[PATCH] drm: Fix deadlock due to getconnector locking changes

[Marc Finet]

On Sun, Feb 22, 2015 at 11:38:36AM +0100, Daniel Vetter wrote:
> In
> 
> daniel at phenom:~/linux/src$ git show ccfc08655
> commit ccfc08655d5fd5076828f45fb09194c070f2f63a
> Author: Rob Clark 
> Date:   Thu Dec 18 16:01:48 2014 -0500
> 
> drm: tweak getconnector locking
> 
> We need to extend the locking to cover connector->state reading for
> atomic drivers, but the above commit was a bit too eager and also
> included the fill_modes callback. Which on i915 on old platforms using
> load detection needs to acquire modeset locks, resulting in a deadlock
> on output probing.
> 
> Reported-by: Marc Finet 
> Cc: Marc Finet 
> Cc: robdclark at gmail.com
> Signed-off-by: Daniel Vetter 

Tested-by: Marc Finet 

Thanks.
> ---
>  drivers/gpu/drm/drm_crtc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> index b15d720eda4c..ce5f1193ecd6 100644
> --- a/drivers/gpu/drm/drm_crtc.c
> +++ b/drivers/gpu/drm/drm_crtc.c
> @@ -2180,7 +2180,6 @@ int drm_mode_getconnector(struct drm_device *dev, void 
> *data,
>   DRM_DEBUG_KMS("[CONNECTOR:%d:?]\n", out_resp->connector_id);
>  
>   mutex_lock(>mode_config.mutex);
> - drm_modeset_lock(>mode_config.connection_mutex, NULL);
>  
>   connector = drm_connector_find(dev, out_resp->connector_id);
>   if (!connector) {
> @@ -2210,6 +2209,8 @@ int drm_mode_getconnector(struct drm_device *dev, void 
> *data,
>   out_resp->mm_height = connector->display_info.height_mm;
>   out_resp->subpixel = connector->display_info.subpixel_order;
>   out_resp->connection = connector->status;
> +
> + drm_modeset_lock(>mode_config.connection_mutex, NULL);
>   encoder = drm_connector_get_encoder(connector);
>   if (encoder)
>   out_resp->encoder_id = encoder->base.id;
> -- 
> 2.1.4
> 

Deadlock in Xorg after commit "drm: tweak getconnector locking"

[Marc Finet]

Hello,

After trying linux-next (next-20150130) on my i915-powered laptop, I failed to
get Xorg running (just a white underscore).
I bisected it to commit ccfc08655 "drm: tweak getconnector locking".

Issuing a Sysrq+T showed that Xorg has the following call-trace:
> drm_modeset_lock+0x30/0xf0 [drm]
> intel_get_load_detect_pipe+0x8f/0x480 [i915]
> intel_crt_get_edid+0x63/0x90 [i915]
> intel_crt_detect_ddc+0x51/0xe0 [i915]
> intel_crt_detect_ddc+0x51/0xe0 [i915]
> intel_crt_detect+0x45e/0x8d0 [i915]
> drm_helper_probe_single_connector_modes_merge_bits+0x278/0x410 
> [drm_kms_helper]
> drm_helper_probe_single_connector_modes+0x17/0x20 [drm_kms_helper]
> drm_mode_getconnector+0x295/0x330 [drm]
> drm_mode_getcrtc+0xd0/0xd0 [drm]
> drm_ioctl+0x1ed/0x560 [drm]
> drm_mode_getcrtc+0xd0/0xd0 [drm]
> drm_getmap+0xc0/0xc0 [drm]

In the mentioned commit, drm_modeset_lock() has been moved (too?) up in
drm_mode_getconnector() hence connector->funcs->fill_modes() 
later calls drm_modeset_lock() too.

Should I provide more info ?

Marc.