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Subject[PATCH AUTOSEL 5.2 30/85] btrfs: qgroup: Don't hold qgroup_ioctl_lock in btrfs_qgroup_inherit()
Date
From: Qu Wenruo <wqu@suse.com>

[ Upstream commit e88439debd0a7f969b3ddba6f147152cd0732676 ]

[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 detect the context by checking
trans->transaction->state.
If we're at TRANS_STATE_COMMIT_DOING, then we're in commit transaction
context and no need to get the mutex.

Reported-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/btrfs/qgroup.c | 24 ++++++++++++++++++++++--
1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index 3e6ffbbd8b0a..f8a3c1b0a15a 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -2614,6 +2614,7 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
int ret = 0;
int i;
u64 *i_qgroups;
+ bool committing = false;
struct btrfs_fs_info *fs_info = trans->fs_info;
struct btrfs_root *quota_root;
struct btrfs_qgroup *srcgroup;
@@ -2621,7 +2622,25 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
u32 level_size = 0;
u64 nums;

- mutex_lock(&fs_info->qgroup_ioctl_lock);
+ /*
+ * There are only two callers of this function.
+ *
+ * One in create_subvol() in the ioctl context, which needs to hold
+ * the qgroup_ioctl_lock.
+ *
+ * The other one in create_pending_snapshot() where no other qgroup
+ * code can modify the fs as they all need to either start a new trans
+ * or hold a trans handler, thus we don't need to hold
+ * qgroup_ioctl_lock.
+ * This would avoid long and complex lock chain and make lockdep happy.
+ */
+ spin_lock(&fs_info->trans_lock);
+ if (trans->transaction->state == TRANS_STATE_COMMIT_DOING)
+ committing = true;
+ spin_unlock(&fs_info->trans_lock);
+
+ if (!committing)
+ mutex_lock(&fs_info->qgroup_ioctl_lock);
if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags))
goto out;

@@ -2785,7 +2804,8 @@ 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);
+ if (!committing)
+ mutex_unlock(&fs_info->qgroup_ioctl_lock);
return ret;
}

--
2.20.1
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