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

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Message-ID: <Zs575QSPazeJRzAy@dread.disaster.area>
Date: Wed, 28 Aug 2024 11:22:45 +1000
From: Dave Chinner <david@...morbit.com>
To: NeilBrown <neilb@...e.de>
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 Wed, Aug 21, 2024 at 07:52:39AM +1000, NeilBrown wrote:
> 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?

It's the encoding of non-trivial logic and code into the macro
parameters that is the problem....

> 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);

.... and this still does it. 

In fact, it's worse, because now I have -zero idea- of what locking
is being performed in this case, and so now I definitely have to go
pull that macro apart to understand what this is actually doing.

Complex macros don't make understanding the code easier - they may
make writing the code faster, but that comes at the expense of
clarity and obviousness of the logic flow of the code...

-Dave.

-- 
Dave Chinner
david@...morbit.com