[PATCH 5.9 237/391] btrfs: drop the path before adding block group sysfs files

From: Greg Kroah-Hartman
Date: Tue Nov 03 2020 - 16:52:49 EST

From: Josef Bacik <josef@xxxxxxxxxxxxxx>

commit 7837fa88704a66257404bb14144c9e4ab631a28a upstream.

Dave reported a problem with my rwsem conversion patch where we got the
following lockdep splat:

======================================================
WARNING: possible circular locking dependency detected
5.9.0-default+ #1297 Not tainted
------------------------------------------------------
kswapd0/76 is trying to acquire lock:
ffff9d5d25df2530 (&delayed_node->mutex){+.+.}-{3:3}, at: __btrfs_release_delayed_node.part.0+0x3f/0x320 [btrfs]

but task is already holding lock:
ffffffffa40cbba0 (fs_reclaim){+.+.}-{0:0}, at: __fs_reclaim_acquire+0x5/0x30

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #4 (fs_reclaim){+.+.}-{0:0}:
__lock_acquire+0x582/0xac0
lock_acquire+0xca/0x430
fs_reclaim_acquire.part.0+0x25/0x30
kmem_cache_alloc+0x30/0x9c0
alloc_inode+0x81/0x90
iget_locked+0xcd/0x1a0
kernfs_get_inode+0x1b/0x130
kernfs_get_tree+0x136/0x210
sysfs_get_tree+0x1a/0x50
vfs_get_tree+0x1d/0xb0
path_mount+0x70f/0xa80
do_mount+0x75/0x90
__x64_sys_mount+0x8e/0xd0
do_syscall_64+0x2d/0x70
entry_SYSCALL_64_after_hwframe+0x44/0xa9

-> #3 (kernfs_mutex){+.+.}-{3:3}:
__lock_acquire+0x582/0xac0
lock_acquire+0xca/0x430
__mutex_lock+0xa0/0xaf0
kernfs_add_one+0x23/0x150
kernfs_create_dir_ns+0x58/0x80
sysfs_create_dir_ns+0x70/0xd0
kobject_add_internal+0xbb/0x2d0
kobject_add+0x7a/0xd0
btrfs_sysfs_add_block_group_type+0x141/0x1d0 [btrfs]
btrfs_read_block_groups+0x1f1/0x8c0 [btrfs]
open_ctree+0x981/0x1108 [btrfs]
btrfs_mount_root.cold+0xe/0xb0 [btrfs]
legacy_get_tree+0x2d/0x60
vfs_get_tree+0x1d/0xb0
fc_mount+0xe/0x40
vfs_kern_mount.part.0+0x71/0x90
btrfs_mount+0x13b/0x3e0 [btrfs]
legacy_get_tree+0x2d/0x60
vfs_get_tree+0x1d/0xb0
path_mount+0x70f/0xa80
do_mount+0x75/0x90
__x64_sys_mount+0x8e/0xd0
do_syscall_64+0x2d/0x70
entry_SYSCALL_64_after_hwframe+0x44/0xa9

-> #2 (btrfs-extent-00){++++}-{3:3}:
__lock_acquire+0x582/0xac0
lock_acquire+0xca/0x430
down_read_nested+0x45/0x220
__btrfs_tree_read_lock+0x35/0x1c0 [btrfs]
__btrfs_read_lock_root_node+0x3a/0x50 [btrfs]
btrfs_search_slot+0x6d4/0xfd0 [btrfs]
check_committed_ref+0x69/0x200 [btrfs]
btrfs_cross_ref_exist+0x65/0xb0 [btrfs]
run_delalloc_nocow+0x446/0x9b0 [btrfs]
btrfs_run_delalloc_range+0x61/0x6a0 [btrfs]
writepage_delalloc+0xae/0x160 [btrfs]
__extent_writepage+0x262/0x420 [btrfs]
extent_write_cache_pages+0x2b6/0x510 [btrfs]
extent_writepages+0x43/0x90 [btrfs]
do_writepages+0x40/0xe0
__writeback_single_inode+0x62/0x610
writeback_sb_inodes+0x20f/0x500
wb_writeback+0xef/0x4a0
wb_do_writeback+0x49/0x2e0
wb_workfn+0x81/0x340
process_one_work+0x233/0x5d0
worker_thread+0x50/0x3b0
kthread+0x137/0x150
ret_from_fork+0x1f/0x30

-> #1 (btrfs-fs-00){++++}-{3:3}:
__lock_acquire+0x582/0xac0
lock_acquire+0xca/0x430
down_read_nested+0x45/0x220
__btrfs_tree_read_lock+0x35/0x1c0 [btrfs]
__btrfs_read_lock_root_node+0x3a/0x50 [btrfs]
btrfs_search_slot+0x6d4/0xfd0 [btrfs]
btrfs_lookup_inode+0x3a/0xc0 [btrfs]
__btrfs_update_delayed_inode+0x93/0x2c0 [btrfs]
__btrfs_commit_inode_delayed_items+0x7de/0x850 [btrfs]
__btrfs_run_delayed_items+0x8e/0x140 [btrfs]
btrfs_commit_transaction+0x367/0xbc0 [btrfs]
btrfs_mksubvol+0x2db/0x470 [btrfs]
btrfs_mksnapshot+0x7b/0xb0 [btrfs]
__btrfs_ioctl_snap_create+0x16f/0x1a0 [btrfs]
btrfs_ioctl_snap_create_v2+0xb0/0xf0 [btrfs]
btrfs_ioctl+0xd0b/0x2690 [btrfs]
__x64_sys_ioctl+0x6f/0xa0
do_syscall_64+0x2d/0x70
entry_SYSCALL_64_after_hwframe+0x44/0xa9

-> #0 (&delayed_node->mutex){+.+.}-{3:3}:
check_prev_add+0x91/0xc60
validate_chain+0xa6e/0x2a20
__lock_acquire+0x582/0xac0
lock_acquire+0xca/0x430
__mutex_lock+0xa0/0xaf0
__btrfs_release_delayed_node.part.0+0x3f/0x320 [btrfs]
btrfs_evict_inode+0x3cc/0x560 [btrfs]
evict+0xd6/0x1c0
dispose_list+0x48/0x70
prune_icache_sb+0x54/0x80
super_cache_scan+0x121/0x1a0
do_shrink_slab+0x16d/0x3b0
shrink_slab+0xb1/0x2e0
shrink_node+0x230/0x6a0
balance_pgdat+0x325/0x750
kswapd+0x206/0x4d0
kthread+0x137/0x150
ret_from_fork+0x1f/0x30

other info that might help us debug this:

Chain exists of:
&delayed_node->mutex --> kernfs_mutex --> fs_reclaim

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(fs_reclaim);
lock(kernfs_mutex);
lock(fs_reclaim);
lock(&delayed_node->mutex);

*** DEADLOCK ***

3 locks held by kswapd0/76:
#0: ffffffffa40cbba0 (fs_reclaim){+.+.}-{0:0}, at: __fs_reclaim_acquire+0x5/0x30
#1: ffffffffa40b8b58 (shrinker_rwsem){++++}-{3:3}, at: shrink_slab+0x54/0x2e0
#2: ffff9d5d322390e8 (&type->s_umount_key#26){++++}-{3:3}, at: trylock_super+0x16/0x50

stack backtrace:
CPU: 2 PID: 76 Comm: kswapd0 Not tainted 5.9.0-default+ #1297
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-59-gc9ba527-rebuilt.opensuse.org 04/01/2014
Call Trace:
dump_stack+0x77/0x97
check_noncircular+0xff/0x110
? save_trace+0x50/0x470
check_prev_add+0x91/0xc60
validate_chain+0xa6e/0x2a20
? save_trace+0x50/0x470
__lock_acquire+0x582/0xac0
lock_acquire+0xca/0x430
? __btrfs_release_delayed_node.part.0+0x3f/0x320 [btrfs]
__mutex_lock+0xa0/0xaf0
? __btrfs_release_delayed_node.part.0+0x3f/0x320 [btrfs]
? __lock_acquire+0x582/0xac0
? __btrfs_release_delayed_node.part.0+0x3f/0x320 [btrfs]
? btrfs_evict_inode+0x30b/0x560 [btrfs]
? __btrfs_release_delayed_node.part.0+0x3f/0x320 [btrfs]
__btrfs_release_delayed_node.part.0+0x3f/0x320 [btrfs]
btrfs_evict_inode+0x3cc/0x560 [btrfs]
evict+0xd6/0x1c0
dispose_list+0x48/0x70
prune_icache_sb+0x54/0x80
super_cache_scan+0x121/0x1a0
do_shrink_slab+0x16d/0x3b0
shrink_slab+0xb1/0x2e0
shrink_node+0x230/0x6a0
balance_pgdat+0x325/0x750
kswapd+0x206/0x4d0
? finish_wait+0x90/0x90
? balance_pgdat+0x750/0x750
kthread+0x137/0x150
? kthread_mod_delayed_work+0xc0/0xc0
ret_from_fork+0x1f/0x30

This happens because we are still holding the path open when we start
adding the sysfs files for the block groups, which creates a dependency
on fs_reclaim via the tree lock. Fix this by dropping the path before
we start doing anything with sysfs.

Reported-by: David Sterba <dsterba@xxxxxxxx>
CC: stable@xxxxxxxxxxxxxxx # 5.8+
Reviewed-by: Anand Jain <anand.jain@xxxxxxxxxx>
Reviewed-by: Filipe Manana <fdmanana@xxxxxxxx>
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
fs/btrfs/block-group.c | 1 +
1 file changed, 1 insertion(+)

--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -2034,6 +2034,7 @@ int btrfs_read_block_groups(struct btrfs
key.offset = 0;
btrfs_release_path(path);
}
+ btrfs_release_path(path);

rcu_read_lock();
list_for_each_entry_rcu(space_info, &info->space_info, list) {