Re: [PATCH RFT v2] gpio: cdev: use raw notifier for line state events

[Kent Gibson <warthog618@xxxxxxxxx>]

On Tue, Mar 11, 2025 at 03:31:43PM +0100, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
>
> We use a notifier to implement the mechanism of informing the user-space
> about changes in GPIO line status. We register with the notifier when
> the GPIO character device file is opened and unregister when the last
> reference to the associated file descriptor is dropped.
>
> Since commit fcc8b637c542 ("gpiolib: switch the line state notifier to
> atomic") we use the atomic notifier variant. Atomic notifiers call
> rcu_synchronize in atomic_notifier_chain_unregister() which caused a
> significant performance regression in some circumstances, observed by
> user-space when calling close() on the GPIO device file descriptor.
>
> Replace the atomic notifier with the raw variant and provide
> synchronization with a read-write spinlock.
>
> Fixes: fcc8b637c542 ("gpiolib: switch the line state notifier to atomic")
> Reported-by: David Jander <david@xxxxxxxxxxx>
> Closes: https://lore.kernel.org/all/20250311110034.53959031@erd003.prtnl/
> Tested-by: David Jander <david@xxxxxxxxxxx>
> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>

Works for me.

Tested-by: Kent Gibson <warthog618@xxxxxxxxx>

Cheers,
Kent.