Input: uinput - fix circular locking dependency with ff-core

Input: uinput - fix circular locking dependency with ff-core

A lockdep circular locking dependency warning can be triggered
reproducibly when using a force-feedback gamepad with uinput (for
example, playing ELDEN RING under Wine with a Flydigi Vader 5
controller):

  ff->mutex -> udev->mutex -> input_mutex -> dev->mutex -> ff->mutex

The cycle is caused by four lock acquisition paths:

1. ff upload: input_ff_upload() holds ff->mutex and calls
   uinput_dev_upload_effect() -> uinput_request_submit() ->
   uinput_request_send(), which acquires udev->mutex.

2. device create: uinput_ioctl_handler() holds udev->mutex and calls
   uinput_create_device() -> input_register_device(), which acquires
   input_mutex.

3. device register: input_register_device() holds input_mutex and
   calls kbd_connect() -> input_register_handle(), which acquires
   dev->mutex.

4. evdev release: evdev_release() calls input_flush_device() under
   dev->mutex, which calls input_ff_flush() acquiring ff->mutex.

Fix this by introducing a new state_lock spinlock to protect
udev->state and udev->dev access in uinput_request_send() instead of
acquiring udev->mutex.  The function only needs to atomically check
device state and queue an input event into the ring buffer via
uinput_dev_event() -- both operations are safe under a spinlock
(ktime_get_ts64() and wake_up_interruptible() do not sleep).  This
breaks the ff->mutex -> udev->mutex link since a spinlock is a leaf in
the lock ordering and cannot form cycles with mutexes.

To keep state transitions visible to uinput_request_send(), protect
writes to udev->state in uinput_create_device() and
uinput_destroy_device() with the same state_lock spinlock.

Additionally, move init_completion(&request->done) from
uinput_request_send() to uinput_request_submit() before
uinput_request_reserve_slot().  Once the slot is allocated,
uinput_flush_requests() may call complete() on it at any time from
the destroy path, so the completion must be initialised before the
request becomes visible.

Lock ordering after the fix:

  ff->mutex -> state_lock (spinlock, leaf)
  udev->mutex -> state_lock (spinlock, leaf)
  udev->mutex -> input_mutex -> dev->mutex -> ff->mutex (no back-edge)

Fixes: ff462551235d ("Input: uinput - switch to the new FF interface")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/CABXGCsMoxag+kEwHhb7KqhuyxfmGGd0P=tHZyb1uKE0pLr8Hkg@mail.gmail.com/
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Link: https://patch.msgid.link/20260407075031.38351-1-mikhail.v.gavrilov@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>