[BUG]
Lockdep will report the following circular locking dependency:
WARNING: possible circular locking dependency detected
5.2.0-rc2-custom #24 Tainted: G O
------------------------------------------------------
btrfs/8631 is trying to acquire lock:
000000002536438c (&fs_info->qgroup_ioctl_lock#2){+.+.}, at: btrfs_qgroup_inherit+0x40/0x620 [btrfs]
but task is already holding lock:
000000003d52cc23 (&fs_info->tree_log_mutex){+.+.}, at: create_pending_snapshot+0x8b6/0xe60 [btrfs]
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 (&fs_info->tree_log_mutex){+.+.}:
__mutex_lock+0x76/0x940
mutex_lock_nested+0x1b/0x20
btrfs_commit_transaction+0x475/0xa00 [btrfs]
btrfs_commit_super+0x71/0x80 [btrfs]
close_ctree+0x2bd/0x320 [btrfs]
btrfs_put_super+0x15/0x20 [btrfs]
generic_shutdown_super+0x72/0x110
kill_anon_super+0x18/0x30
btrfs_kill_super+0x16/0xa0 [btrfs]
deactivate_locked_super+0x3a/0x80
deactivate_super+0x51/0x60
cleanup_mnt+0x3f/0x80
__cleanup_mnt+0x12/0x20
task_work_run+0x94/0xb0
exit_to_usermode_loop+0xd8/0xe0
do_syscall_64+0x210/0x240
entry_SYSCALL_64_after_hwframe+0x49/0xbe
-> #1 (&fs_info->reloc_mutex){+.+.}:
__mutex_lock+0x76/0x940
mutex_lock_nested+0x1b/0x20
btrfs_commit_transaction+0x40d/0xa00 [btrfs]
btrfs_quota_enable+0x2da/0x730 [btrfs]
btrfs_ioctl+0x2691/0x2b40 [btrfs]
do_vfs_ioctl+0xa9/0x6d0
ksys_ioctl+0x67/0x90
__x64_sys_ioctl+0x1a/0x20
do_syscall_64+0x65/0x240
entry_SYSCALL_64_after_hwframe+0x49/0xbe
-> #0 (&fs_info->qgroup_ioctl_lock#2){+.+.}:
lock_acquire+0xa7/0x190
__mutex_lock+0x76/0x940
mutex_lock_nested+0x1b/0x20
btrfs_qgroup_inherit+0x40/0x620 [btrfs]
create_pending_snapshot+0x9d7/0xe60 [btrfs]
create_pending_snapshots+0x94/0xb0 [btrfs]
btrfs_commit_transaction+0x415/0xa00 [btrfs]
btrfs_mksubvol+0x496/0x4e0 [btrfs]
btrfs_ioctl_snap_create_transid+0x174/0x180 [btrfs]
btrfs_ioctl_snap_create_v2+0x11c/0x180 [btrfs]
btrfs_ioctl+0xa90/0x2b40 [btrfs]
do_vfs_ioctl+0xa9/0x6d0
ksys_ioctl+0x67/0x90
__x64_sys_ioctl+0x1a/0x20
do_syscall_64+0x65/0x240
entry_SYSCALL_64_after_hwframe+0x49/0xbe
other info that might help us debug this:
Chain exists of:
&fs_info->qgroup_ioctl_lock#2 --> &fs_info->reloc_mutex --> &fs_info->tree_log_mutex
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&fs_info->tree_log_mutex);
lock(&fs_info->reloc_mutex);
lock(&fs_info->tree_log_mutex);
lock(&fs_info->qgroup_ioctl_lock#2);
*** DEADLOCK ***
6 locks held by btrfs/8631:
#0: 00000000ed8f23f6 (sb_writers#12){.+.+}, at: mnt_want_write_file+0x28/0x60
#1: 000000009fb1597a (&type->i_mutex_dir_key#10/1){+.+.}, at: btrfs_mksubvol+0x70/0x4e0 [btrfs]
#2: 0000000088c5ad88 (&fs_info->subvol_sem){++++}, at: btrfs_mksubvol+0x128/0x4e0 [btrfs]
#3: 000000009606fc3e (sb_internal#2){.+.+}, at: start_transaction+0x37a/0x520 [btrfs]
#4: 00000000f82bbdf5 (&fs_info->reloc_mutex){+.+.}, at: btrfs_commit_transaction+0x40d/0xa00 [btrfs]
#5: 000000003d52cc23 (&fs_info->tree_log_mutex){+.+.}, at: create_pending_snapshot+0x8b6/0xe60 [btrfs]
[CAUSE]
Due to the delayed subvolume creation, we need to call
btrfs_qgroup_inherit() inside commit transaction code, with a lot of
other mutex hold.
This hell of lock chain can lead to above problem.
[FIX]
On the other hand, we don't really need to hold qgroup_ioctl_lock if
we're in the context of create_pending_snapshot().
As in that context, we're the only one being able to modify qgroup.
All other qgroup functions which needs qgroup_ioctl_lock are either
holding a transaction handle, or will start a new transaction:
Functions will start a new transaction():
* btrfs_quota_enable()
* btrfs_quota_disable()
Functions hold a transaction handler:
* btrfs_add_qgroup_relation()
* btrfs_del_qgroup_relation()
* btrfs_create_qgroup()
* btrfs_remove_qgroup()
* btrfs_limit_qgroup()
* btrfs_qgroup_inherit() call inside create_subvol()
So we have a higher level protection provided by transaction, thus we
don't need to always hold qgroup_ioctl_lock in btrfs_qgroup_inherit().
Only the btrfs_qgroup_inherit() call in create_subvol() needs to hold
qgroup_ioctl_lock, while the btrfs_qgroup_inherit() call in
create_pending_snapshot() is already protected by transaction.
So the fix is to manually hold qgroup_ioctl_lock inside create_subvol()
while skip the lock inside create_pending_snapshot.
Reported-by: Nikolay Borisov <nborisov@xxxxxxxx>
Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
---
fs/btrfs/ioctl.c | 2 ++
fs/btrfs/qgroup.c | 9 +++++++--
2 files changed, 9 insertions(+), 2 deletions(-)
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 6dafa857bbb9..5a526f38b446 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -618,7 +618,9 @@ static noinline int create_subvol(struct inode *dir,
trans->block_rsv = &block_rsv;
trans->bytes_reserved = block_rsv.size;
+ mutex_lock(&fs_info->qgroup_ioctl_lock);
ret = btrfs_qgroup_inherit(trans, 0, objectid, inherit);
+ mutex_unlock(&fs_info->qgroup_ioctl_lock);
if (ret)
goto fail;
diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index 3e6ffbbd8b0a..26485a73911a 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -2607,6 +2607,13 @@ int btrfs_run_qgroups(struct btrfs_trans_handle *trans)
* when a snapshot or a subvolume is created. Throwing an error will
* cause a transaction abort so we take extra care here to only error
* when a readonly fs is a reasonable outcome.
+ *
+ * NOTE: Caller outside of commit transaction code should hold
+ * qgroup_ioctl_lock.
+ * For caller inside commit transaction, all other qgroup code will be
+ * blocked by transaction, thus no need to hold that mutex. This will
+ * avoid complex lock chain in commit transaction context and make lockdep
+ * happier.
*/
int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
u64 objectid, struct btrfs_qgroup_inherit *inherit)
@@ -2621,7 +2628,6 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
u32 level_size = 0;
u64 nums;
- mutex_lock(&fs_info->qgroup_ioctl_lock);
if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags))
goto out;
@@ -2785,7 +2791,6 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
unlock:
spin_unlock(&fs_info->qgroup_lock);
out:
- mutex_unlock(&fs_info->qgroup_ioctl_lock);
return ret;
}
--
2.22.0
