lkml.org 
[lkml]   [2012]   [Aug]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    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


    \
     
     \ /
      Last update: 2012-08-09 19:02    [W:0.026 / U:88.740 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site