[PATCH] hwmon: sht15: Fix wrong assumptions in device remove callback

[PATCH] hwmon: sht15: Fix wrong assumptions in device remove callback

[Uwe Kleine-König]

Taking a lock at the beginning of .remove() doesn't prevent new readers.
With the existing approach it can happen, that a read occurs just when
the lock was taken blocking the reader until the lock is released at the
end of the remove callback which then accessed *data that is already
freed then.

To actually fix this problem the hwmon core needs some adaption. Until
this is implemented take the optimistic approach of assuming that all
readers are gone after hwmon_device_unregister() and
sysfs_remove_group() as most other drivers do. (And once the core
implements that, taking the lock would deadlock.)

So drop the lock, move the reset to after device unregistration to keep
the device in a workable state until it's deregistered. Also add a error
message in case the reset fails and return 0 anyhow. (Returning an error
code, doesn't stop the platform device unregistration and only results
in a little helpful error message before the devm cleanup handlers are
called.)

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
---
Hello,

the motivation for this patch is to fix the driver to not return an
error code in .remove(). The long term goal is to make remove callbacks
return void as returning an error is nearly always wrong and doesn't
have the effect that driver authors think it has. This patch is a
preparation for this conversion.

Best regards
Uwe

 drivers/hwmon/sht15.c | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/drivers/hwmon/sht15.c b/drivers/hwmon/sht15.c
index 7f4a63959730..ae4d14257a11 100644
--- a/drivers/hwmon/sht15.c
+++ b/drivers/hwmon/sht15.c
@@ -1020,25 +1020,20 @@ static int sht15_probe(struct platform_device *pdev)
 static int sht15_remove(struct platform_device *pdev)
 {
 	struct sht15_data *data = platform_get_drvdata(pdev);
+	int ret;
 
-	/*
-	 * Make sure any reads from the device are done and
-	 * prevent new ones beginning
-	 */
-	mutex_lock(&data->read_lock);
-	if (sht15_soft_reset(data)) {
-		mutex_unlock(&data->read_lock);
-		return -EFAULT;
-	}
 	hwmon_device_unregister(data->hwmon_dev);
 	sysfs_remove_group(&pdev->dev.kobj, &sht15_attr_group);
+
+	ret = sht15_soft_reset(data);
+	if (ret)
+		dev_err(&pdev->dev, "Failed to reset device (%pe)\n", ERR_PTR(ret));
+
 	if (!IS_ERR(data->reg)) {
 		regulator_unregister_notifier(data->reg, &data->nb);
 		regulator_disable(data->reg);
 	}
 
-	mutex_unlock(&data->read_lock);
-
 	return 0;
 }
 
-- 
2.36.1

Re: [PATCH] hwmon: sht15: Fix wrong assumptions in device remove callback

[Guenter Roeck]

On Mon, Jul 25, 2022 at 09:43:44PM +0200, Uwe Kleine-König wrote:
> Taking a lock at the beginning of .remove() doesn't prevent new readers.
> With the existing approach it can happen, that a read occurs just when
> the lock was taken blocking the reader until the lock is released at the
> end of the remove callback which then accessed *data that is already
> freed then.
> 
> To actually fix this problem the hwmon core needs some adaption. Until
> this is implemented take the optimistic approach of assuming that all
> readers are gone after hwmon_device_unregister() and
> sysfs_remove_group() as most other drivers do. (And once the core
> implements that, taking the lock would deadlock.)
> 
> So drop the lock, move the reset to after device unregistration to keep
> the device in a workable state until it's deregistered. Also add a error
> message in case the reset fails and return 0 anyhow. (Returning an error
> code, doesn't stop the platform device unregistration and only results
> in a little helpful error message before the devm cleanup handlers are
> called.)
> 
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

Applied.

Thanks,
Guenter

> ---
> Hello,
> 
> the motivation for this patch is to fix the driver to not return an
> error code in .remove(). The long term goal is to make remove callbacks
> return void as returning an error is nearly always wrong and doesn't
> have the effect that driver authors think it has. This patch is a
> preparation for this conversion.
> 
> Best regards
> Uwe
> 
>  drivers/hwmon/sht15.c | 17 ++++++-----------
>  1 file changed, 6 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/hwmon/sht15.c b/drivers/hwmon/sht15.c
> index 7f4a63959730..ae4d14257a11 100644
> --- a/drivers/hwmon/sht15.c
> +++ b/drivers/hwmon/sht15.c
> @@ -1020,25 +1020,20 @@ static int sht15_probe(struct platform_device *pdev)
>  static int sht15_remove(struct platform_device *pdev)
>  {
>  	struct sht15_data *data = platform_get_drvdata(pdev);
> +	int ret;
>  
> -	/*
> -	 * Make sure any reads from the device are done and
> -	 * prevent new ones beginning
> -	 */
> -	mutex_lock(&data->read_lock);
> -	if (sht15_soft_reset(data)) {
> -		mutex_unlock(&data->read_lock);
> -		return -EFAULT;
> -	}
>  	hwmon_device_unregister(data->hwmon_dev);
>  	sysfs_remove_group(&pdev->dev.kobj, &sht15_attr_group);
> +
> +	ret = sht15_soft_reset(data);
> +	if (ret)
> +		dev_err(&pdev->dev, "Failed to reset device (%pe)\n", ERR_PTR(ret));
> +
>  	if (!IS_ERR(data->reg)) {
>  		regulator_unregister_notifier(data->reg, &data->nb);
>  		regulator_disable(data->reg);
>  	}
>  
> -	mutex_unlock(&data->read_lock);
> -
>  	return 0;
>  }
>