On Tue, 20 Aug 2024, Dave Chinner wrote:
> On Mon, Aug 19, 2024 at 03:20:39PM +1000, NeilBrown wrote:
> > bd_prepare_to_claim() current uses a bit waitqueue with a matching
> > wake_up_bit() in bd_clear_claiming(). However it is really waiting on a
> > "var", not a "bit".
> >
> > So change to wake_up_var(), and use ___wait_var_event() for the waiting.
> > Using the triple-underscore version allows us to drop the mutex across
> > the schedule() call.
> ....
> > @@ -535,33 +535,23 @@ int bd_prepare_to_claim(struct block_device *bdev, void *holder,
> > const struct blk_holder_ops *hops)
> > {
> > struct block_device *whole = bdev_whole(bdev);
> > + int err = 0;
> >
> > if (WARN_ON_ONCE(!holder))
> > return -EINVAL;
> > -retry:
> > - mutex_lock(&bdev_lock);
> > - /* if someone else claimed, fail */
> > - if (!bd_may_claim(bdev, holder, hops)) {
> > - mutex_unlock(&bdev_lock);
> > - return -EBUSY;
> > - }
> > -
> > - /* if claiming is already in progress, wait for it to finish */
> > - if (whole->bd_claiming) {
> > - wait_queue_head_t *wq = bit_waitqueue(&whole->bd_claiming, 0);
> > - DEFINE_WAIT(wait);
> >
> > - prepare_to_wait(wq, &wait, TASK_UNINTERRUPTIBLE);
> > - mutex_unlock(&bdev_lock);
> > - schedule();
> > - finish_wait(wq, &wait);
> > - goto retry;
> > - }
> > + mutex_lock(&bdev_lock);
> > + ___wait_var_event(&whole->bd_claiming,
> > + (err = bd_may_claim(bdev, holder, hops)) != 0 || !whole->bd_claiming,
> > + TASK_UNINTERRUPTIBLE, 0, 0,
> > + mutex_unlock(&bdev_lock); schedule(); mutex_lock(&bdev_lock));
>
> That's not an improvement. Instead of nice, obvious, readable code,
> I now have to go look at a macro and manually substitute the
> parameters to work out what this abomination actually does.
Interesting - I thought the function as a whole was more readable this
way.
I agree that the ___wait_var_event macro isn't the best part.
Is your dislike simply that it isn't a macro that you are familar with,
or is there something specific that you don't like?
Suppose we could add a new macro so that it read:
wait_var_event_mutex(&whole->bd_claiming,
(err = bd_may_claim(bdev, holder, hops)) != 0 || !whole->bd_claiming,
&bdev_lock);
would that be less abominable?
Thanks,
NeilBrown
