From: Chris Wilson <chris@chris-wilson.co.uk>
Date: Tue, 28 Jan 2020 17:16:14 +0000 (+0000)
Subject: drm/i915/execlist: Mark up racy read of execlists->pending[0]
X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=8a5746982e0fdabae85990d90a9b80e107fcd450;p=linux.git

drm/i915/execlist: Mark up racy read of execlists->pending[0]

We write to execlists->pending[0] in process_csb() to acknowledge the
completion of the ESLP update, outside of the main spinlock. When we
check the current status of the previous submission in
__execlists_submission_tasklet() we should therefore use READ_ONCE() to
reflect and document the unsynchronized read.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200128171614.3845825-1-chris@chris-wilson.co.uk
---

diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index f1ea2138cbba2..677efdc7c3017 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -2347,7 +2347,7 @@ static void process_csb(struct intel_engine_cs *engine)
 static void __execlists_submission_tasklet(struct intel_engine_cs *const engine)
 {
 	lockdep_assert_held(&engine->active.lock);
-	if (!engine->execlists.pending[0]) {
+	if (!READ_ONCE(engine->execlists.pending[0])) {
 		rcu_read_lock(); /* protect peeking at execlists->active */
 		execlists_dequeue(engine);
 		rcu_read_unlock();