linux-kernel - Re: [PATCH v2 3/3] pmbus

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Date:   Tue, 25 Jul 2023 08:06:45 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Naresh Solanki <naresh.solanki@...ements.com>
Cc:     linux-hwmon@...r.kernel.org, Jean Delvare <jdelvare@...e.com>,
        Patrick Rudolph <patrick.rudolph@...ements.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] pmbus_core: Fix Deadlock

On Tue, Jul 25, 2023 at 02:54:27PM +0200, Naresh Solanki wrote:
> From: Patrick Rudolph <patrick.rudolph@...ements.com>
> 
> pmbus_regulator_get_error_flags() will also acquire the update_lock,
> thus unlock the mutex before trying to lock it again from within
> the same thread.
> 
> Fixes a deadlock when trying to read the regulator status.
> 

The idea was that the lock would cover both pmbus_get_status()
and pmbus_regulator_get_error_flags() to avoid inconsistencies.
This change defeats that purpose.

> Signed-off-by: Patrick Rudolph <patrick.rudolph@...ements.com>
> Signed-off-by: Naresh Solanki <Naresh.Solanki@...ements.com>
> ---
>  drivers/hwmon/pmbus/pmbus_core.c | 30 ++++++++++--------------------
>  1 file changed, 10 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/hwmon/pmbus/pmbus_core.c b/drivers/hwmon/pmbus/pmbus_core.c
> index 30aeb59062a5..42f4250c53ed 100644
> --- a/drivers/hwmon/pmbus/pmbus_core.c
> +++ b/drivers/hwmon/pmbus/pmbus_core.c
> @@ -2949,37 +2949,27 @@ static int pmbus_regulator_get_status(struct regulator_dev *rdev)
>  
>  	mutex_lock(&data->update_lock);
>  	status = pmbus_get_status(client, page, PMBUS_STATUS_WORD);
> -	if (status < 0) {
> -		ret = status;
> -		goto unlock;
> -	}
> +	mutex_unlock(&data->update_lock);
> +	if (status < 0)
> +		return status;
>  
> -	if (status & PB_STATUS_OFF) {
> -		ret = REGULATOR_STATUS_OFF;
> -		goto unlock;
> -	}
> +	if (status & PB_STATUS_OFF)
> +		return REGULATOR_STATUS_OFF;
>  
>  	/* If regulator is ON & reports power good then return ON */
> -	if (!(status & PB_STATUS_POWER_GOOD_N)) {
> -		ret = REGULATOR_STATUS_ON;
> -		goto unlock;
> -	}
> +	if (!(status & PB_STATUS_POWER_GOOD_N))
> +		return REGULATOR_STATUS_ON;
>  
>  	ret = pmbus_regulator_get_error_flags(rdev, &status);

Why not just call _pmbus_get_flags() here instead ?

Guenter

>  	if (ret)
> -		goto unlock;
> +		return ret;
>  
>  	if (status & (REGULATOR_ERROR_UNDER_VOLTAGE | REGULATOR_ERROR_OVER_CURRENT |
>  	   REGULATOR_ERROR_REGULATION_OUT | REGULATOR_ERROR_FAIL | REGULATOR_ERROR_OVER_TEMP)) {
> -		ret = REGULATOR_STATUS_ERROR;
> -		goto unlock;
> +		return REGULATOR_STATUS_ERROR;
>  	}
>  
> -	ret = REGULATOR_STATUS_UNDEFINED;
> -
> -unlock:
> -	mutex_unlock(&data->update_lock);
> -	return ret;
> +	return REGULATOR_STATUS_UNDEFINED;
>  }
>  
>  static int pmbus_regulator_get_low_margin(struct i2c_client *client, int page)