On Sat, Jul 11, 2015 at 04:36:52PM -0400, Waiman Long wrote:
> @@ -181,9 +187,9 @@ static void pv_wait_node(struct mcs_spinlock *node)
> pv_wait(&pn->state, vcpu_halted);
>
> /*
> - * Reset the vCPU state to avoid unncessary CPU kicking
> + * Reset the state except when vcpu_hashed is set.
> */
> - WRITE_ONCE(pn->state, vcpu_running);
> + cmpxchg(&pn->state, vcpu_halted, vcpu_running);
Why? Suppose we did get advanced into the hashed state, and then get a
(spurious) wakeup, this means we'll observe our ->locked == 1 condition
and fall out of pv_wait_node().
We'll then enter pv_wait_head(), which with your modification:
> @@ -229,19 +244,42 @@ static void pv_wait_head(struct qspinlock *lock, struct
> mcs_spinlock *node)
> {
> struct pv_node *pn = (struct pv_node *)node;
> struct __qspinlock *l = (void *)lock;
> - struct qspinlock **lp = NULL;
> + struct qspinlock **lp;
> int loop;
>
> + /*
> + * Initialize lp to a non-NULL value if it has already been in the
> + * pv_hashed state so that pv_hash() won't be called again.
> + */
> + lp = (READ_ONCE(pn->state) == vcpu_hashed) ? (struct qspinlock **)1
> + : NULL;
> for (;;) {
> + WRITE_ONCE(pn->state, vcpu_running);
Will instantly and unconditionally write vcpu_running.
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