RFC: mutex: hung tasks on SMP platforms with asm-generic/mutex-xchg.h

[Nicolas Pitre]

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