[3.19.y-ckt stable] Patch "Btrfs: check if previous transaction aborted to avoid fs corruption" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    Btrfs: check if previous transaction aborted to avoid fs corruption

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt8.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

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>From 32768523703e055ec615d10b87f0b304396380b9 Mon Sep 17 00:00:00 2001
From: Filipe Manana <fdmanana at suse.com>
Date: Wed, 12 Aug 2015 11:54:35 +0100
Subject: Btrfs: check if previous transaction aborted to avoid fs corruption

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.

Signed-off-by: Filipe Manana <fdmanana at suse.com>
Reviewed-by: Josef Bacik <jbacik at fb.com>
Reviewed-by: Liu Bo <bo.li.liu at oracle.com>
Signed-off-by: Chris Mason <clm at fb.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 fs/btrfs/transaction.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 2818994..e64306a 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1819,8 +1819,11 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
 			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);
 		}
--
1.9.1