[PATCH 5.4 08/86] btrfs: fix lockdep splat in btrfs_recover_relocation

Greg Kroah-Hartman
2021-01-26T08:47:07Z

From: Josef Bacik <josef@toxicpanda.com>

commit fb286100974e7239af243bc2255a52f29442f9c8 upstream.

While testing the error paths of relocation I hit the following lockdep
splat:

======================================================
WARNING: possible circular locking dependency detected
5.10.0-rc6+ #217 Not tainted
------------------------------------------------------
mount/779 is trying to acquire lock:
ffffa0e676945418 (&fs_info->balance_mutex){+.+.}-{3:3}, at: btrfs_recover_balance+0x2f0/0x340

but task is already holding lock:
ffffa0e60ee31da8 (btrfs-root-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x27/0x100

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (btrfs-root-00){++++}-{3:3}:
down_read_nested+0x43/0x130
__btrfs_tree_read_lock+0x27/0x100
btrfs_read_lock_root_node+0x31/0x40
btrfs_search_slot+0x462/0x8f0
btrfs_update_root+0x55/0x2b0
btrfs_drop_snapshot+0x398/0x750
clean_dirty_subvols+0xdf/0x120
btrfs_recover_relocation+0x534/0x5a0
btrfs_start_pre_rw_mount+0xcb/0x170
open_ctree+0x151f/0x1726
btrfs_mount_root.cold+0x12/0xea
legacy_get_tree+0x30/0x50
vfs_get_tree+0x28/0xc0
vfs_kern_mount.part.0+0x71/0xb0
btrfs_mount+0x10d/0x380
legacy_get_tree+0x30/0x50
vfs_get_tree+0x28/0xc0
path_mount+0x433/0xc10
__x64_sys_mount+0xe3/0x120
do_syscall_64+0x33/0x40
entry_SYSCALL_64_after_hwframe+0x44/0xa9

-> #1 (sb_internal#2){.+.+}-{0:0}:
start_transaction+0x444/0x700
insert_balance_item.isra.0+0x37/0x320
btrfs_balance+0x354/0xf40
btrfs_ioctl_balance+0x2cf/0x380
__x64_sys_ioctl+0x83/0xb0
do_syscall_64+0x33/0x40
entry_SYSCALL_64_after_hwframe+0x44/0xa9

-> #0 (&fs_info->balance_mutex){+.+.}-{3:3}:
__lock_acquire+0x1120/0x1e10
lock_acquire+0x116/0x370
__mutex_lock+0x7e/0x7b0
btrfs_recover_balance+0x2f0/0x340
open_ctree+0x1095/0x1726
btrfs_mount_root.cold+0x12/0xea
legacy_get_tree+0x30/0x50
vfs_get_tree+0x28/0xc0
vfs_kern_mount.part.0+0x71/0xb0
btrfs_mount+0x10d/0x380
legacy_get_tree+0x30/0x50
vfs_get_tree+0x28/0xc0
path_mount+0x433/0xc10
__x64_sys_mount+0xe3/0x120
do_syscall_64+0x33/0x40
entry_SYSCALL_64_after_hwframe+0x44/0xa9

other info that might help us debug this:

Chain exists of:
&fs_info->balance_mutex --> sb_internal#2 --> btrfs-root-00

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(btrfs-root-00);
lock(sb_internal#2);
lock(btrfs-root-00);
lock(&fs_info->balance_mutex);

*** DEADLOCK ***

2 locks held by mount/779:
#0: ffffa0e60dc040e0 (&type->s_umount_key#47/1){+.+.}-{3:3}, at: alloc_super+0xb5/0x380
#1: ffffa0e60ee31da8 (btrfs-root-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x27/0x100

stack backtrace:
CPU: 0 PID: 779 Comm: mount Not tainted 5.10.0-rc6+ #217
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014
Call Trace:
dump_stack+0x8b/0xb0
check_noncircular+0xcf/0xf0
? trace_call_bpf+0x139/0x260
__lock_acquire+0x1120/0x1e10
lock_acquire+0x116/0x370
? btrfs_recover_balance+0x2f0/0x340
__mutex_lock+0x7e/0x7b0
? btrfs_recover_balance+0x2f0/0x340
? btrfs_recover_balance+0x2f0/0x340
? rcu_read_lock_sched_held+0x3f/0x80
? kmem_cache_alloc_trace+0x2c4/0x2f0
? btrfs_get_64+0x5e/0x100
btrfs_recover_balance+0x2f0/0x340
open_ctree+0x1095/0x1726
btrfs_mount_root.cold+0x12/0xea
? rcu_read_lock_sched_held+0x3f/0x80
legacy_get_tree+0x30/0x50
vfs_get_tree+0x28/0xc0
vfs_kern_mount.part.0+0x71/0xb0
btrfs_mount+0x10d/0x380
? __kmalloc_track_caller+0x2f2/0x320
legacy_get_tree+0x30/0x50
vfs_get_tree+0x28/0xc0
? capable+0x3a/0x60
path_mount+0x433/0xc10
__x64_sys_mount+0xe3/0x120
do_syscall_64+0x33/0x40
entry_SYSCALL_64_after_hwframe+0x44/0xa9

This is straightforward to fix, simply release the path before we setup
the balance_ctl.

CC: stable@vger.kernel.org # 4.4+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/btrfs/volumes.c | 2 ++
1 file changed, 2 insertions(+)

--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4409,6 +4409,8 @@ int btrfs_recover_balance(struct btrfs_f
btrfs_warn(fs_info,
"balance: cannot set exclusive op status, resume manually");

+ btrfs_release_path(path);
+
mutex_lock(&fs_info->balance_mutex);
BUG_ON(fs_info->balance_ctl);
spin_lock(&fs_info->balance_lock);