On Mon, Jul 13, 2015 at 03:48:28PM +0200, Peter Zijlstra wrote:
> @@ -239,9 +265,16 @@ static void pv_wait_head(struct qspinloc
> cpu_relax();
> }
>
> - WRITE_ONCE(pn->state, vcpu_halted);
> + /*
> + * Either pv_kick_node() advanced us and ->state is already
> + * vcpu_hashed and this store is superfluous, or this is the
> + * first in which case the below cmpxchg() provides the
> + * required barriers.
> + */
> + WRITE_ONCE(pn->state, vcpu_hashed);
The easier option is of course to just move this store into the branch
below. That immediately clarifies the logic and avoids the superfluous
store.
> if (!lp) { /* ONCE */
> lp = pv_hash(lock, pn);
> +
> /*
> * lp must be set before setting _Q_SLOW_VAL
> *
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