drm/i915: Protect the request->global_seqno with the engine->timeline lock

A request is assigned a global seqno only when it is on the hardware
execution queue. The global seqno can be used to maintain a list of
requests on the same engine in retirement order, for example for
constructing a priority queue for waiting. Prior to its execution, or
if it is subsequently removed in the event of preemption, its global
seqno is zero. As both insertion and removal from the execution queue
may operate in IRQ context, it is not guarded by the usual struct_mutex
BKL. Instead those relying on the global seqno must be prepared for its
value to change between reads. Only when the request is complete can
the global seqno be stable (due to the memory barriers on submitting
the commands to the hardware to write the breadcrumb, if the HWS shows
that it has passed the global seqno and the global seqno is unchanged
after the read, it is indeed complete).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170223074422.4125-9-chris@chris-wilson.co.uk

1 files changed, 38 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index b91c2c7ef5a6..04e7d5a9ee3a 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -582,10 +582,47 @@ static inline u32 intel_hws_seqno_address(struct intel_engine_cs *engine)
 /* intel_breadcrumbs.c -- user interrupt bottom-half for waiters */
 int intel_engine_init_breadcrumbs(struct intel_engine_cs *engine);
 
-static inline void intel_wait_init(struct intel_wait *wait, u32 seqno)
+static inline void intel_wait_init(struct intel_wait *wait)
 {
 	wait->tsk = current;
+}
+
+static inline void intel_wait_init_for_seqno(struct intel_wait *wait, u32 seqno)
+{
+	wait->tsk = current;
+	wait->seqno = seqno;
+}
+
+static inline bool intel_wait_has_seqno(const struct intel_wait *wait)
+{
+	return wait->seqno;
+}
+
+static inline bool
+intel_wait_update_seqno(struct intel_wait *wait, u32 seqno)
+{
 	wait->seqno = seqno;
+	return intel_wait_has_seqno(wait);
+}
+
+static inline bool
+intel_wait_update_request(struct intel_wait *wait,
+			  const struct drm_i915_gem_request *rq)
+{
+	return intel_wait_update_seqno(wait, i915_gem_request_global_seqno(rq));
+}
+
+static inline bool
+intel_wait_check_seqno(const struct intel_wait *wait, u32 seqno)
+{
+	return wait->seqno == seqno;
+}
+
+static inline bool
+intel_wait_check_request(const struct intel_wait *wait,
+			 const struct drm_i915_gem_request *rq)
+{
+	return intel_wait_check_seqno(wait, i915_gem_request_global_seqno(rq));
 }
 
 static inline bool intel_wait_complete(const struct intel_wait *wait)