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Commit 4c274bc6 authored by Liu Bo's avatar Liu Bo Committed by David Sterba
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Btrfs: document rules about bio async submit 



These rules have been hidden in several if-else and are not
straightforward to follow, for example, dio submit hook's nocsum case
has a bug , i.e. doing async submit instead of sync submit, which has
been fixed recently.

This is documenting the rules for reference.

Signed-off-by: default avatarLiu Bo <bo.li.liu@oracle.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 057aac3e

fs/btrfs/inode.c

+16 −1
Original line number Diff line number Diff line
@@ -1951,7 +1951,21 @@ static blk_status_t __btrfs_submit_bio_done(void *private_data, struct bio *bio, 


/*

 * extent_io.c submission hook. This does the right thing for csum calculation

 * on write, or reading the csums from the tree before a read

 * on write, or reading the csums from the tree before a read.

 *

 * Rules about async/sync submit,

 * a) read:				sync submit

 *

 * b) write without checksum:		sync submit

 *

 * c) write with checksum:

 *    c-1) if bio is issued by fsync:	sync submit

 *         (sync_writers != 0)

 *

 *    c-2) if root is reloc root:	sync submit

 *         (only in case of buffered IO)

 *

 *    c-3) otherwise:			async submit

 */

static blk_status_t btrfs_submit_bio_hook(void *private_data, struct bio *bio,

				 int mirror_num, unsigned long bio_flags,
@@ -8456,6 +8470,7 @@ __btrfs_submit_dio_bio(struct bio *bio, struct inode *inode, u64 file_offset, 
	bool write = bio_op(bio) == REQ_OP_WRITE;

	blk_status_t ret;



	/* Check btrfs_submit_bio_hook() for rules about async submit. */

	if (async_submit)

		async_submit = !atomic_read(&BTRFS_I(inode)->sync_writers);