Patch "drm: Allow determining if current task is output poll worker" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

drm: Allow determining if current task is output poll worker

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
drm-allow-determining-if-current-task-is-output-poll-worker.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Lukas Wunner <[email protected]>
Date: Wed, 14 Feb 2018 06:41:25 +0100
Subject: drm: Allow determining if current task is output poll worker

From: Lukas Wunner <[email protected]>

commit 25c058ccaf2ebbc3e250ec1e199e161f91fe27d4 upstream.

Introduce a helper to determine if the current task is an output poll worker.

This allows us to fix a long-standing deadlock in several DRM drivers wherein the ->runtime_suspend callback waits for the output poll worker to finish and the worker in turn calls a ->detect callback which waits for runtime suspend to finish. The ->detect callback is invoked from multiple call sites and waiting for runtime suspend to finish is the correct thing to do except if it's executing in the context of the worker.

v2: Expand kerneldoc to specifically mention deadlock between
output poll worker and autosuspend worker as use case. (Lyude)

---
 drivers/gpu/drm/drm_probe_helper.c |   20 ++++++++++++++++++++
 include/drm/drm_crtc_helper.h      |    1 +
 2 files changed, 21 insertions(+)

@@ -412,6 +412,26 @@ out:
 }
 
 /**
+ * drm_kms_helper_is_poll_worker - is %current task an output poll worker?
+ *
+ * Determine if %current task is an output poll worker.  This can be used
+ * to select distinct code paths for output polling versus other contexts.
+ *
+ * One use case is to avoid a deadlock between the output poll worker and
+ * the autosuspend worker wherein the latter waits for polling to finish
+ * upon calling drm_kms_helper_poll_disable(), while the former waits for
+ * runtime suspend to finish upon calling pm_runtime_get_sync() in a
+ * connector ->detect hook.
+ */
+bool drm_kms_helper_is_poll_worker(void)
+{
+	struct work_struct *work = current_work();
+
+	return work && work->func == output_poll_execute;
+}
+EXPORT_SYMBOL(drm_kms_helper_is_poll_worker);
+
+/**
  * drm_kms_helper_poll_disable - disable output polling
  * @dev: drm_device
  *

@@ -241,5 +241,6 @@ extern void drm_kms_helper_hotplug_event
 extern void drm_kms_helper_poll_disable(struct drm_device *dev);
 extern void drm_kms_helper_poll_enable(struct drm_device *dev);
 extern void drm_kms_helper_poll_enable_locked(struct drm_device *dev);
+extern bool drm_kms_helper_is_poll_worker(void);
 
 #endif

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/workqueue-allow-retrieval-of-current-task-s-work-struct.patch queue-4.4/drm-radeon-fix-deadlock-on-runtime-suspend.patch queue-4.4/drm-amdgpu-fix-deadlock-on-runtime-suspend.patch queue-4.4/drm-allow-determining-if-current-task-is-output-poll-worker.patch queue-4.4/drm-nouveau-fix-deadlock-on-runtime-suspend.patch