[PATCH] Btrfs: fix deadlock when enabling quotas due to concurrent snapshot creation

[fdmanana@xxxxxxxxxx]

From: Filipe Manana <fdmanana@xxxxxxxx>

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. The following time diagram shows how this happens.

           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()

So fix this by adding a flag to the transaction handle that signals if the
transaction is being used for enabling quotas (only seen by the task doing
it) and do not lock the mutex qgroup_ioctl_lock at btrfs_qgroup_inherit()
if the transaction handle corresponds to the one being used to enable the
quotas.

Fixes: 6426c7ad697d ("btrfs: qgroup: Fix qgroup accounting when creating snapshot")
Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
---
 fs/btrfs/qgroup.c      | 10 ++++++++--
 fs/btrfs/transaction.h |  1 +
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index d4917c0cddf5..3aec3bfa3d70 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -908,6 +908,7 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
 		trans = NULL;
 		goto out;
 	}
+	trans->enabling_quotas = true;
 
 	fs_info->qgroup_ulist = ulist_alloc(GFP_KERNEL);
 	if (!fs_info->qgroup_ulist) {
@@ -2250,7 +2251,11 @@ 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 (trans->enabling_quotas)
+		lockdep_assert_held(&fs_info->qgroup_ioctl_lock);
+	else
+		mutex_lock(&fs_info->qgroup_ioctl_lock);
+
 	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags))
 		goto out;
 
@@ -2413,7 +2418,8 @@ 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 (!trans->enabling_quotas)
+		mutex_unlock(&fs_info->qgroup_ioctl_lock);
 	return ret;
 }
 
diff --git a/fs/btrfs/transaction.h b/fs/btrfs/transaction.h
index 703d5116a2fc..a5553a1dee30 100644
--- a/fs/btrfs/transaction.h
+++ b/fs/btrfs/transaction.h
@@ -122,6 +122,7 @@ struct btrfs_trans_handle {
 	bool reloc_reserved;
 	bool sync;
 	bool dirty;
+	bool enabling_quotas;
 	struct btrfs_root *root;
 	struct btrfs_fs_info *fs_info;
 	struct list_head new_bgs;
-- 
2.11.0