[PATCH 01/17] btrfs: drop path before adding new uuid tree entry

[Josef Bacik <josef@toxicpanda.com>]

With the conversion to the rwsemaphore I got the following lockdep splat

======================================================
WARNING: possible circular locking dependency detected
5.8.0-rc7-00167-g0d7ba0c5b375-dirty #925 Not tainted
------------------------------------------------------
btrfs-uuid/7955 is trying to acquire lock:
ffff88bfbafec0f8 (btrfs-root-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x39/0x180

but task is already holding lock:
ffff88bfbafef2a8 (btrfs-uuid-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x39/0x180

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (btrfs-uuid-00){++++}-{3:3}:
       down_read_nested+0x3e/0x140
       __btrfs_tree_read_lock+0x39/0x180
       __btrfs_read_lock_root_node+0x3a/0x50
       btrfs_search_slot+0x4bd/0x990
       btrfs_uuid_tree_add+0x89/0x2d0
       btrfs_uuid_scan_kthread+0x330/0x390
       kthread+0x133/0x150
       ret_from_fork+0x1f/0x30

-> #0 (btrfs-root-00){++++}-{3:3}:
       __lock_acquire+0x1272/0x2310
       lock_acquire+0x9e/0x360
       down_read_nested+0x3e/0x140
       __btrfs_tree_read_lock+0x39/0x180
       __btrfs_read_lock_root_node+0x3a/0x50
       btrfs_search_slot+0x4bd/0x990
       btrfs_find_root+0x45/0x1b0
       btrfs_read_tree_root+0x61/0x100
       btrfs_get_root_ref.part.50+0x143/0x630
       btrfs_uuid_tree_iterate+0x207/0x314
       btrfs_uuid_rescan_kthread+0x12/0x50
       kthread+0x133/0x150
       ret_from_fork+0x1f/0x30

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(btrfs-uuid-00);
                               lock(btrfs-root-00);
                               lock(btrfs-uuid-00);
  lock(btrfs-root-00);

 *** DEADLOCK ***

1 lock held by btrfs-uuid/7955:
 #0: ffff88bfbafef2a8 (btrfs-uuid-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x39/0x180

stack backtrace:
CPU: 73 PID: 7955 Comm: btrfs-uuid Kdump: loaded Not tainted 5.8.0-rc7-00167-g0d7ba0c5b375-dirty #925
Hardware name: Quanta Tioga Pass Single Side 01-0030993006/Tioga Pass Single Side, BIOS F08_3A18 12/20/2018
Call Trace:
 dump_stack+0x78/0xa0
 check_noncircular+0x165/0x180
 __lock_acquire+0x1272/0x2310
 lock_acquire+0x9e/0x360
 ? __btrfs_tree_read_lock+0x39/0x180
 ? btrfs_root_node+0x1c/0x1d0
 down_read_nested+0x3e/0x140
 ? __btrfs_tree_read_lock+0x39/0x180
 __btrfs_tree_read_lock+0x39/0x180
 __btrfs_read_lock_root_node+0x3a/0x50
 btrfs_search_slot+0x4bd/0x990
 btrfs_find_root+0x45/0x1b0
 btrfs_read_tree_root+0x61/0x100
 btrfs_get_root_ref.part.50+0x143/0x630
 btrfs_uuid_tree_iterate+0x207/0x314
 ? btree_readpage+0x20/0x20
 btrfs_uuid_rescan_kthread+0x12/0x50
 kthread+0x133/0x150
 ? kthread_create_on_node+0x60/0x60
 ret_from_fork+0x1f/0x30

This problem exists because we have two different rescan threads,
btrfs_uuid_scan_kthread which creates the uuid tree, and
btrfs_uuid_tree_iterate that goes through and updates or deletes any out
of date roots.  The problem is they both do things in different order.
btrfs_uuid_scan_kthread() reads the tree_root, and then inserts entries
into the uuid_root.  btrfs_uuid_tree_iterate() scans the uuid_root, but
then does a btrfs_get_fs_root() which can read from the tree_root.

It's actually easy enough to not be holding the path in
btrfs_uuid_scan_kthread() when we add a uuid entry, as we already drop
it further down and re-start the search when we loop.  So simply move
the path release before we add our entry to the uuid tree.

This also fixes a problem where we're holding a path open after we do
btrfs_end_transaction(), which has it's own problems.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/volumes.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index d7670e2a9f39..3ac44dad58bb 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4462,6 +4462,7 @@ int btrfs_uuid_scan_kthread(void *data)
 			goto skip;
 		}
 update_tree:
+		btrfs_release_path(path);
 		if (!btrfs_is_empty_uuid(root_item.uuid)) {
 			ret = btrfs_uuid_tree_add(trans, root_item.uuid,
 						  BTRFS_UUID_KEY_SUBVOL,
@@ -4486,6 +4487,7 @@ int btrfs_uuid_scan_kthread(void *data)
 		}
 
 skip:
+		btrfs_release_path(path);
 		if (trans) {
 			ret = btrfs_end_transaction(trans);
 			trans = NULL;
@@ -4493,7 +4495,6 @@ int btrfs_uuid_scan_kthread(void *data)
 				break;
 		}
 
-		btrfs_release_path(path);
 		if (key.offset < (u64)-1) {
 			key.offset++;
 		} else if (key.type < BTRFS_ROOT_ITEM_KEY) {
-- 
2.24.1