Commit 9e769bd7 authored by Josef Bacik's avatar Josef Bacik Committed by David Sterba
Browse files

btrfs: unlock locked extent area if we have contention 



In production we hit the following deadlock

task 1			task 2			task 3
------			------			------
fiemap(file)		falloc(file)		fsync(file)
						  write(0, 1MiB)
						  btrfs_commit_transaction()
						    wait_on(!pending_ordered)
			  lock(512MiB, 1GiB)
			  start_transaction
			    wait_on_transaction

  lock(0, 1GiB)
    wait_extent_bit(512MiB)

task 4
------
finish_ordered_extent(0, 1MiB)
  lock(0, 1MiB)
  **DEADLOCK**

This occurs because when task 1 does it's lock, it locks everything from
0-512MiB, and then waits for the 512MiB chunk to unlock.  task 2 will
never unlock because it's waiting on the transaction commit to happen,
the transaction commit is waiting for the outstanding ordered extents,
and then the ordered extent thread is blocked waiting on the 0-1MiB
range to unlock.

To fix this we have to clear anything we've locked so far, wait for the
extent_state that we contended on, and then try to re-lock the entire
range again.

CC: stable@vger.kernel.org # 5.15+
Reviewed-by: default avatarFilipe Manana <fdmanana@suse.com>
Signed-off-by: default avatarJosef Bacik <josef@toxicpanda.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent c86eab81

fs/btrfs/extent-io-tree.c

+8 −7
Original line number Diff line number Diff line
@@ -1641,16 +1641,17 @@ int lock_extent(struct extent_io_tree *tree, u64 start, u64 end, 
	int err;

	u64 failed_start;



	while (1) {

	err = __set_extent_bit(tree, start, end, EXTENT_LOCKED, &failed_start,

			       cached_state, NULL, GFP_NOFS);

	while (err == -EEXIST) {

		if (failed_start != start)

			clear_extent_bit(tree, start, failed_start - 1,

					 EXTENT_LOCKED, cached_state);



		wait_extent_bit(tree, failed_start, end, EXTENT_LOCKED);

		err = __set_extent_bit(tree, start, end, EXTENT_LOCKED,

				       &failed_start, cached_state, NULL,

				       GFP_NOFS);

		if (err == -EEXIST) {

			wait_extent_bit(tree, failed_start, end, EXTENT_LOCKED);

			start = failed_start;

		} else

			break;

		WARN_ON(start > end);

	}

	return err;

}