On 15.05.2018 20:37, Liu Bo wrote:
> If a btree block, aka. extent buffer, is not available in the extent
> buffer cache, it'll be read out from the disk instead, i.e.
>
> btrfs_search_slot()
> read_block_for_search() # hold parent and its lock, go to read child
> btrfs_release_path()
> read_tree_block() # read child
>
> Unfortunately, the parent lock got released before reading child, so
> commit 5bdd3536cbbe ("Btrfs: Fix block generation verification race") had
> used 0 as parent transid to read the child block. It forces
> read_tree_block() not to check if parent transid is different with the
> generation id of the child that it reads out from disk.
>
> A simple PoC is included in btrfs/124,
>
> 0. A two-disk raid1 btrfs,
>
> 1. Right after mkfs.btrfs, block A is allocated to be device tree's root.
>
> 2. Mount this filesystem and put it in use, after a while, device tree's
> root got COW but block A hasn't been allocated/overwritten yet.
>
> 3. Umount it and reload the btrfs module to remove both disks from the
> global @fs_devices list.
>
> 4. mount -odegraded dev1 and write some data, so now block A is allocated
> to be a leaf in checksum tree. Note that only dev1 has the latest
> metadata of this filesystem.
>
> 5. Umount it and mount it again normally (with both disks), since raid1
> can pick up one disk by the writer task's pid, if btrfs_search_slot()
> needs to read block A, dev2 which does NOT have the latest metadata
> might be read for block A, then we got a stale block A.
>
> 6. As parent transid is not checked, block A is marked as uptodate and
> put into the extent buffer cache, so the future search won't bother
> to read disk again, which means it'll make changes on this stale
> one and make it dirty and flush it onto disk.
>
> To avoid the problem, parent transid needs to be passed to
> read_tree_block().
>
> In order to get a valid parent transid, we need to hold the parent's
> lock until finish reading child.
>
> Signed-off-by: Liu Bo <bo.liu@xxxxxxxxxxxxxxxxx>
Doesn't this warrant a stable tag and
Fixes: 5bdd3536cbbe ("Btrfs: Fix block generation verification race")
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