[PATCH 3.16.y-ckt 052/133] Btrfs: check if previous transaction aborted to avoid fs corruption

From: Luis Henriques
Date: Wed Sep 30 2015 - 06:25:27 EST

3.16.7-ckt18 -stable review patch. If anyone has any objections, please let me know.


From: Filipe Manana <fdmanana@xxxxxxxx>

commit 1f9b8c8fbc9a4d029760b16f477b9d15500e3a34 upstream.

While we are committing a transaction, it's possible the previous one is
still finishing its commit and therefore we wait for it to finish first.
However we were not checking if that previous transaction ended up getting
aborted after we waited for it to commit, so we ended up committing the
current transaction which can lead to fs corruption because the new
superblock can point to trees that have had one or more nodes/leafs that
were never durably persisted.
The following sequence diagram exemplifies how this is possible:


transaction N starts



cur_trans->state = TRANS_STATE_COMMIT_START;
cur_trans->state = TRANS_STATE_COMMIT_DOING;

cur_trans->state = TRANS_STATE_UNBLOCKED;
root->fs_info->running_transaction = NULL;

--> starts transaction N + 1

btrfs_write_and_wait_transaction(trans, root);
--> starts writing all new or COWed ebs created
at transaction N

creates some new ebs, COWs some
existing ebs but doesn't COW or
deletes eb X

btrfs_commit_transaction(N + 1)
cur_trans->state = TRANS_STATE_COMMIT_START;
wait_for_commit(root, prev_trans);
--> prev_trans == transaction N

btrfs_write_and_wait_transaction() continues
writing ebs
--> fails writing eb X, we abort transaction N
and set bit BTRFS_FS_STATE_ERROR on
fs_info->fs_state, so no new transactions
can start after setting that bit

wakes up task at CPU 1

continues, doesn't abort because
cur_trans->aborted (transaction N + 1)
is zero, and no checks for bit
BTRFS_FS_STATE_ERROR in fs_info->fs_state
are made

btrfs_write_and_wait_transaction(trans, root);
--> succeeds, no errors during writeback

write_ctree_super(trans, root, 0);
--> succeeds
--> we have now a superblock that points us
to some root that uses eb X, which was
never written to disk

In this scenario future attempts to read eb X from disk results in an
error message like "parent transid verify failed on X wanted Y found Z".

So fix this by aborting the current transaction if after waiting for the
previous transaction we verify that it was aborted.

Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
Reviewed-by: Josef Bacik <jbacik@xxxxxx>
Reviewed-by: Liu Bo <bo.li.liu@xxxxxxxxxx>
Signed-off-by: Chris Mason <clm@xxxxxx>
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
fs/btrfs/transaction.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 3807fffb1c33..0309af907e21 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1733,8 +1733,11 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,

wait_for_commit(root, prev_trans);
+ ret = prev_trans->aborted;

+ if (ret)
+ goto cleanup_transaction;
} else {
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