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>
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index c8a3e84..f259d89 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -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);