[PATCH AUTOSEL 4.20 074/117] Btrfs: fix deadlock when enabling quotas due to concurrent snapshot creation

Sasha Levin
2019-01-08T20:03:31Z

From: Filipe Manana <fdmanana@suse.com>

[ Upstream commit 9a6f209e36500efac51528132a3e3083586eda5f ]

If the quota enable and snapshot creation ioctls are called concurrently
we can get into a deadlock where the task enabling quotas will deadlock
on the fs_info->qgroup_ioctl_lock mutex because it attempts to lock it
twice, or the task creating a snapshot tries to commit the transaction
while the task enabling quota waits for the former task to commit the
transaction while holding the mutex. The following time diagrams show how
both cases happen.

First scenario:

CPU 0 CPU 1

btrfs_ioctl()
btrfs_ioctl_quota_ctl()
btrfs_quota_enable()
mutex_lock(fs_info->qgroup_ioctl_lock)
btrfs_start_transaction()

btrfs_ioctl()
btrfs_ioctl_snap_create_v2
create_snapshot()
--> adds snapshot to the
list pending_snapshots
of the current
transaction

btrfs_commit_transaction()
create_pending_snapshots()
create_pending_snapshot()
qgroup_account_snapshot()
btrfs_qgroup_inherit()
mutex_lock(fs_info->qgroup_ioctl_lock)
--> deadlock, mutex already locked
by this task at
btrfs_quota_enable()

Second scenario:

CPU 0 CPU 1

btrfs_ioctl()
btrfs_ioctl_quota_ctl()
btrfs_quota_enable()
mutex_lock(fs_info->qgroup_ioctl_lock)
btrfs_start_transaction()

btrfs_ioctl()
btrfs_ioctl_snap_create_v2
create_snapshot()
--> adds snapshot to the
list pending_snapshots
of the current
transaction

btrfs_commit_transaction()
--> waits for task at
CPU 0 to release
its transaction
handle

btrfs_commit_transaction()
--> sees another task started
the transaction commit first
--> releases its transaction
handle
--> waits for the transaction
commit to be completed by
the task at CPU 1

create_pending_snapshot()
qgroup_account_snapshot()
btrfs_qgroup_inherit()
mutex_lock(fs_info->qgroup_ioctl_lock)
--> deadlock, task at CPU 0
has the mutex locked but
it is waiting for us to
finish the transaction
commit

So fix this by setting the quota enabled flag in fs_info after committing
the transaction at btrfs_quota_enable(). This ends up serializing quota
enable and snapshot creation as if the snapshot creation happened just
before the quota enable request. The quota rescan task, scheduled after
committing the transaction in btrfs_quote_enable(), will do the accounting.

Fixes: 6426c7ad697d ("btrfs: qgroup: Fix qgroup accounting when creating snapshot")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/btrfs/qgroup.c | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index f70825af6438..9e419f6878c5 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -1013,16 +1013,22 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
btrfs_abort_transaction(trans, ret);
goto out_free_path;
}
- spin_lock(&fs_info->qgroup_lock);
- fs_info->quota_root = quota_root;
- set_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags);
- spin_unlock(&fs_info->qgroup_lock);

ret = btrfs_commit_transaction(trans);
trans = NULL;
if (ret)
goto out_free_path;

+ /*
+ * Set quota enabled flag after committing the transaction, to avoid
+ * deadlocks on fs_info->qgroup_ioctl_lock with concurrent snapshot
+ * creation.
+ */
+ spin_lock(&fs_info->qgroup_lock);
+ fs_info->quota_root = quota_root;
+ set_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags);
+ spin_unlock(&fs_info->qgroup_lock);
+
ret = qgroup_rescan_init(fs_info, 0, 1);
if (!ret) {
qgroup_rescan_zero_tracking(fs_info);
--
2.19.1