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Tue, 07 Apr 2026 21:56:52 -0700 (PDT) Date: Tue, 7 Apr 2026 21:56:49 -0700 From: Dmitry Torokhov To: Mikhail Gavrilov Cc: linux-input@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH v2] Input: uinput - fix circular locking dependency with ff-core Message-ID: References: <20260407075031.38351-1-mikhail.v.gavrilov@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260407075031.38351-1-mikhail.v.gavrilov@gmail.com> On Tue, Apr 07, 2026 at 12:50:31PM +0500, Mikhail Gavrilov wrote: > 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 Applied, thank you. -- Dmitry