linux-kernel - Re: [PATCH] hwmon: pmbus: protect read-modify-write with lock

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Date:   Thu, 30 May 2019 10:21:20 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     "Adamski, Krzysztof (Nokia - PL/Wroclaw)" 
        <krzysztof.adamski@...ia.com>
Cc:     Jean Delvare <jdelvare@...e.com>,
        "linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] hwmon: pmbus: protect read-modify-write with lock

Hi,

On Thu, May 30, 2019 at 06:45:48AM +0000, Adamski, Krzysztof (Nokia - PL/Wroclaw) wrote:
> The operation done in the pmbus_update_fan() function is a
> read-modify-write operation but it lacks any kind of lock protection
> which may cause problems if run more than once simultaneously. This
> patch uses an existing update_lock mutex to fix this problem.
> 
> Signed-off-by: Krzysztof Adamski <krzysztof.adamski@...ia.com>
> ---
> 
> I'm resending this patch to proper recipients this time. Sorry if the
> previous submission confused anybody.
> 
>  drivers/hwmon/pmbus/pmbus_core.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/hwmon/pmbus/pmbus_core.c b/drivers/hwmon/pmbus/pmbus_core.c
> index ef7ee90ee785..94adbede7912 100644
> --- a/drivers/hwmon/pmbus/pmbus_core.c
> +++ b/drivers/hwmon/pmbus/pmbus_core.c
> @@ -268,6 +268,7 @@ int pmbus_update_fan(struct i2c_client *client, int page, int id,
>  	int rv;
>  	u8 to;
>  
> +	mutex_lock(&data->update_lock);
>  	from = pmbus_read_byte_data(client, page,
>  				    pmbus_fan_config_registers[id]);
>  	if (from < 0)
> @@ -278,11 +279,15 @@ int pmbus_update_fan(struct i2c_client *client, int page, int id,
>  		rv = pmbus_write_byte_data(client, page,
>  					   pmbus_fan_config_registers[id], to);
>  		if (rv < 0)
> -			return rv;
> +			goto out;
>  	}
>  
> -	return _pmbus_write_word_data(client, page,
> -				      pmbus_fan_command_registers[id], command);
> +	rv = _pmbus_write_word_data(client, page,
> +				    pmbus_fan_command_registers[id], command);
> +
> +out:
> +	mutex_lock(&data->update_lock);

Should be mutex_unlock(), meaning you have not tested this ;-).

Either case, I think this is unnecessary. The function is (or should be)
always called with the lock already taken (ie with pmbus_set_sensor()
in the call path). If not, we would need a locked and an unlocked version
of this function to avoid lock recursion.

Thanks,
Guenter

> +	return rv;
>  }
>  EXPORT_SYMBOL_GPL(pmbus_update_fan);
>  
> -- 
> 2.20.1
>