[PATCH] btrfs: fix error handling in btrfs_dev_replace_start

[jeffm]

From: Jeff Mahoney <je...@suse.com>

When we fail to start a transaction in btrfs_dev_replace_start,
we leave dev_replace->replace_start set to STARTED but clear
->srcdev and ->tgtdev.  Later, that can result in an Oops in
btrfs_dev_replace_progress when having state set to STARTED or
SUSPENDED implies that ->srcdev is valid.

Also fix error handling when the state is already STARTED or
SUSPENDED while starting.  That, too, will clear ->srcdev and ->tgtdev
even though it doesn't own them.  This should be an impossible case to
hit since we should be protected by the BTRFS_FS_EXCL_OP bit being set.
Let's add an ASSERT there while we're at it.

Fixes: e93c89c1aaaaa (Btrfs: add new sources for device replace code)
Signed-off-by: Jeff Mahoney <je...@suse.com>
---
 fs/btrfs/dev-replace.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
index e2ba0419297a..0581c8570a05 100644
--- a/fs/btrfs/dev-replace.c
+++ b/fs/btrfs/dev-replace.c
@@ -445,6 +445,7 @@ int btrfs_dev_replace_start(struct btrfs_fs_info *fs_info,
                break;
        case BTRFS_IOCTL_DEV_REPLACE_STATE_STARTED:
        case BTRFS_IOCTL_DEV_REPLACE_STATE_SUSPENDED:
+               ASSERT(0);
                ret = BTRFS_IOCTL_DEV_REPLACE_RESULT_ALREADY_STARTED;
                goto leave;
        }
@@ -487,6 +488,10 @@ int btrfs_dev_replace_start(struct btrfs_fs_info *fs_info,
        if (IS_ERR(trans)) {
                ret = PTR_ERR(trans);
                btrfs_dev_replace_write_lock(dev_replace);
+               dev_replace->replace_state =
+                       BTRFS_IOCTL_DEV_REPLACE_STATE_NEVER_STARTED;
+               dev_replace->srcdev = NULL;
+               dev_replace->tgtdev = NULL;
                goto leave;
        }
 
@@ -508,8 +513,6 @@ int btrfs_dev_replace_start(struct btrfs_fs_info *fs_info,
        return ret;
 
 leave:
-       dev_replace->srcdev = NULL;
-       dev_replace->tgtdev = NULL;
        btrfs_dev_replace_write_unlock(dev_replace);
        btrfs_destroy_dev_replace_tgtdev(fs_info, tgt_device);
        return ret;
-- 
2.12.3