On Tue, 2009-07-07 at 18:27 +0300, Artem Bityutskiy wrote:
> Hi Jens, I've noticed the following _possible_ issue.
>
> Jens Axboe wrote:
> > +static int bdi_forker_task(void *ptr)
> > +{
> > + struct backing_dev_info *me = ptr;
> > +
> > + for (;;) {
> > + struct backing_dev_info *bdi, *tmp;
> > +
> > + /*
> > + * Temporary measure, we want to make sure we don't see
> > + * dirty data on the default backing_dev_info
> > + */
> > + if (bdi_has_dirty_io(me))
> > + bdi_flush_io(me);
> > +
> > + spin_lock(&bdi_lock);
> > +
> > + /*
> > + * Check if any existing bdi's have dirty data without
> > + * a thread registered. If so, set that up.
> > + */
> > + list_for_each_entry_safe(bdi, tmp, &bdi_list, bdi_list) {
> > + if (bdi->task || !bdi_has_dirty_io(bdi))
> > + continue;
> > +
> > + bdi_add_default_flusher_task(bdi);
> > + }
> > +
> > + set_current_state(TASK_INTERRUPTIBLE);
> > +
> What happens if we are preempted here? Since we have TASK_INTERRUPTIBLE
> state, we will not come back unless some other task wakes us up. Who
> would wake us up in this case?
If it's preempted (CONFIG_PREEMPT=y), it will stay in runqueue. Only when
it calls schedule initiatively or calls schedule when exiting to user space,
it will be moved out of runqueue if its state isn't TASK_RUNNING.
See flag PREEMPT_ACTIVE.
>
> > + if (list_empty(&bdi_pending_list)) {
> > + unsigned long wait;
> > +
> > + spin_unlock(&bdi_lock);
> > + wait = msecs_to_jiffies(dirty_writeback_interval * 10);
> > + schedule_timeout(wait);
> > + try_to_freeze();
> > + continue;
> > + }
> > +
> > + __set_current_state(TASK_RUNNING);
> > +
> > + /*
> > + * This is our real job - check for pending entries in
> > + * bdi_pending_list, and create the tasks that got added
> > + */
> > + bdi = list_entry(bdi_pending_list.next, struct backing_dev_info,
> > + bdi_list);
> > + list_del_init(&bdi->bdi_list);
> > + spin_unlock(&bdi_lock);
> > +
> > + BUG_ON(bdi->task);
> > +
> > + bdi->task = kthread_run(bdi_start_fn, bdi, "flush-%s",
> > + dev_name(bdi->dev));
> > + /*
> > + * If task creation fails, then readd the bdi to
> > + * the pending list and force writeout of the bdi
> > + * from this forker thread. That will free some memory
> > + * and we can try again.
> > + */
> > + if (IS_ERR(bdi->task)) {
> > + bdi->task = NULL;
> > +
> > + /*
> > + * Add this 'bdi' to the back, so we get
> > + * a chance to flush other bdi's to free
> > + * memory.
> > + */
> > + spin_lock(&bdi_lock);
> > + list_add_tail(&bdi->bdi_list, &bdi_pending_list);
> > + spin_unlock(&bdi_lock);
> > +
> > + bdi_flush_io(bdi);
> > + }
> > + }
> > +
> > + return 0;
> > +}
>
>
>
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