Patch "drm/i915: Snapshot seqno of most recently submitted request." has been added to the 4.1-stable tree

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

drm/i915: Snapshot seqno of most recently submitted request.

to the 4.1-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-i915-snapshot-seqno-of-most-recently-submitted-request.patch
and it can be found in the queue-4.1 subdirectory.

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

From: Tomas Elf <[email protected]>
Date: Thu, 9 Jul 2015 15:30:57 +0100
Subject: drm/i915: Snapshot seqno of most recently submitted request.

From: Tomas Elf <[email protected]>

commit 94f7bbe1509731bdef651d7fb235b2c31fb23be8 upstream.

The hang checker needs to inspect whether or not the ring request list is empty as well as if the given engine has reached or passed the most recently submitted request. The problem with this is that the hang checker cannot grab the struct_mutex, which is required in order to safely inspect requests since requests might be deallocated during inspection. In the past we've had kernel panics due to this very unsynchronized access in the hang checker.

One solution to this problem is to not inspect the requests directly since we're only interested in the seqno of the most recently submitted request - not the request itself. Instead the seqno of the most recently submitted request is stored separately, which the hang checker then inspects, circumventing the issue of synchronization from the hang checker entirely.

This fixes a regression introduced in

commit 44cdd6d219bc64f6810b8ed0023a4d4db9e0fe68

Author: John Harrison <[email protected]>
Date:   Mon Nov 24 18:49:40 2014 +0000

drm/i915: Convert 'ring_idle()' to use requests not seqnos

v2 (Chris Wilson):
- Pass current engine seqno to ring_idle() from i915_hangcheck_elapsed() rather than compute it over again.
- Remove extra whitespace.

Issue: VIZ-5998

---
 drivers/gpu/drm/i915/i915_gem.c         |    1 +
 drivers/gpu/drm/i915/i915_irq.c         |   13 +++----------
 drivers/gpu/drm/i915/intel_ringbuffer.h |    7 +++++++
 3 files changed, 11 insertions(+), 10 deletions(-)

@@ -2401,6 +2401,7 @@ int __i915_add_request(struct intel_engi
 	}
 
 	request->emitted_jiffies = jiffies;
+	ring->last_submitted_seqno = request->seqno;
 	list_add_tail(&request->list, &ring->request_list);
 	request->file_priv = NULL;
 

@@ -2650,18 +2650,11 @@ static void gen8_disable_vblank(struct d
 	spin_unlock_irqrestore(&dev_priv->irq_lock, irqflags);
 }
 
-static struct drm_i915_gem_request *
-ring_last_request(struct intel_engine_cs *ring)
-{
-	return list_entry(ring->request_list.prev,
-			  struct drm_i915_gem_request, list);
-}
-
 static bool
-ring_idle(struct intel_engine_cs *ring)
+ring_idle(struct intel_engine_cs *ring, u32 seqno)
 {
 	return (list_empty(&ring->request_list) ||
-		i915_gem_request_completed(ring_last_request(ring), false));
+		i915_seqno_passed(seqno, ring->last_submitted_seqno));
 }
 
 static bool

@@ -2883,7 +2876,7 @@ static void i915_hangcheck_elapsed(struc
 		acthd = intel_ring_get_active_head(ring);
 
 		if (ring->hangcheck.seqno == seqno) {
-			if (ring_idle(ring)) {
+			if (ring_idle(ring, seqno)) {
 				ring->hangcheck.action = HANGCHECK_IDLE;
 
 				if (waitqueue_active(&ring->irq_queue)) {

@@ -266,6 +266,13 @@ struct  intel_engine_cs {
 	 * Do we have some not yet emitted requests outstanding?
 	 */
 	struct drm_i915_gem_request *outstanding_lazy_request;
+	/**
+	 * Seqno of request most recently submitted to request_list.
+	 * Used exclusively by hang checker to avoid grabbing lock while
+	 * inspecting request list.
+	 */
+	u32 last_submitted_seqno;
+
 	bool gpu_caches_dirty;
 
 	wait_queue_head_t irq_queue;

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

queue-4.1/drm-i915-snapshot-seqno-of-most-recently-submitted-request.patch