drm/i915: Use lockdep_pin_lock() over the construction of the request

During request construction, we take the timeline->mutex to ensure
exclusive access to the ringbuffer (for command emission) and the
timeline itself (for command ordering). The timeline->mutex should not
be dropped by callers until we release it in i915_request_add().

lockdep provides a pin/unpin lock facility to detect accidental unlocks
inside critical sections, so put it to use for request construction.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190403082132.327-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_request.c

@@ -751,7 +751,10 @@ i915_request_alloc(struct intel_engine_cs *engine, struct i915_gem_context *ctx)
 	rq->infix = rq->ring->emit; /* end of header; start of user payload */
 
 	/* Check that we didn't interrupt ourselves with a new request */
+	lockdep_assert_held(&rq->timeline->mutex);
 	GEM_BUG_ON(rq->timeline->seqno != rq->fence.seqno);
+	rq->cookie = lockdep_pin_lock(&rq->timeline->mutex);
+
 	return rq;
 
 err_unwind:
@@ -1070,6 +1073,8 @@ void i915_request_add(struct i915_request *request)
 		  engine->name, request->fence.context, request->fence.seqno);
 
 	lockdep_assert_held(&request->timeline->mutex);
+	lockdep_unpin_lock(&request->timeline->mutex, request->cookie);
+
 	trace_i915_request_add(request);
 
 	/*

drivers/gpu/drm/i915/i915_request.h

@@ -26,6 +26,7 @@
 #define I915_REQUEST_H
 
 #include <linux/dma-fence.h>
+#include <linux/lockdep.h>
 
 #include "i915_gem.h"
 #include "i915_scheduler.h"
@@ -120,6 +121,15 @@ struct i915_request {
 	 */
 	unsigned long rcustate;
 
+	/*
+	 * We pin the timeline->mutex while constructing the request to
+	 * ensure that no caller accidentally drops it during construction.
+	 * The timeline->mutex must be held to ensure that only this caller
+	 * can use the ring and manipulate the associated timeline during
+	 * construction.
+	 */
+	struct pin_cookie cookie;
+
 	/*
 	 * Fences for the various phases in the request's lifetime.
 	 *