Re: [PATCH] drm: bridge: cdns-mhdp8546: Fix PM reference leak in cdns_mhdp_probe()

[Johan Hovold]

On Mon, May 17, 2021 at 11:27:38AM +0200, Robert Foss wrote:
> Hey Yu,
> 
> On Mon, 17 May 2021 at 10:08, Yu Kuai <yukuai3@xxxxxxxxxx> wrote:
> >
> > pm_runtime_get_sync will increment pm usage counter even it failed.
> > Forgetting to putting operation will result in reference leak here.
> > Fix it by replacing it with pm_runtime_resume_and_get to keep usage
> > counter balanced.
> >
> > Reported-by: Hulk Robot <hulkci@xxxxxxxxxx>
> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > ---
> >  drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
> > index 0cd8f40fb690..305489d48c16 100644
> > --- a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
> > +++ b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
> > @@ -2478,7 +2478,7 @@ static int cdns_mhdp_probe(struct platform_device *pdev)
> >         clk_prepare_enable(clk);
> >
> >         pm_runtime_enable(dev);
> > -       ret = pm_runtime_get_sync(dev);
> > +       ret = pm_runtime_resume_and_get(dev);
> >         if (ret < 0) {
> >                 dev_err(dev, "pm_runtime_get_sync failed\n");
> >                 pm_runtime_disable(dev);
> 
> The code is correct as it is. If pm_runtime_get_sync() fails and
> increments[1] the pm.usage_count variable, that isn't a problem since
> pm_runtime_disable() disables pm, and resets pm.usage_count variable
> to zero[2].

No it doesn't; pm_runtime_disable() does not reset the counter and you
still need to decrement the usage count when pm_runtime_get_sync()
fails.

> [1] https://elixir.bootlin.com/linux/latest/source/include/linux/pm_runtime.h#L384
> [2] https://elixir.bootlin.com/linux/latest/source/drivers/base/power/runtime.c#L1383

Johan