[Intel-gfx] [PATCH 25/29] drm/i915: Spinlock protection for request list

[John.C.Harrison at Intel.com]

From: John Harrison <John.C.Harrison at Intel.com>

The completion status for all entries in the request list is updated on demand.
This occurs whenever the code queries the completion status of a given request
and a new seqno value has popped out of the hardware. Unfortuntately, not all
such queries are done with the driver mutex lock held. Therefore there is a race
condition between the query processing and the retired request removal code
which can result in a dodgy pointer dereference.

The solution is to spinlock around those two points - the actual list entry
removal and the potentially asynchronous query. This ensures that the query will
never see a node that is in the process of being destroyed.

For: VIZ-4377
Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
---
 drivers/gpu/drm/i915/i915_gem.c |    8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 035735a..c4facbd 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2491,7 +2491,12 @@ static void i915_set_reset_status(struct drm_i915_private *dev_priv,
 
 static void i915_gem_free_request(struct drm_i915_gem_request *request)
 {
+	unsigned long flags;
+
+	spin_lock_irqsave(&request->ring->reqlist_lock, flags);
 	list_del(&request->list);
+	spin_unlock_irqrestore(&request->ring->reqlist_lock, flags);
+
 	i915_gem_request_remove_from_client(request);
 
 	i915_gem_request_unreference(request);
@@ -2647,12 +2652,14 @@ void i915_gem_complete_requests_ring(struct intel_engine_cs *ring,
 				     bool lazy_coherency)
 {
 	struct drm_i915_gem_request *req;
+	unsigned long flags;
 	u32 seqno;
 
 	seqno = ring->get_seqno(ring, lazy_coherency);
 	if (seqno == ring->last_read_seqno)
 		return;
 
+	spin_lock_irqsave(&ring->reqlist_lock, flags);
 	list_for_each_entry(req, &ring->request_list, list) {
 		if (req->complete)
 			continue;
@@ -2660,6 +2667,7 @@ void i915_gem_complete_requests_ring(struct intel_engine_cs *ring,
 		if (i915_seqno_passed(seqno, req->seqno))
 			req->complete = true;
 	}
+	spin_unlock_irqrestore(&ring->reqlist_lock, flags);
 
 	ring->last_read_seqno = seqno;
 }
-- 
1.7.9.5