Re: [PATCH v2] Input: serio - don't sleep on serio_mutex from drvctl_store()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] sysfs: write returning -EAGAIN breaks userspace ABI
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commit 587df4e4a0dc7a2a601093d20c02612cf36f0cbd
Author: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>

Input: serio - don't sleep on serio_mutex from drvctl_store()

Writing to the drvctl attribute takes serio_mutex while holding the
attribute's kernfs active reference. serio_unregister_port() does the
opposite: it holds serio_mutex and device_del() then waits for active
references to drain in kernfs_drain(). If a drvctl write is in flight
when the port is unregistered, the writer waits for serio_mutex and the
unregistering task waits for the writer, and neither makes progress.

> diff --git a/drivers/input/serio/serio.c b/drivers/input/serio/serio.c
> index 54dd26249b027..5b650421e0663 100644
> --- a/drivers/input/serio/serio.c
> +++ b/drivers/input/serio/serio.c
> @@ -357,7 +357,8 @@ static ssize_t drvctl_store(struct device *dev, struct device_attribute *attr, c
>  	struct device_driver *drv;
>  	int error;
>  
> -	scoped_cond_guard(mutex_intr, return -EINTR, &serio_mutex) {
> +	/* Port removal holds serio_mutex and drains us: don't wait for it. */
> +	scoped_cond_guard(mutex_try, return -EAGAIN, &serio_mutex) {

[Severity: High]
Does returning -EAGAIN directly to the VFS here break the userspace ABI?

Standard userspace tools (like echo) performing blocking writes do not
automatically retry on -EAGAIN. This causes them to fail outright with
'Resource temporarily unavailable' when the trylock fails in drvctl_store()
during concurrent port removal.

Could this be resolved by using sysfs_break_active_protection() or returning
restart_syscall() (which sets TIF_SIGPENDING and returns -ERESTARTNOINTR)
instead, to allow transparent restarting?

>  		if (!strncmp(buf, "none", count)) {
>  			serio_disconnect_port(serio);
>  		} else if (!strncmp(buf, "reconnect", count)) {

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Sashiko AI review · https://sashiko.dev/#/patchset/20260924040430.711439-1-ngocthang2710.1999@gmail.com?part=1