Re: [PATCH] gpio: cdev: fix waitqueue deadlock on chip deregistration

[Kent Gibson <warthog618@gmail.com>]

On Thu, Nov 05, 2026 at 03:47:59PM +0100, Bartosz Golaszewski wrote:
> The three calls to wait_event_interruptible_locked() sleep while holding
> gdev->srcu. In gpiochip_remove(), synchronize_srcu() runs before
> gcdev_unregister(). The latter fires the waitqueue wakeup events from
> the notifier callbacks, so a reader blocked on an empty FIFO never leaves
> its SRCU section and the removal will block forever in the former.
> 
> Since rcu_access_pointer() is a plain READ_ONCE(), we don't really need
> to take the SRCU read lock. We don't access the chip structure after
> checking it. Remove the lock and instead make the waitqueue aware of the
> possible chip deregistration. Keep the initial gdev->chip checks as
> removing them may change the error codes visible to user-space, for
> instance in cases of a non-blocking read on an empty fifo.
> 
> Ideally, we'd factor out the common code into a dedicated helper but
> there doesn't seem to exist a clean way of passing a kfifo as argument
> so let's open-code the three call sites.
> 
> Cc: stable@vger.kernel.org
> Fixes: d83cee3d2bb1 ("gpio: protect the pointer to gpio_chip in gpio_device with SRCU")
> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
> ---
> I've had it on my TODO list for a while. I vaguely remember someone
> reporting it to me but can't find the reference anymore. Please feel
> free to add a Reported-by: and Closes:.
> 

Isn't this the same as patch 4/7 in Adrian's series[1]?

Cheers,
Kent.

[1] https://lore.kernel.org/linux-gpio/20261103-gpiolib-chip-removal-v5-4-aambrozewicz@nvidia.com/