drm/i915/gt: Protect defer_request() from new waiters

Mika spotted

<4>[17436.705441] general protection fault: 0000 [#1] PREEMPT SMP PTI
<4>[17436.705447] CPU: 2 PID: 0 Comm: swapper/2 Not tainted 5.5.0+ #1
<4>[17436.705449] Hardware name: System manufacturer System Product Name/Z170M-PLUS, BIOS 3805 05/16/2018
<4>[17436.705512] RIP: 0010:__execlists_submission_tasklet+0xc4d/0x16e0 [i915]
<4>[17436.705516] Code: c5 4c 8d 60 e0 75 17 e9 8c 07 00 00 49 8b 44 24 20 49 39 c5 4c 8d 60 e0 0f 84 7a 07 00 00 49 8b 5c 24 08 49 8b 87 80 00 00 00 <48> 39 83 d8 fe ff ff 75 d9 48 8b 83 88 fe ff ff a8 01 0f 84 b6 05
<4>[17436.705518] RSP: 0018:ffffc9000012ce80 EFLAGS: 00010083
<4>[17436.705521] RAX: ffff88822ae42000 RBX: 5a5a5a5a5a5a5a5a RCX: dead000000000122
<4>[17436.705523] RDX: ffff88822ae42588 RSI: ffff8881e32a7908 RDI: ffff8881c429fd48
<4>[17436.705525] RBP: ffffc9000012cf00 R08: ffff88822ae42588 R09: 00000000fffffffe
<4>[17436.705527] R10: ffff8881c429fb80 R11: 00000000a677cf08 R12: ffff8881c42a0aa8
<4>[17436.705529] R13: ffff8881c429fd38 R14: ffff88822ae42588 R15: ffff8881c429fb80
<4>[17436.705532] FS:  0000000000000000(0000) GS:ffff88822ed00000(0000) knlGS:0000000000000000
<4>[17436.705534] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
<4>[17436.705536] CR2: 00007f858c76d000 CR3: 0000000005610003 CR4: 00000000003606e0
<4>[17436.705538] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
<4>[17436.705540] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
<4>[17436.705542] Call Trace:
<4>[17436.705545]  <IRQ>
<4>[17436.705603]  execlists_submission_tasklet+0xc0/0x130 [i915]

which is us consuming a partially initialised new waiter in
defer_requests(). We can prevent this by initialising the i915_dependency
prior to making it visible, and since we are using a concurrent
list_add/iterator mark them up to the compiler.

Fixes: 8ee36e04 ("drm/i915/execlists: Minimalistic timeslicing")
Signed-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/20200206204915.2636606-2-chris@chris-wilson.co.uk

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

@@ -1615,6 +1615,11 @@ last_active(const struct intel_engine_execlists *execlists)
 	return *last;
 }
 
+#define for_each_waiter(p__, rq__) \
+	list_for_each_entry_lockless(p__, \
+				     &(rq__)->sched.waiters_list, \
+				     wait_link)
+
 static void defer_request(struct i915_request *rq, struct list_head * const pl)
 {
 	LIST_HEAD(list);
@@ -1632,7 +1637,7 @@ static void defer_request(struct i915_request *rq, struct list_head * const pl)
 		GEM_BUG_ON(i915_request_is_active(rq));
 		list_move_tail(&rq->sched.link, pl);
 
-		list_for_each_entry(p, &rq->sched.waiters_list, wait_link) {
+		for_each_waiter(p, rq) {
 			struct i915_request *w =
 				container_of(p->waiter, typeof(*w), sched);
 

drivers/gpu/drm/i915/i915_scheduler.c

@@ -423,8 +423,6 @@ bool __i915_sched_node_add_dependency(struct i915_sched_node *node,
 
 	if (!node_signaled(signal)) {
 		INIT_LIST_HEAD(&dep->dfs_link);
-		list_add(&dep->wait_link, &signal->waiters_list);
-		list_add(&dep->signal_link, &node->signalers_list);
 		dep->signaler = signal;
 		dep->waiter = node;
 		dep->flags = flags;
@@ -434,6 +432,10 @@ bool __i915_sched_node_add_dependency(struct i915_sched_node *node,
 		    !node_started(signal))
 			node->flags |= I915_SCHED_HAS_SEMAPHORE_CHAIN;
 
+		/* All set, now publish. Beware the lockless walkers. */
+		list_add(&dep->signal_link, &node->signalers_list);
+		list_add_rcu(&dep->wait_link, &signal->waiters_list);
+
 		/*
 		 * As we do not allow WAIT to preempt inflight requests,
 		 * once we have executed a request, along with triggering