Re: [PATCH] drm/sysfs: Do not drop device reference twice

[Paulo Zanoni]

2013/10/30 Thierry Reding <thierry.reding@xxxxxxxxx>:
> device_unregister() already drops its reference to the struct device, so
> explicitly calling put_device() before device_unregister() can cause the
> device to have been freed before it can be unregistered.
>
> Signed-off-by: Thierry Reding <treding@xxxxxxxxxx>

I started investigating this problem yesterday and reached the same
conclusion. The connector path can be easily reproduced on i915.ko:
get a machine that has an eDP panel, physically disconnect the panel,
boot the machine, "modprobe i915" and watch the segfault.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@xxxxxxxxx>
Tested-by: Paulo Zanoni <paulo.r.zanoni@xxxxxxxxx>

I didn't really bisect, but I believe this is probably a regression
from "drm/sysfs: sort out minor and connector device object
lifetimes".

And kudos to whoever invented CONFIG_DEBUG_KOBJECT :)

> ---
>  drivers/gpu/drm/drm_sysfs.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_sysfs.c b/drivers/gpu/drm/drm_sysfs.c
> index dae42c7..db1c8f9 100644
> --- a/drivers/gpu/drm/drm_sysfs.c
> +++ b/drivers/gpu/drm/drm_sysfs.c
> @@ -439,7 +439,6 @@ err_out_files:
>                 device_remove_file(connector->kdev, &connector_attrs_opt1[i]);
>         for (i = 0; i < attr_cnt; i++)
>                 device_remove_file(connector->kdev, &connector_attrs[i]);
> -       put_device(connector->kdev);
>         device_unregister(connector->kdev);
>
>  out:
> @@ -472,7 +471,6 @@ void drm_sysfs_connector_remove(struct drm_connector *connector)
>         for (i = 0; i < ARRAY_SIZE(connector_attrs); i++)
>                 device_remove_file(connector->kdev, &connector_attrs[i]);
>         sysfs_remove_bin_file(&connector->kdev->kobj, &edid_attr);
> -       put_device(connector->kdev);
>         device_unregister(connector->kdev);
>         connector->kdev = NULL;
>  }
> --
> 1.8.4
>
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-- 
Paulo Zanoni
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