On Mon, Jul 13, 2015 at 02:02:02PM +0200, Peter Zijlstra wrote:
> 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;
Because that ^
> > for (;;) {
> > + WRITE_ONCE(pn->state, vcpu_running);
>
> Will instantly and unconditionally write vcpu_running.
>
>
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