On Mon, Jun 4, 2018 at 8:54 AM Jens Axboe <axboe@xxxxxxxxx> wrote:
>
> On 6/4/18 9:51 AM, Linus Torvalds wrote:
> >
> > Why the hell doesn't it just do bioset_exit() on the originals instead,
> > before freeing the mddev?
>
> CC'ing Neil to get his input on how best to clean that up, I'd
> be much more comfortable with that logic improved rather than
> just catering to the stack usage issue. Also include Arnd, as
> he had a test patch for it.
Also adding Tejun, because the only reason for that odd sequencing
seems to be that
- we want to queue the deletion work *inside* the spinlock, because
it actually has synchronization between the workqueue and the
'all_mddevs_lock' spinlock.
- we want to bioset_exit() the fields *outside* the spinlock.
So what it does now is to copy those big 'struct bio_set' fields
because they might go away from under it.
Tejun, the code in question is mddev_put() in drivers/md/md.c, and it
basically does
INIT_WORK(&mddev->del_work, mddev_delayed_delete);
queue_work(md_misc_wq, &mddev->del_work);
inside a spinlock, but then wants to do some stuff *outside* the
spinlock before that mddev_delayed_delete() thing actually gets
called.
Is there some way to "half-queue" the work - enough that a
flush_workqueue() will wait for it, but still guaranteeing that it
won't actually run until released?
IOW, what I think that code really wants to do is perhaps something like
/* under &all_mddevs_lock spinlock */
.. remove from all lists so that it can't be found ..
.. but add it to the md_misc_wq so that people will wait for it ..
INIT_WORK_LOCKED(&mddev->del_work, mddev_delayed_delete);
queue_work(md_misc_wq, &mddev->del_work);
...
spin_unlock(&all_mddevs_lock);
/* mddev is still guaranteed live */
bioset_exit(&mddev->bio_set);
bioset_exit(&mddev->sync_set);
/* NOW we can release it */
if (queued)
unlock_work(&mddev->del_work);
else
kfree(mddev);
or something like that?
The above is *ENTIRELY* hand-wavy garbage - don't take it seriously
per se, consider it just pseudo-code for documentation reasons, not
serious in any other way.
Linus
