WARNING: suspicious RCU usage in modeset_lock

[Steven Rostedt]

On Thu, 17 Dec 2020 11:03:20 +0100
Daniel Vetter <daniel.vetter at ffwll.ch> wrote:

> I think we're tripping over the might_sleep() all the mutexes have,
> and that's not as good as yours, but good enough to catch a missing
> rcu_read_unlock(). That's kinda why I'm baffled, since like almost
> every 2nd function in the backtrace grabbed a mutex and it was all
> fine until the very last.
> 
> I think it would be really nice if the rcu checks could retain (in
> debugging only) the backtrace of the outermost rcu_read_lock, so we
> could print that when something goes wrong in cases where it's leaked.
> For normal locks lockdep does that already (well not full backtrace I
> think, just the function that acquired the lock, but that's often
> enough). I guess that doesn't exist yet?
> 
> Also yes without reproducer this is kinda tough nut to crack.

I'm looking at drm_client_modeset_commit_atomic(), where it triggered after
the "retry:" label, which to get to, does a bit of goto spaghetti, with
a -EDEADLK detected and a goto backoff, which calls goto retry, and then
the next mutex taken is the one that triggers the bug.

As this is hard to reproduce, but reproducible by a fuzzer, I'm guessing
there's some error return path somewhere in there that doesn't release an
rcu_read_lock().

-- Steve