From: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Date: Mon, 23 Apr 2018 14:37:54 +0300
Subject: drm/i915: Add compiler barrier to wait_for
Git-commit: 1c3c1dc66a9664a9b7048c3869fa7863be9123a0
Patch-mainline: v4.18-rc1
References: FATE#326289 FATE#326079 FATE#326049 FATE#322398 FATE#326166

We need to be careful to not let compiler evaluate
the expiration and the operation on it's terms.

Document and enforce that COND will be evaluated
before checking timeout expiration.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180423113754.28424-2-mika.kuoppala@linux.intel.com

Acked-by: Petr Tesarik <ptesarik@suse.com>
---
 drivers/gpu/drm/i915/intel_drv.h |    4 ++++
 1 file changed, 4 insertions(+)

--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -56,6 +56,8 @@
 	for (;;) {							\
 		const bool expired__ = ktime_after(ktime_get_raw(), end__); \
 		OP;							\
+		/* Guarantee COND check prior to timeout */		\
+		barrier();						\
 		if (COND) {						\
 			ret__ = 0;					\
 			break;						\
@@ -96,6 +98,8 @@
 		u64 now = local_clock(); \
 		if (!(ATOMIC)) \
 			preempt_enable(); \
+		/* Guarantee COND check prior to timeout */ \
+		barrier(); \
 		if (COND) { \
 			ret = 0; \
 			break; \