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

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Message-ID: <ZsQZHZ0y6qMJGaLQ@dread.disaster.area>
Date: Tue, 20 Aug 2024 14:18:37 +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 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.

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