drm/i915/gem: Take timeline->mutex to walk list-of-requests

Though the context is closed and so no more requests can be added to the
timeline, retirement can still be removing requests. It can even be
removing the very request we are inspecting and so cause us to wander
into dead links.

Serialise with the retirement by taking the timeline->mutex used for
guarding the timeline->requests list.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112404
Fixes: 4a3174152147 ("drm/i915/gem: Refine occupancy test in kill_context()")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191129151845.1092933-1-chris@chris-wilson.co.uk
(cherry picked from commit 7ce596a8036cf3a4cb9ffa0c4edd8a76a7a43cc3)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
index 255ab040022ec01ca1784dc2f0296b9eac1b53a8..4237a2887ff212a591d3ec859060ef8275efe4b8 100644 (file)
--- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
@@ -368,7 +368,7 @@ static struct intel_engine_cs *active_engine(struct intel_context *ce)
        if (!ce->timeline)
                return NULL;
 
-       rcu_read_lock();
+       mutex_lock(&ce->timeline->mutex);
        list_for_each_entry_reverse(rq, &ce->timeline->requests, link) {
                if (i915_request_completed(rq))
                        break;
@@ -378,7 +378,7 @@ static struct intel_engine_cs *active_engine(struct intel_context *ce)
                if (engine)
                        break;
        }
-       rcu_read_unlock();
+       mutex_unlock(&ce->timeline->mutex);
 
        return engine;
 }