Re: [Xen-devel] [PATCH 8/9] qspinlock: Generic paravirt support

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Thu, Apr 02, 2015 at 09:48:34PM +0200, Peter Zijlstra wrote:
> @@ -158,20 +257,20 @@ static void pv_wait_head(struct qspinloc
>  void __pv_queue_spin_unlock(struct qspinlock *lock)
>  {
>       struct __qspinlock *l = (void *)lock;
> +     struct pv_hash_bucket *hb;
>  
>       if (xchg(&l->locked, 0) != _Q_SLOW_VAL)
>               return;
>  
>       /*
>        * At this point the memory pointed at by lock can be freed/reused,
> +      * however we can still use the pointer value to search in our hash
> +      * table.
>        *
> +      * Also, if we observe _Q_SLOW_VAL we _must_ now observe 'our' hash
> +      * bucket. See pv_wait_head().
>        */
> +     hb = pv_hash_find(lock);
> +     pv_kick(hb->cpu);
> +     WRITE_ONCE(hb->lock, NULL); /* unhash */
>  }

So I _think_ I found a problem with this approach :/

If, as per the above, the lock does indeed get freed, it can get
re-allocated and stuck in the hash table (again) before we get the
lookup and unhash it.

I'll have to ponder that a bit more.

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