linux-kernel - Re: [PATCH] counter: interrupt-cnt: Protect enable/disable OPs with mutex

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Message-ID: <8394ef31a75fdf9122331ecb97dd6f986d5026f1.camel@siemens.com>
Date: Fri, 2 May 2025 09:24:20 +0000
From: "Sverdlin, Alexander" <alexander.sverdlin@...mens.com>
To: "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>
CC: "o.rempel@...gutronix.de" <o.rempel@...gutronix.de>,
	"kernel@...gutronix.de" <kernel@...gutronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"wbg@...nel.org" <wbg@...nel.org>
Subject: Re: [PATCH] counter: interrupt-cnt: Protect enable/disable OPs with
 mutex

Dear maintainers,

On Mon, 2025-03-31 at 18:36 +0200, A. Sverdlin wrote:
> From: Alexander Sverdlin <alexander.sverdlin@...mens.com>
> 
> Enable/disable seems to be racy on SMP, consider the following scenario:
> 
> CPU0					CPU1
> 
> interrupt_cnt_enable_write(true)
> {
> 	if (priv->enabled == enable)
> 		return 0;
> 
> 	if (enable) {
> 		priv->enabled = true;
> 					interrupt_cnt_enable_write(false)
> 					{
> 						if (priv->enabled == enable)
> 							return 0;
> 
> 						if (enable) {
> 							priv->enabled = true;
> 							enable_irq(priv->irq);
> 						} else {
> 							disable_irq(priv->irq)
> 							priv->enabled = false;
> 						}
> 		enable_irq(priv->irq);
> 	} else {
> 		disable_irq(priv->irq);
> 		priv->enabled = false;
> 	}
> 
> The above would result in priv->enabled == false, but IRQ left enabled.
> Protect both write (above race) and read (to propagate the value on SMP)
> callbacks with a mutex.
> 
> Signed-off-by: Alexander Sverdlin <alexander.sverdlin@...mens.com>

I've noticed that the patch has been marked as "Changes Requested" in
the patchwork, could it be a mistake? Because I never received any
change request.

> ---
>  drivers/counter/interrupt-cnt.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/counter/interrupt-cnt.c b/drivers/counter/interrupt-cnt.c
> index 949598d51575a..d83848d0fe2af 100644
> --- a/drivers/counter/interrupt-cnt.c
> +++ b/drivers/counter/interrupt-cnt.c
> @@ -3,12 +3,14 @@
>   * Copyright (c) 2021 Pengutronix, Oleksij Rempel <kernel@...gutronix.de>
>   */
>  
> +#include <linux/cleanup.h>
>  #include <linux/counter.h>
>  #include <linux/gpio/consumer.h>
>  #include <linux/interrupt.h>
>  #include <linux/irq.h>
>  #include <linux/mod_devicetable.h>
>  #include <linux/module.h>
> +#include <linux/mutex.h>
>  #include <linux/platform_device.h>
>  #include <linux/types.h>
>  
> @@ -19,6 +21,7 @@ struct interrupt_cnt_priv {
>  	struct gpio_desc *gpio;
>  	int irq;
>  	bool enabled;
> +	struct mutex lock;
>  	struct counter_signal signals;
>  	struct counter_synapse synapses;
>  	struct counter_count cnts;
> @@ -41,6 +44,8 @@ static int interrupt_cnt_enable_read(struct counter_device *counter,
>  {
>  	struct interrupt_cnt_priv *priv = counter_priv(counter);
>  
> +	guard(mutex)(&priv->lock);
> +
>  	*enable = priv->enabled;
>  
>  	return 0;
> @@ -51,6 +56,8 @@ static int interrupt_cnt_enable_write(struct counter_device *counter,
>  {
>  	struct interrupt_cnt_priv *priv = counter_priv(counter);
>  
> +	guard(mutex)(&priv->lock);
> +
>  	if (priv->enabled == enable)
>  		return 0;
>  
> @@ -227,6 +234,8 @@ static int interrupt_cnt_probe(struct platform_device *pdev)
>  	if (ret)
>  		return ret;
>  
> +	mutex_init(&priv->lock);
> +
>  	ret = devm_counter_add(dev, counter);
>  	if (ret < 0)
>  		return dev_err_probe(dev, ret, "Failed to add counter\n");

-- 
Alexander Sverdlin
Siemens AG
www.siemens.com