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SubjectRe: RFC: mutex: hung tasks on SMP platforms with asm-generic/mutex-xchg.h
On Thu, 9 Aug 2012, Will Deacon wrote:

> I think we could actually fix this entirely in mutex-xchg.h by doing
> something in fastpath_lock similar to what we do for trylock:
>
>
> diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h
> index 580a6d3..c082e99 100644
> --- a/include/asm-generic/mutex-xchg.h
> +++ b/include/asm-generic/mutex-xchg.h
> @@ -25,8 +25,19 @@
> static inline void
> __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
> {
> - if (unlikely(atomic_xchg(count, 0) != 1))
> - fail_fn(count);
> + int prev = atomic_xchg(count, 0);
> +
> + if (unlikely(prev != 1)) {
> + if (prev < 0)
> + /*
> + * The lock was contended, so we need to restore
> + * its original state to ensure that any waiting
> + * tasks are woken up by the unlock slow path.
> + */
> + prev = atomic_xchg(count, prev);
> + if (prev != 1)
> + fail_fn(count);
> + }
> }
>
> What do you reckon?

Yes, that looks fine. I'd remove that if (prev < 0) entirely though.
We'll just swap a 0 for a 0 if the count wasn't < 0, or a 0 for a 1 if
the mutex just got unlocked which is also fine. This is especially
beneficial when a native xchg processor instruction is used.
> > Of course that might tilt the balance towards using mutex-dec.h on ARM
> > v6 and above instead of mutex-xchg.h. But that is an orthogonal issue,
> > and that doesn't remove the need for fixing the xchg based case for
> > correctness.
>
> I'll do some hackbench runs against mutex-dec once we've decided on the final
> xchg code. If it's faster, I'll submit a patch for ARM.

I don't think it would be faster. It is just potentially more fair.


Nicolas


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