_and_fetch(addr, v);
> smp_mb();
> ret;
>
> get_cycles(): use gettimeofday.
>
> This is based on the assumption that all architecture's __sync_*() primitives
> are right, modulo possibly incorrect memory barriers (so we provide our own).
> This would be basically showing extreme distrust in gcc's __sync_*() memory
> barriers.
>
> Thoughts ?
Interesting... I just managed to convince myself that "unknown" should
refuse to build. I will think some more about it and send a patch
tomorrow. ;-)
Thanx, Paul
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