From 749085a2131f7e328dd33635eb3d37b519f451f9 Mon Sep 17 00:00:00 2001
From: Chris Wilson <chris@chris-wilson.co.uk>
Date: Wed, 9 Oct 2019 11:09:54 +0100
Subject: drm/i915/execlists: Protect peeking at execlists->active
Git-commit: 749085a2131f7e328dd33635eb3d37b519f451f9
Patch-mainline: v5.4-rc3
References: bsc#1152489

Now that we dropped the engine->active.lock serialisation from around
process_csb(), direct submission can run concurrently to the interrupt
handler. As such execlists->active may be advanced as we dequeue,
dropping the reference to the request. We need to employ our RCU request
protection to ensure that the request is not freed too early.

Fixes: df403069029d ("drm/i915/execlists: Lift process_csb() out of the irq-off spinlock")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191009100955.21477-1-chris@chris-wilson.co.uk
(cherry picked from commit c949ae431467764277cdd88d7c26ff963a9db40a)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 drivers/gpu/drm/i915/gt/intel_lrc.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index b09bd1729d39..bdfcc7bdadbf 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -903,7 +903,7 @@ static void virtual_xfer_breadcrumbs(struct virtual_engine *ve,
 static struct i915_request *
 last_active(const struct intel_engine_execlists *execlists)
 {
-	struct i915_request * const *last = execlists->active;
+	struct i915_request * const *last = READ_ONCE(execlists->active);
 
 	while (*last && i915_request_completed(*last))
 		last++;
@@ -1608,8 +1608,11 @@ 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 (!engine->execlists.pending[0]) {
+		rcu_read_lock(); /* protect peeking at execlists->active */
 		execlists_dequeue(engine);
+		rcu_read_unlock();
+	}
 }
 
 /*
-- 
2.28.0