Re: [PATCH 1/2] drm/i915/gt: Add to timeline requires the timeline mutex

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On 25/07/2019 14:14, Chris Wilson wrote:
Modifying a remote context requires careful serialisation with requests
on that context, and that serialisation requires us to take their
timeline->mutex. Make it so.

Note that while struct_mutex rules, we can't create more than one
request in parallel, but that age is soon coming to an end.

v2: Though it doesn't affect the current users, contexts may share
timelines so check if we already hold the right mutex.

Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
  drivers/gpu/drm/i915/gt/intel_context.c | 23 ++++++++++++++++++-----
  1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
index 9292b6ca5e9c..d64b45f7ec6d 100644
--- a/drivers/gpu/drm/i915/gt/intel_context.c
+++ b/drivers/gpu/drm/i915/gt/intel_context.c
@@ -254,10 +254,18 @@ int intel_context_prepare_remote_request(struct intel_context *ce,
  	/* Only suitable for use in remotely modifying this context */
  	GEM_BUG_ON(rq->hw_context == ce);
- /* Queue this switch after all other activity by this context. */
-	err = i915_active_request_set(&tl->last_request, rq);
-	if (err)
-		return err;
+	if (rq->timeline != tl) { /* beware timeline sharing */
+		err = mutex_lock_interruptible_nested(&tl->mutex,
+						      SINGLE_DEPTH_NESTING);
+		if (err)
+			return err;
+
+		/* Queue this switch after current activity by this context. */
+		err = i915_active_request_set(&tl->last_request, rq);
+		if (err)
+			goto unlock;
+	}
+	lockdep_assert_held(&tl->mutex);
/*
  	 * Guarantee context image and the timeline remains pinned until the
@@ -267,7 +275,12 @@ int intel_context_prepare_remote_request(struct intel_context *ce,
  	 * words transfer the pinned ce object to tracked active request.
  	 */
  	GEM_BUG_ON(i915_active_is_idle(&ce->active));
-	return i915_active_ref(&ce->active, rq->fence.context, rq);
+	err = i915_active_ref(&ce->active, rq->fence.context, rq);
+
+unlock:
+	if (rq->timeline != tl)
+		mutex_unlock(&tl->mutex);
+	return err;
  }
struct i915_request *intel_context_create_request(struct intel_context *ce)


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

Regards,

Tvrtko
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