john stultz wrote:
>
> The following additional patch should correct this issue. Although since
> we weren't actually hitting it, the issue is a bit theoretical, so I've
> not been able to prove it really fixes anything.
Could you please look at the message below? I sent it privately near a month
ago, but I think these problems are not fixed yet.
Oleg.
---------------------------------------------------------------------------
> +__tasklet_action(struct softirq_action *a, struct tasklet_struct *list)
> +{
> + int loops = 1000000;
>
> while (list) {
> struct tasklet_struct *t = list;
>
> list = list->next;
> + /*
> + * Should always succeed - after a tasklist got on the
> + * list (after getting the SCHED bit set from 0 to 1),
> + * nothing but the tasklet softirq it got queued to can
> + * lock it:
> + */
> + if (!tasklet_trylock(t)) {
> + WARN_ON(1);
> + continue;
> + }
> +
> + t->next = NULL;
> +
> + /*
> + * If we cannot handle the tasklet because it's disabled,
> + * mark it as pending. tasklet_enable() will later
> + * re-schedule the tasklet.
> + */
> + if (unlikely(atomic_read(&t->count))) {
> +out_disabled:
> + /* implicit unlock: */
> + wmb();
> + t->state = TASKLET_STATEF_PENDING;
What if tasklet_enable() happens just before this line ?
After the next schedule_tasklet() we have all bits set: SCHED, RUN, PENDING.
The next invocation of __tasklet_action() clears SCHED, but tasklet_tryunlock()
below can never succeed because of PENDING.
> +again:
> + t->func(t->data);
> +
> + /*
> + * Try to unlock the tasklet. We must use cmpxchg, because
> + * another CPU might have scheduled or disabled the tasklet.
> + * We only allow the STATE_RUN -> 0 transition here.
> + */
> + while (!tasklet_tryunlock(t)) {
> + /*
> + * If it got disabled meanwhile, bail out:
> + */
> + if (atomic_read(&t->count))
> + goto out_disabled;
> + /*
> + * If it got scheduled meanwhile, re-execute
> + * the tasklet function:
> + */
> + if (test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
> + goto again;
TASKLET_STATE_SCHED could be set by tasklet_kill(), in this case it is not nice
to call t->func() again (but probably harmless).
-
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