linux-kernel - Re: [PATCH] drm/bridge: fix reference count leaks due to pm_runtime_get

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Date:   Sun, 14 Jun 2020 16:46:55 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Aditya Pakki <pakki001@....edu>
Cc:     kjlu@....edu, wu000273@....edu,
        Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH] drm/bridge: fix reference count leaks due to
 pm_runtime_get_sync()

Hi Aditya,

(CC'ing Rafael)

Thank you for the patch.

On Sat, Jun 13, 2020 at 09:40:05PM -0500, Aditya Pakki wrote:
> On calling pm_runtime_get_sync() the reference count of the device
> is incremented. In case of failure, decrement the
> reference count before returning the error.
> 
> Signed-off-by: Aditya Pakki <pakki001@....edu>

I've seen lots of similar patches recently. Instead of mass-patching the
drivers this way, shouldn't pm_runtime_get_sync() (and similar
functions) decrease the refcount on their failure path ? That would
require patching drivers that already handle this issue, but I believe
that would cause less churn, and more importantly, avoid the issue once
and for good for new code.

> ---
>  drivers/gpu/drm/bridge/cdns-dsi.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/cdns-dsi.c b/drivers/gpu/drm/bridge/cdns-dsi.c
> index 69c3892caee5..583cb8547106 100644
> --- a/drivers/gpu/drm/bridge/cdns-dsi.c
> +++ b/drivers/gpu/drm/bridge/cdns-dsi.c
> @@ -788,8 +788,10 @@ static void cdns_dsi_bridge_enable(struct drm_bridge *bridge)
>  	u32 tmp, reg_wakeup, div;
>  	int nlanes;
>  
> -	if (WARN_ON(pm_runtime_get_sync(dsi->base.dev) < 0))
> +	if (WARN_ON(pm_runtime_get_sync(dsi->base.dev) < 0)) {
> +		pm_runtime_put(dsi->base.dev);
>  		return;
> +	}
>  
>  	mode = &bridge->encoder->crtc->state->adjusted_mode;
>  	nlanes = output->dev->lanes;
> @@ -1028,8 +1030,10 @@ static ssize_t cdns_dsi_transfer(struct mipi_dsi_host *host,
>  	int ret, i, tx_len, rx_len;
>  
>  	ret = pm_runtime_get_sync(host->dev);
> -	if (ret < 0)
> +	if (ret < 0) {
> +		pm_runtime_put(host->dev);
>  		return ret;
> +	}
>  
>  	cdns_dsi_init_link(dsi);
>  

-- 
Regards,

Laurent Pinchart