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

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Message-Id: <20240820114016.873-1-hdanton@sina.com>
Date: Tue, 20 Aug 2024 19:40:16 +0800
From: Hillf Danton <hdanton@...a.com>
To: NeilBrown <neilb@...e.de>
Cc: Dave Chinner <david@...morbit.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/9] Block: switch bd_prepare_to_claim to use ___wait_var_event()

On Mon, 19 Aug 2024 15:20:39 +1000 NeilBrown <neilb@...e.de>
> 
> @@ -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));
>  
At the first glance you add the coding pattern not recommended for the block
directory. Second, you abuse ___wait_var_event() simply because it is available.

> -	/* yay, all mine */
> -	whole->bd_claiming = holder;
> +	/* if someone else claimed, fail */
> +	if (!err)
> +		/* yay, all mine */
> +		whole->bd_claiming = holder;
>  	mutex_unlock(&bdev_lock);
> -	return 0;
> +	return err;
>  }
>  EXPORT_SYMBOL_GPL(bd_prepare_to_claim); /* only for the loop driver */
>  
> @@ -571,7 +561,8 @@ static void bd_clear_claiming(struct block_device *whole, void *holder)
>  	/* tell others that we're done */
>  	BUG_ON(whole->bd_claiming != holder);
>  	whole->bd_claiming = NULL;
> -	wake_up_bit(&whole->bd_claiming, 0);
> +	smp_mb();
> +	wake_up_var(&whole->bd_claiming);

Third, worse, you have no real idea why mb is needed.
>  }
>  
>  /**
> -- 
> 2.44.0