Re: [PATCH v4 1/2] pinctrl: stm32: Fix the unit of the hwspinlock timeout

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] pinctrl: stm32: same-CPU starvation with HWLOCK_IN_ATOMIC on PREEMPT_RT
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commit 635cf6ace8c829d5e8fd3222bc79311f9ac07fc2
Author: Ju Nan <junan76@163.com>

pinctrl: stm32: Fix the unit of the hwspinlock timeout

This commit updates the hardware spinlock timeout parameter passed to
hwspin_lock_timeout_in_atomic(). It modifies the timeout constant name and
changes its value from microseconds to milliseconds across the stm32 pinctrl
driver to match the expected API units.

> diff --git a/drivers/pinctrl/stm32/pinctrl-stm32.c b/drivers/pinctrl/stm32/pinctrl-stm32.c
> --- a/drivers/pinctrl/stm32/pinctrl-stm32.c
> +++ b/drivers/pinctrl/stm32/pinctrl-stm32.c
[ ... ]
> @@ -946,7 +946,7 @@ static int stm32_pmx_set_mode(struct stm32_gpio_bank *bank,
>  
>  	if (pctl->hwlock) {
>  		err = hwspin_lock_timeout_in_atomic(pctl->hwlock,
> -						    HWSPNLCK_TIMEOUT);
> +						    HWSPNLCK_TIMEOUT_MS);

[Severity: High]
Does calling hwspin_lock_timeout_in_atomic() here cause a same-CPU starvation
issue on PREEMPT_RT?

Because hwspin_lock_timeout_in_atomic() uses the HWLOCK_IN_ATOMIC mode,
it implements the timeout by busy-waiting via udelay() and assumes preemption
is disabled by the caller.

However, on PREEMPT_RT, spin_lock_irqsave(&bank->lock) acquires an rt_mutex
which leaves preemption enabled.

If thread A acquires pctl->hwlock and is preempted, and thread B on the
same CPU subsequently acquires a different bank's lock and attempts to acquire
this shared pctl->hwlock, thread B will busy-wait.

Since thread B loops in a busy-wait for the entire 1ms timeout, thread A is
starved and cannot run to release the hardware lock, resulting in a guaranteed
spurious -ETIMEDOUT failure for thread B.

>  		if (err) {
>  			dev_err(pctl->dev, "Can't get hwspinlock\n");
>  			goto unlock;
[ ... ]

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Sashiko AI review · https://sashiko.dev/#/patchset/20261014160917.139241-2-junan76@163.com?part=1