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 e46e83e87600..734866ab5194 100644 --- a/fs/btrfs/qgroup.c +++ b/fs/btrfs/qgroup.c @@ -2249,6 +2249,7 @@ 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; @@ -2256,7 +2257,25 @@ 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;
@@ -2420,7 +2439,8 @@ 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; }