Re: [PATCH] btrfs: Silence a static checker locking warning

[David Sterba <dsterba@suse.cz>]

On Mon, Feb 11, 2019 at 05:36:13PM +0100, David Sterba wrote:
> On Sat, Feb 09, 2019 at 12:02:55PM +0300, Dan Carpenter wrote:
> > Back in the day, before commit 0b246afa62b0 ("btrfs: root->fs_info
> > cleanup, add fs_info convenience variables") then we used to take
> > different locks.
> 
> Nope, it's the same per-filesystem lock, just the old code got there
> in two different ways (ie. two subvolume roots).
> 
> > But now it's just one lock and the static checkers
> > think we can call down_read(&fs_info->subvol_sem); twice in a row which
> > would lead to a deadlock.
> 
> Why? It's read side of a semaphore.
> 
> > That code is several years old now so presumably both (old_ino ==
> > BTRFS_FIRST_FREE_OBJECTID) and (new_ino == BTRFS_FIRST_FREE_OBJECTID)
> > conditions can't be true at the same time or the bug would have showed
> > up in testing.
> 
> Why do you think it's a bug? If you are sure that there's a bug we've
> overlooked, please state it in the changelog, the rationale you've
> provided is very vague.
> 
> And I believe also wrong. The rename-exchange cannot work between two
> subvolumes, but we still can cross-rename two subvolumes. In this
> example hierarchy:
> 
> /
> - subvol1 (inode number 256, ie. BTRFS_FIRST_FREE_OBJECTID)
>   - file1
> - subvol2 (inode number 256, ie. BTRFS_FIRST_FREE_OBJECTID)
>   - file2
> 
> btrfs_rename_exchange leads to this:
> 
> /
> - subvol1
>   - file2
> - subvol2
>   - file1
> 
> There's no common tool that supports renameat2, so I'm using the one
> from fstests/src/renameat2.c to verify that, and it does indeed work as
> expected.

Lockdep was forgiving and did not deadlock, that I would notice while
running the test. There's a warning in the log about the recursive
locking. So we need to add the lock annotation or merge them to a single
location as you suggest.

And also add a test to fstests, as the subvolume related testcases are
completely missing.