On Thursday 05 January 2006 18:21, Linus Torvalds wrote:
> On Thu, 5 Jan 2006, Ingo Molnar wrote:
> >
> > the patch below adds the barriers to the asm-generic mutex routines, so
> > it's not like i'm lazy ;), but i really think this is unnecessary.
> > Adding this patch would add a second, unnecessary barrier for all the
> > arches that have barrier-less atomic ops.
> >
> > it also makes sense: the moment you are interested in the 'previous
> > value' of the atomic counter in an atomic fashion, you very likely want
> > to use it for a critical section. (e.g. all the put-the-resource ops
> > that use atomic_dec_test() rely on this implicit barrier.)
>
> So I _think_ your argument is bogus, and your patch is bogus. The use of
> "atomic_dec_return()" in a mutex is _not_ the same barrier as using it for
> reference counting. Not at all. Memory barriers aren't just one thing:
> they are semi-permeable things in two different directions and with two
> different operations: there are several different kinds of them.
>
> > #define __mutex_fastpath_lock(count, fail_fn) \
> > do { \
> > + smp_mb__before_atomic_dec(); \
> > if (unlikely(atomic_dec_return(count) < 0)) \
> > fail_fn(count); \
> > } while (0)
>
> So I think the barrier has to come _after_ the atomic decrement (or
> exchange).
>
> Because as it is written now, any writes in the locked region could
> percolate up to just before the atomic dec - ie _outside_ the region.
> Which is against the whole point of a lock - it would allow another CPU to
> see the write even before it sees that the lock was successful, as far as
> I can tell.
>
> But memory ordering is subtle, so maybe I'm missing something..
We mere humans^W device driver people get more confused with barriers
every day, as CPUs get more subtle in their out-of-order-ness.
I think adding longer-named-but-self-explanatory aliases for memory and io
barrier functions can help.
mmiowb => barrier_memw_iow
.... => barrier_memw_memw (a store-store barrier to mem)
....
General template for the name may be something like
[smp]barrier_{mem,io,memio}{r,w,rw}_{mem,io,memio}{r,w,rw}
Are there even more subtle cases?
--
vda
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
