Commit ae5e070e authored by Qu Wenruo's avatar Qu Wenruo Committed by David Sterba
Browse files

btrfs: qgroup: don't try to wait flushing if we're already holding a transaction 



There is a chance of racing for qgroup flushing which may lead to
deadlock:

	Thread A		|	Thread B
   (not holding trans handle)	|  (holding a trans handle)
--------------------------------+--------------------------------
__btrfs_qgroup_reserve_meta()   | __btrfs_qgroup_reserve_meta()
|- try_flush_qgroup()		| |- try_flush_qgroup()
   |- QGROUP_FLUSHING bit set   |    |
   |				|    |- test_and_set_bit()
   |				|    |- wait_event()
   |- btrfs_join_transaction()	|
   |- btrfs_commit_transaction()|

			!!! DEAD LOCK !!!

Since thread A wants to commit transaction, but thread B is holding a
transaction handle, blocking the commit.
At the same time, thread B is waiting for thread A to finish its commit.

This is just a hot fix, and would lead to more EDQUOT when we're near
the qgroup limit.

The proper fix would be to make all metadata/data reservations happen
without holding a transaction handle.

CC: stable@vger.kernel.org # 5.9+
Reviewed-by: default avatarFilipe Manana <fdmanana@suse.com>
Signed-off-by: default avatarQu Wenruo <wqu@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 9a664971

fs/btrfs/qgroup.c

+20 −10
Original line number Diff line number Diff line
@@ -3530,16 +3530,6 @@ static int try_flush_qgroup(struct btrfs_root *root) 
	int ret;

	bool can_commit = true;



	/*

	 * We don't want to run flush again and again, so if there is a running

	 * one, we won't try to start a new flush, but exit directly.

	 */

	if (test_and_set_bit(BTRFS_ROOT_QGROUP_FLUSHING, &root->state)) {

		wait_event(root->qgroup_flush_wait,

			!test_bit(BTRFS_ROOT_QGROUP_FLUSHING, &root->state));

		return 0;

	}



	/*

	 * If current process holds a transaction, we shouldn't flush, as we

	 * assume all space reservation happens before a transaction handle is
@@ -3554,6 +3544,26 @@ static int try_flush_qgroup(struct btrfs_root *root) 
	    current->journal_info != BTRFS_SEND_TRANS_STUB)

		can_commit = false;



	/*

	 * We don't want to run flush again and again, so if there is a running

	 * one, we won't try to start a new flush, but exit directly.

	 */

	if (test_and_set_bit(BTRFS_ROOT_QGROUP_FLUSHING, &root->state)) {

		/*

		 * We are already holding a transaction, thus we can block other

		 * threads from flushing.  So exit right now. This increases

		 * the chance of EDQUOT for heavy load and near limit cases.

		 * But we can argue that if we're already near limit, EDQUOT is

		 * unavoidable anyway.

		 */

		if (!can_commit)

			return 0;



		wait_event(root->qgroup_flush_wait,

			!test_bit(BTRFS_ROOT_QGROUP_FLUSHING, &root->state));

		return 0;

	}



	ret = btrfs_start_delalloc_snapshot(root);

	if (ret < 0)

		goto out;