Re: [PATCH 06/10] btrfs: transfer the bio counter reference to the raid submission helpers

[Christoph Hellwig <hch@lst.de>]

On Sun, Jun 19, 2022 at 06:45:11PM +0800, Qu Wenruo wrote:
>> Transfer the bio counter reference acquired by btrfs_submit_bio to
>> raid56_parity_write and raid56_parity_recovery together with the bio that
>> the reference was acuired for instead of acquiring another reference in
>> those helpers and dropping the original one in btrfs_submit_bio.
>
> Btrfs_submit_bio() has called btrfs_bio_counter_inc_blocked(), then call
> btrfs_bio_counter_dec() in its out_dec: tag.
>
> Thus the bio counter is already paired.
>
> Then why we want to dec the counter again in RAID56 path?
>
> Or did I miss some patches in the past modifying the behavior?

The behviour before this patch is:

btrfs_submit_bio does:

	btrfs_bio_counter_inc_blocked
	call raid56_parity_write / raid56_parity_recover
	btrfs_bio_counter_dec

raid56_parity_write / raid56_parity_recover then do:

	btrfs_bio_counter_inc_noblocked
	btrfs_bio_counter_dec on error only

The new behavior is:

btrfs_submit_bio does:

	btrfs_bio_counter_inc_blocked
	call raid56_parity_write / raid56_parity_recover
	return

raid56_parity_write / raid56_parity_recover then do:

	btrfs_bio_counter_dec on error only

so no change in the final number of reference, but on less inc/dec
pair for the successful submission fast path.