linux-kernel - Re: [PATCH 5/9] Block: switch bd_prepare_to_claim to use ___wait_var

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Message-id: <172419075958.6062.14405334545688254538@noble.neil.brown.name>
Date: Wed, 21 Aug 2024 07:52:39 +1000
From: "NeilBrown" <neilb@...e.de>
To: "Dave Chinner" <david@...morbit.com>
Cc: "Ingo Molnar" <mingo@...hat.com>, "Peter Zijlstra" <peterz@...radead.org>,
 "Linus Torvalds" <torvalds@...ux-foundation.org>,
 linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject:
 Re: [PATCH 5/9] Block: switch bd_prepare_to_claim to use ___wait_var_event()

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