Re: [btrfs/rt] lockdep false positive

[Mike Galbraith]

On Wed, 2017-01-25 at 18:02 +0100, Sebastian Andrzej Siewior wrote:

> > [  341.960794]        CPU0
> > [  341.960795]        ----
> > [  341.960795]   lock(btrfs-tree-00);
> > [  341.960795]   lock(btrfs-tree-00);
> > [  341.960796] 
> > [  341.960796]  *** DEADLOCK ***
> > [  341.960796]
> > [  341.960796]  May be due to missing lock nesting notation
> > [  341.960796]
> > [  341.960796] 6 locks held by kworker/u8:9/2039:
> > [  341.960797]  #0:  ("%s-%s""btrfs", name){.+.+..}, at: [] process_one_work+0x171/0x700
> > [  341.960812]  #1:  ((&work->normal_work)){+.+...}, at: [] process_one_work+0x171/0x700
> > [  341.960815]  #2:  (sb_internal){.+.+..}, at: [] start_transaction+0x2a7/0x5a0 [btrfs]
> > [  341.960825]  #3:  (btrfs-tree-02){+.+...}, at: [] btrfs_clear_lock_blocking_rw+0x55/0x100 [btrfs]
> > [  341.960835]  #4:  (btrfs-tree-01){+.+...}, at: [] btrfs_clear_lock_blocking_rw+0x55/0x100 [btrfs]
> > [  341.960854]  #5:  (btrfs-tree-00){+.+...}, at: [] btrfs_clear_lock_blocking_rw+0x55/0x100 [btrfs]
> > 
> > Attempting to describe RT rwlock semantics to lockdep prevents this.
> 
> and this is what I don't get. I stumbled upon this myself [0] but didn't
> fully understand the problem (assuming this is the same problem colored
> differently).

Yeah, [0] looks like it, though I haven't met an 'fs' variant, my
encounters were always either 'tree' or 'csum' flavors.

> With your explanation I am not sure if I get what is happening. If btrfs
> is taking here read-locks on random locks then it may deadlock if
> another btrfs-thread is doing the same and need each other's locks.

I don't know if a real RT deadlock is possible.  I haven't met one,
only variants of this bogus recursion gripe.
 
> If btrfs takes locks recursively which it already holds (in the same
> context / process) then it shouldn't be visible here because lockdep
> does not account this on -RT.

If what lockdep gripes about were true, we would never see the splat,
we'd zip straight through that (illusion) recursive read_lock() with
lockdep being none the wiser. 

> If btrfs takes the locks in a special order for instance only ascending
> according to inode's number then it shouldn't deadlock.

No idea.  Locking fancy enough to require dynamic key assignment to
appease lockdep is too fancy for me.

	-Mike