drm/i915/gt: Defend against concurrent updates to execlists->active

[  206.875637] BUG: KCSAN: data-race in __i915_schedule+0x7fc/0x930 [i915]
[  206.875654]
[  206.875666] race at unknown origin, with read to 0xffff8881f7644480 of 8 bytes by task 703 on cpu 3:
[  206.875901]  __i915_schedule+0x7fc/0x930 [i915]
[  206.876130]  __bump_priority+0x63/0x80 [i915]
[  206.876361]  __i915_sched_node_add_dependency+0x258/0x300 [i915]
[  206.876593]  i915_sched_node_add_dependency+0x50/0xa0 [i915]
[  206.876824]  i915_request_await_dma_fence+0x1da/0x530 [i915]
[  206.877057]  i915_request_await_object+0x2fe/0x470 [i915]
[  206.877287]  i915_gem_do_execbuffer+0x45dc/0x4c20 [i915]
[  206.877517]  i915_gem_execbuffer2_ioctl+0x2c3/0x580 [i915]
[  206.877535]  drm_ioctl_kernel+0xe4/0x120
[  206.877549]  drm_ioctl+0x297/0x4c7
[  206.877563]  ksys_ioctl+0x89/0xb0
[  206.877577]  __x64_sys_ioctl+0x42/0x60
[  206.877591]  do_syscall_64+0x6e/0x2c0
[  206.877606]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

v2: Be safe and include mb

References: https://gitlab.freedesktop.org/drm/intel/issues/1318Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200309170540.10332-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_engine.h

@@ -107,7 +107,20 @@ execlists_num_ports(const struct intel_engine_execlists * const execlists)
 static inline struct i915_request *
 execlists_active(const struct intel_engine_execlists *execlists)
 {
-	return *READ_ONCE(execlists->active);
+	struct i915_request * const *cur, * const *old, *active;
+
+	cur = READ_ONCE(execlists->active);
+	smp_rmb(); /* pairs with overwrite protection in process_csb() */
+	do {
+		old = cur;
+
+		active = READ_ONCE(*cur);
+		cur = READ_ONCE(execlists->active);
+
+		smp_rmb(); /* and complete the seqlock retry */
+	} while (unlikely(cur != old));
+
+	return active;
 }
 
 static inline void

drivers/gpu/drm/i915/gt/intel_lrc.c

@@ -2232,6 +2232,7 @@ cancel_port_requests(struct intel_engine_execlists * const execlists)
 		execlists_schedule_out(*port);
 	clear_ports(execlists->inflight, ARRAY_SIZE(execlists->inflight));
 
+	smp_wmb(); /* complete the seqlock for execlists_active() */
 	WRITE_ONCE(execlists->active, execlists->inflight);
 }
 
@@ -2386,6 +2387,7 @@ static void process_csb(struct intel_engine_cs *engine)
 
 			/* Point active to the new ELSP; prevent overwriting */
 			WRITE_ONCE(execlists->active, execlists->pending);
+			smp_wmb(); /* notify execlists_active() */
 
 			/* cancel old inflight, prepare for switch */
 			trace_ports(execlists, "preempted", old);
@@ -2393,11 +2395,12 @@ static void process_csb(struct intel_engine_cs *engine)
 				execlists_schedule_out(*old++);
 
 			/* switch pending to inflight */
-			WRITE_ONCE(execlists->active,
-				   memcpy(execlists->inflight,
-					  execlists->pending,
-					  execlists_num_ports(execlists) *
-					  sizeof(*execlists->pending)));
+			memcpy(execlists->inflight,
+			       execlists->pending,
+			       execlists_num_ports(execlists) *
+			       sizeof(*execlists->pending));
+			smp_wmb(); /* complete the seqlock */
+			WRITE_ONCE(execlists->active, execlists->inflight);
 
 			WRITE_ONCE(execlists->pending[0], NULL);
 		} else {