Re: [PATCH 02/10] writeback: switch to per-bdi threads for flushing data

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

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;
> > +}
> 
> 
>