[PATCH] hwmon (ina3221) Add mutex lock to shunt nodes

[PATCH] hwmon (ina3221) Add mutex lock to shunt nodes

[Nicolin Chen]

The shunt resistor values are used to calculate shunt voltages
and currents. As a part of sysfs nodes, it would be better to
get protected with the same mutex too as other sysfs ABI nodes,
although this is not very critical because the mutex was added
to mainly protect register access.

So this patch adds the mutex lock to protect the shunt node.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
---
 drivers/hwmon/ina3221.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/hwmon/ina3221.c b/drivers/hwmon/ina3221.c
index 86281afd2619..f7a09ab6c440 100644
--- a/drivers/hwmon/ina3221.c
+++ b/drivers/hwmon/ina3221.c
@@ -532,8 +532,15 @@ static ssize_t ina3221_show_shunt(struct device *dev,
 	struct ina3221_data *ina = dev_get_drvdata(dev);
 	unsigned int channel = sd_attr->index;
 	struct ina3221_input *input = &ina->inputs[channel];
+	ssize_t ret;
 
-	return snprintf(buf, PAGE_SIZE, "%d\n", input->shunt_resistor);
+	mutex_lock(&ina->lock);
+
+	ret = snprintf(buf, PAGE_SIZE, "%d\n", input->shunt_resistor);
+
+	mutex_unlock(&ina->lock);
+
+	return ret;
 }
 
 static ssize_t ina3221_set_shunt(struct device *dev,
@@ -547,14 +554,20 @@ static ssize_t ina3221_set_shunt(struct device *dev,
 	int val;
 	int ret;
 
+	mutex_lock(&ina->lock);
+
 	ret = kstrtoint(buf, 0, &val);
-	if (ret)
+	if (ret) {
+		mutex_unlock(&ina->lock);
 		return ret;
+	}
 
 	val = clamp_val(val, 1, INT_MAX);
 
 	input->shunt_resistor = val;
 
+	mutex_unlock(&ina->lock);
+
 	return count;
 }
 
-- 
2.17.1

Re: [PATCH] hwmon (ina3221) Add mutex lock to shunt nodes

[Guenter Roeck]

On Mon, Nov 12, 2018 at 08:23:24PM -0800, Nicolin Chen wrote:
> The shunt resistor values are used to calculate shunt voltages
> and currents. As a part of sysfs nodes, it would be better to
> get protected with the same mutex too as other sysfs ABI nodes,
> although this is not very critical because the mutex was added
> to mainly protect register access.
> 
> So this patch adds the mutex lock to protect the shunt node.
> 

I am missing something here. I don't see the point of this mutex.
It just reads a variable and reports the result. When setting,
it just writes a value. Protecting the conversion of the passed
value with a mutex is pointless. Protecting writing the value
against reading it is just as pointless.

Guenter

> Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
> ---
>  drivers/hwmon/ina3221.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hwmon/ina3221.c b/drivers/hwmon/ina3221.c
> index 86281afd2619..f7a09ab6c440 100644
> --- a/drivers/hwmon/ina3221.c
> +++ b/drivers/hwmon/ina3221.c
> @@ -532,8 +532,15 @@ static ssize_t ina3221_show_shunt(struct device *dev,
>  	struct ina3221_data *ina = dev_get_drvdata(dev);
>  	unsigned int channel = sd_attr->index;
>  	struct ina3221_input *input = &ina->inputs[channel];
> +	ssize_t ret;
>  
> -	return snprintf(buf, PAGE_SIZE, "%d\n", input->shunt_resistor);
> +	mutex_lock(&ina->lock);
> +
> +	ret = snprintf(buf, PAGE_SIZE, "%d\n", input->shunt_resistor);
> +
> +	mutex_unlock(&ina->lock);
> +
> +	return ret;
>  }
>  
>  static ssize_t ina3221_set_shunt(struct device *dev,
> @@ -547,14 +554,20 @@ static ssize_t ina3221_set_shunt(struct device *dev,
>  	int val;
>  	int ret;
>  
> +	mutex_lock(&ina->lock);
> +
>  	ret = kstrtoint(buf, 0, &val);
> -	if (ret)
> +	if (ret) {
> +		mutex_unlock(&ina->lock);
>  		return ret;
> +	}
>  
>  	val = clamp_val(val, 1, INT_MAX);
>  
>  	input->shunt_resistor = val;
>  
> +	mutex_unlock(&ina->lock);
> +
>  	return count;
>  }
>  
> -- 
> 2.17.1
> 

Re: [PATCH] hwmon (ina3221) Add mutex lock to shunt nodes

[Nicolin Chen]

On Mon, Nov 12, 2018 at 08:31:22PM -0800, Guenter Roeck wrote:
> On Mon, Nov 12, 2018 at 08:23:24PM -0800, Nicolin Chen wrote:
> > The shunt resistor values are used to calculate shunt voltages
> > and currents. As a part of sysfs nodes, it would be better to
> > get protected with the same mutex too as other sysfs ABI nodes,
> > although this is not very critical because the mutex was added
> > to mainly protect register access.
> > 
> > So this patch adds the mutex lock to protect the shunt node.
> > 
> 
> I am missing something here. I don't see the point of this mutex.
> It just reads a variable and reports the result. When setting,
> it just writes a value. Protecting the conversion of the passed
> value with a mutex is pointless. Protecting writing the value
> against reading it is just as pointless.

The penalty here is that the resistor value would be mismatched
between here and the ongoing calculation when a race happens to
the sysfs accesses. But I feel it might not be really that hurt
to allow it happen.

Let's just drop it then: the mutex was used to protect register
read/writes any way.

Thanks
Nicolin