Re: [PATCH v4 1/4] gpio: shared-proxy: Fix race between concurrent first users

[Bartosz Golaszewski]

On Sat, 17 Oct 2026 06:36:02 +0200, Shawn Guo
<shengchao.guo@xxxxxxxxxxxxxxxx> said:
> The proxy lets a consumer set the line direction and reset the
> default value and vote tally only when usecnt == 1, i.e. when it is
> the sole requester. But request() and direction_output() take the
> descriptor mutex separately. When two consumers probe concurrently,
> both request the line before either configures it, both see
> usecnt == 2, both fall into the shared path, read back the still
> unconfigured direction (input) and fail with -EPERM.
>
> On Nord RRD the two DAC supplies are reg-fixed-voltage regulators
> sharing tlmm GPIO 41. They probe asynchronously and intermittently
> both fail, leaving the codecs without supplies:
>
>   reg-fixed-voltage regulator-3p3-dac: setup of GPIO (default) failed: -1
>   reg-fixed-voltage regulator-5p0-dac: setup of GPIO (default) failed: -1
>
> Requesting the line is not evidence that a consumer has claimed it.
> Instead, count proxies that have actually configured the line (set its
> direction or voted for a value) in a new cfgcnt, with a per-proxy
> configured flag, and allow (re)initialization only while no other
> proxy has done so. A consumer obtained with GPIOD_ASIS that votes via
> gpiod_set_value() without ever setting the direction counts as
> configured, so a later user can never reset def_val/votecnt behind its
> vote. The count is dropped when the proxy frees the line.
>
> Fixes: e992d54c6f97 ("gpio: shared-proxy: implement the shared GPIO proxy driver")
> Assisted-by: LLM
> Signed-off-by: Shawn Guo <shengchao.guo@xxxxxxxxxxxxxxxx>
> ---
>  drivers/gpio/gpio-shared-proxy.c | 74 ++++++++++++++++++++++++++++----
>  drivers/gpio/gpiolib-shared.h    |  1 +
>  2 files changed, 66 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/gpio/gpio-shared-proxy.c b/drivers/gpio/gpio-shared-proxy.c
> index bc69b8729d19..0d1d724a6a37 100644
> --- a/drivers/gpio/gpio-shared-proxy.c
> +++ b/drivers/gpio/gpio-shared-proxy.c
> @@ -21,9 +21,33 @@ struct gpio_shared_proxy_data {
>  	struct gpio_chip gc;
>  	struct gpio_shared_desc *shared_desc;
>  	struct device *dev;
> +	bool configured;
>  	bool voted_change;
>  };
>
> +/*
> + * A proxy may (re)initialize the line - set its direction and reset the
> + * default value and the vote tally - only while no other proxy has set the
> + * direction or voted for a value. Requesting the line is not enough: consumers
> + * may request concurrently, so counting requests lets every one of them
> + * conclude that it is not first.
> + */
> +static bool gpio_shared_proxy_may_init(struct gpio_shared_proxy_data *proxy)
> +{
> +	unsigned int cfgcnt = proxy->shared_desc->cfgcnt;
> +

We expect the shared lock to be taken, so can you add lockdep assert here ...

> +	return !cfgcnt || (cfgcnt == 1 && proxy->configured);
> +}
> +
> +static void gpio_shared_proxy_mark_configured(struct gpio_shared_proxy_data *proxy)
> +{
> +	if (proxy->configured)
> +		return;

... and here?

Thanks,
Bartosz