Patch "Btrfs: check if previous transaction aborted to avoid fs corruption" has been added to the 4.2-stable tree

This is a note to let you know that I've just added the patch titled

Btrfs: check if previous transaction aborted to avoid fs corruption

to the 4.2-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
btrfs-check-if-previous-transaction-aborted-to-avoid-fs-corruption.patch
and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Filipe Manana <[email protected]>
Date: Wed, 12 Aug 2015 11:54:35 +0100
Subject: Btrfs: check if previous transaction aborted to avoid fs corruption

From: Filipe Manana <[email protected]>

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:

CPU 0 CPU 1

transaction N starts

(...)

btrfs_commit_transaction(N)

    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;

                                                              btrfs_start_transaction()
                                                                 --> 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

       cleanup_transaction()
         btrfs_cleanup_one_transaction()
           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.

---
 fs/btrfs/transaction.c |    3 +++
 1 file changed, 3 insertions(+)

@@ -1893,8 +1893,11 @@ int btrfs_commit_transaction(struct btrf
 			spin_unlock(&root->fs_info->trans_lock);
 
 			wait_for_commit(root, prev_trans);
+			ret = prev_trans->aborted;
 
 			btrfs_put_transaction(prev_trans);
+			if (ret)
+				goto cleanup_transaction;
 		} else {
 			spin_unlock(&root->fs_info->trans_lock);
 		}

Patches currently in stable-queue which might be from [email protected] are

queue-4.2/btrfs-check-if-previous-transaction-aborted-to-avoid-fs-corruption.patch