Re: [PATCH] soundwire: bus: Fix unbalanced pm_runtime_put() causing usage count underflow

[Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>]

On 2/27/23 06:29, Richard Fitzgerald wrote:
> This reverts commit
> 443a98e649b4 ("soundwire: bus: use pm_runtime_resume_and_get()")
> 
> Change calls to pm_runtime_resume_and_get() back to pm_runtime_get_sync().
> This fixes a usage count underrun caused by doing a pm_runtime_put() even
> though pm_runtime_resume_and_get() returned an error.
> 
> The three affected functions ignore -EACCES error from trying to get
> pm_runtime, and carry on, including a put at the end of the function.
> But pm_runtime_resume_and_get() does not increment the usage count if it
> returns an error. So in the -EACCES case you must not call
> pm_runtime_put().
> 
> The documentation for pm_runtime_get_sync() says:
>  "Consider using pm_runtime_resume_and_get() ...  as this is likely to
>  result in cleaner code."
> 
> In this case I don't think it results in cleaner code because the
> pm_runtime_put() at the end of the function would have to be conditional on
> the return value from pm_runtime_resume_and_get() at the top of the
> function.
> 
> pm_runtime_get_sync() doesn't have this problem because it always
> increments the count, so always needs a put. The code can just flow through
> and do the pm_runtime_put() unconditionally.
> 
> Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>

Agreed, in hindsight the move to use pm_runtime_resume_and_get() mostly
added issues left and right with limited benefits - just too hard to
review and figure out what cases work and which ones don't.

Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>



> ---
>  drivers/soundwire/bus.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/soundwire/bus.c b/drivers/soundwire/bus.c
> index 76515c33e639..4fd221d0cc81 100644
> --- a/drivers/soundwire/bus.c
> +++ b/drivers/soundwire/bus.c
> @@ -571,9 +571,11 @@ int sdw_nread(struct sdw_slave *slave, u32 addr, size_t count, u8 *val)
>  {
>  	int ret;
>  
> -	ret = pm_runtime_resume_and_get(&slave->dev);
> -	if (ret < 0 && ret != -EACCES)
> +	ret = pm_runtime_get_sync(&slave->dev);
> +	if (ret < 0 && ret != -EACCES) {
> +		pm_runtime_put_noidle(&slave->dev);
>  		return ret;
> +	}
>  
>  	ret = sdw_nread_no_pm(slave, addr, count, val);
>  
> @@ -595,9 +597,11 @@ int sdw_nwrite(struct sdw_slave *slave, u32 addr, size_t count, const u8 *val)
>  {
>  	int ret;
>  
> -	ret = pm_runtime_resume_and_get(&slave->dev);
> -	if (ret < 0 && ret != -EACCES)
> +	ret = pm_runtime_get_sync(&slave->dev);
> +	if (ret < 0 && ret != -EACCES) {
> +		pm_runtime_put_noidle(&slave->dev);
>  		return ret;
> +	}
>  
>  	ret = sdw_nwrite_no_pm(slave, addr, count, val);
>  
> @@ -1565,9 +1569,10 @@ static int sdw_handle_slave_alerts(struct sdw_slave *slave)
>  
>  	sdw_modify_slave_status(slave, SDW_SLAVE_ALERT);
>  
> -	ret = pm_runtime_resume_and_get(&slave->dev);
> +	ret = pm_runtime_get_sync(&slave->dev);
>  	if (ret < 0 && ret != -EACCES) {
>  		dev_err(&slave->dev, "Failed to resume device: %d\n", ret);
> +		pm_runtime_put_noidle(&slave->dev);
>  		return ret;
>  	}
>