Re: [PATCH] - Fix memory ordering problem in wake_futex()

[Jack Steiner <steiner@sgi.com>]

On Fri, Dec 23, 2005 at 02:48:22PM -0600, Olof Johansson wrote:
> On Fri, Dec 23, 2005 at 10:38:16AM -0600, Jack Steiner wrote:
> > 
> > Here is a fix for a ugly race condition that occurs in wake_futex() on IA64.
> > 
> > On IA64, locks are released using a "st.rel" instruction. This ensures that
> > preceding "stores" are visible before the lock is released but does NOT prevent
> > a "store" that follows the "st.rel" from becoming visible before the "st.rel".
> > The result is that the task that owns the futex_q continues prematurely. 
> > 
> > The failure I saw is the task that owned the futex_q resumed prematurely and
> > was context-switch off of the cpu. The task's switch_stack occupied the same
> > space of the futex_q. The store to q->lock_ptr overwrote the ar.bspstore in the
> > switch_stack. When the task resumed, it ran with a corrupted ar.bspstore.
> > Things went downhill from there.
> > 
> > Without the fix, the application fails roughly every 10 minutes. With
> > the fix, it ran 16 hours without a failure.
> 
> So what happened to what the comment 10 lines above your patch says?
> 
>         /*
>          * The lock in wake_up_all() is a crucial memory barrier after
>          * the list_del_init() and also before assigning to q->lock_ptr.
>          */
> 
> On PPC64, the spinlock unlock path has a sync in there for the very
> purpose of adding the write barrier. Maybe the ia64 unlock path is
> missing something similar?

On IA64, the "sync" instructions are actually part of the ld.acq ot st.rel
instructions that are used to set/clear spinlocks.

The program order of the instructions is:

        cmpxchg.acq             // set lock
          ..
          .. do things
          ..
        st.rel                  // release lock

        st NULL -> lock_ptr     // set flag to allow futex_q to be freed


IA64 implements fencing of ld.acq or st.rel instructions as one-directional
barriers.

For example, st.rel allows stores that FOLLOW the st.rel to be reordered
and become visible before the st.rel
(I hope this picture survives the various mailers)


        ld      |
        st      |   |        ^     ^
               - fence  -    |     |
        st.rel --------------|-----|------------------- fence
                             |     |
        ld                   |     |
        st                         |

I don't understand PPC64 but I suspect it has different memory ordering semantics.

-- 
Thanks

Jack Steiner (steiner@sgi.com)          651-683-5302
Principal Engineer                      SGI - Silicon Graphics, Inc.


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