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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: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Liu Bo 2017-11-01 17:19:27 -06:00 committed by David Sterba
parent 057aac3e62
commit 4c274bc67b
1 changed files with 16 additions and 1 deletions
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17
fs/btrfs/inode.c
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@ -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);