[PATCH 4/5] btrfs: do not resolve backrefs for roots that are being deleted

Josef Bacik
2020-03-20T18:34:52Z

Zygo reported a deadlock where a task was stuck in the inode logical
resolve code. The deadlock looks like this

Task 1
btrfs_ioctl_logical_to_ino
->iterate_inodes_from_logical
->iterate_extent_inodes
->path->search_commit_root isn't set, so a transaction is started
->resolve_indirect_ref for a root that's being deleted
->search for our key, attempt to lock a node, DEADLOCK

Task 2
btrfs_drop_snapshot
->walk down to a leaf, lock it, walk up, lock node
->end transaction
->start transaction
-> wait_cur_trans

Task 3
btrfs_commit_transaction
->wait_event(cur_trans->write_wait, num_writers == 1) DEADLOCK

We are holding a transaction open in btrfs_ioctl_logical_to_ino while we
try to resolve our references. btrfs_drop_snapshot() holds onto its
locks while it stops and starts transaction handles, because it assumes
nobody is going to touch the root now. Commit just does what commit
does, waiting for the writers to finish, blocking any new trans handles
from starting.

Fix this by making the backref code not try to resolve backrefs of roots
that are currently being deleted. This will keep us from walking into a
snapshot that's currently being deleted.

This problem was harder to hit before because we rarely broke out of the
snapshot delete halfway through, but with my delayed ref throttling code
it happened much more often. However we've always been able to do this,
so it's not a new problem.

Fixes: 8da6d5815c59 ("Btrfs: added btrfs_find_all_roots()")
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
fs/btrfs/backref.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index 9d0f87df2c35..0dcc11644be4 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -523,6 +523,12 @@ static int resolve_indirect_ref(struct btrfs_fs_info *fs_info,
goto out_free;
}

+ if (!path->search_commit_root &&
+ test_bit(BTRFS_ROOT_DELETING, &root->state)) {
+ ret = -ENOENT;
+ goto out;
+ }
+
if (btrfs_is_testing(fs_info)) {
ret = -ENOENT;
goto out;
--
2.24.1