[PATCH] drm: Fix getconnector connection_mutex locking

[PATCH] drm: Fix getconnector connection_mutex locking

[Daniel Vetter]

I've fumbled my own idea and enthusiastically wrapped all the
getconnector code with the connection_mutex. But we only need it to
chase the connector->encoder link. Even there it's not really needed
since races with userspace won't matter, but better paranoid and
consistent about this stuff.

If we grap it everywhere connector probe callbacks can't grab it
themselves, which means they'll deadlock. i915 does that for the load
detect pipe. Furthermore i915 needs to do a ww dance since we also
need to grab the mutex of the load detect crtc.

This is a regression from

commit 6e9f798d91c526982cca0026cd451e8fdbf18aaf
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu May 29 23:54:47 2014 +0200

    drm: Split connection_mutex out of mode_config.mutex (v3)

Cc: Rob Clark <robdclark@gmail.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
 drivers/gpu/drm/drm_crtc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index b8cc4fe0a0f7..fe94cc10cd35 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -1838,7 +1838,6 @@ int drm_mode_getconnector(struct drm_device *dev, void *data,
 	DRM_DEBUG_KMS("[CONNECTOR:%d:?]\n", out_resp->connector_id);
 
 	mutex_lock(&dev->mode_config.mutex);
-	drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
 
 	connector = drm_connector_find(dev, out_resp->connector_id);
 	if (!connector) {
@@ -1872,10 +1871,12 @@ 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(&dev->mode_config.connection_mutex, NULL);
 	if (connector->encoder)
 		out_resp->encoder_id = connector->encoder->base.id;
 	else
 		out_resp->encoder_id = 0;
+	drm_modeset_unlock(&dev->mode_config.connection_mutex);
 
 	/*
 	 * This ioctl is called twice, once to determine how much space is
@@ -1937,7 +1938,6 @@ int drm_mode_getconnector(struct drm_device *dev, void *data,
 	out_resp->count_encoders = encoders_count;
 
 out:
-	drm_modeset_unlock(&dev->mode_config.connection_mutex);
 	mutex_unlock(&dev->mode_config.mutex);
 
 	return ret;
-- 
2.0.0

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Re: [PATCH] drm: Fix getconnector connection_mutex locking

[Rob Clark]

On Thu, Jun 5, 2014 at 5:12 AM, Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> I've fumbled my own idea and enthusiastically wrapped all the
> getconnector code with the connection_mutex. But we only need it to
> chase the connector->encoder link. Even there it's not really needed
> since races with userspace won't matter, but better paranoid and
> consistent about this stuff.
>
> If we grap it everywhere connector probe callbacks can't grab it
> themselves, which means they'll deadlock. i915 does that for the load
> detect pipe. Furthermore i915 needs to do a ww dance since we also
> need to grab the mutex of the load detect crtc.
>
> This is a regression from
>
> commit 6e9f798d91c526982cca0026cd451e8fdbf18aaf
> Author: Daniel Vetter <daniel.vetter@ffwll.ch>
> Date:   Thu May 29 23:54:47 2014 +0200
>
>     drm: Split connection_mutex out of mode_config.mutex (v3)
>
> Cc: Rob Clark <robdclark@gmail.com>
> Cc: Dave Airlie <airlied@redhat.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Reviewed-by: Rob Clark <robdclark@gmail.com>


> ---
>  drivers/gpu/drm/drm_crtc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> index b8cc4fe0a0f7..fe94cc10cd35 100644
> --- a/drivers/gpu/drm/drm_crtc.c
> +++ b/drivers/gpu/drm/drm_crtc.c
> @@ -1838,7 +1838,6 @@ int drm_mode_getconnector(struct drm_device *dev, void *data,
>         DRM_DEBUG_KMS("[CONNECTOR:%d:?]\n", out_resp->connector_id);
>
>         mutex_lock(&dev->mode_config.mutex);
> -       drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
>
>         connector = drm_connector_find(dev, out_resp->connector_id);
>         if (!connector) {
> @@ -1872,10 +1871,12 @@ 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(&dev->mode_config.connection_mutex, NULL);
>         if (connector->encoder)
>                 out_resp->encoder_id = connector->encoder->base.id;
>         else
>                 out_resp->encoder_id = 0;
> +       drm_modeset_unlock(&dev->mode_config.connection_mutex);
>
>         /*
>          * This ioctl is called twice, once to determine how much space is
> @@ -1937,7 +1938,6 @@ int drm_mode_getconnector(struct drm_device *dev, void *data,
>         out_resp->count_encoders = encoders_count;
>
>  out:
> -       drm_modeset_unlock(&dev->mode_config.connection_mutex);
>         mutex_unlock(&dev->mode_config.mutex);
>
>         return ret;
> --
> 2.0.0
>
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