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Commit 99d16cbc authored by Sage Weil's avatar Sage Weil Committed by Chris Mason
Browse files

Btrfs: fix deadlock in btrfs_commit_transaction 



We calculate timeout (either 1 or MAX_SCHEDULE_TIMEOUT) based on whether
num_writers > 1 or should_grow at the top of the loop.  Then, much much
later, we wait for that timeout if either num_writers or should_grow is
true.  However, it's possible for a racing process (calling
btrfs_end_transaction()) to decrement num_writers such that we wait
forever instead of for 1.

Fix this by deciding how long to wait when we wait.  Include a smp_mb()
before checking if the waitqueue is active to ensure the num_writers
is visible.

Signed-off-by: default avatarSage Weil <sage@newdream.net>
Signed-off-by: default avatarChris Mason <chris.mason@oracle.com>
parent fccdae43

fs/btrfs/transaction.c

+5 −8
Original line number Diff line number Diff line
@@ -402,6 +402,7 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans, 
	WARN_ON(cur_trans->num_writers < 1);

	cur_trans->num_writers--;



	smp_mb();

	if (waitqueue_active(&cur_trans->writer_wait))

		wake_up(&cur_trans->writer_wait);

	put_transaction(cur_trans);
@@ -1010,7 +1011,6 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans, 
			     struct btrfs_root *root)

{

	unsigned long joined = 0;

	unsigned long timeout = 1;

	struct btrfs_transaction *cur_trans;

	struct btrfs_transaction *prev_trans = NULL;

	DEFINE_WAIT(wait);
@@ -1081,11 +1081,6 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans, 
			snap_pending = 1;



		WARN_ON(cur_trans != trans->transaction);

		if (cur_trans->num_writers > 1)

			timeout = MAX_SCHEDULE_TIMEOUT;

		else if (should_grow)

			timeout = 1;



		mutex_unlock(&root->fs_info->trans_mutex);



		if (flush_on_commit || snap_pending) {
@@ -1107,8 +1102,10 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans, 
				TASK_UNINTERRUPTIBLE);



		smp_mb();

		if (cur_trans->num_writers > 1 || should_grow)

			schedule_timeout(timeout);

		if (cur_trans->num_writers > 1)

			schedule_timeout(MAX_SCHEDULE_TIMEOUT);

		else if (should_grow)

			schedule_timeout(1);



		mutex_lock(&root->fs_info->trans_mutex);

		finish_wait(&cur_trans->writer_wait, &wait);