[PATCH 3/4] iopoll: Reorder the timeout handling in poll_timeout_us()

[Ville Syrjala]

From: Ville Syrjälä <ville.syrjala at linux.intel.com>

Currently poll_timeout_us() evaluates 'op' and 'cond' twice
within the loop, once at the start, and a second time after
the timeout check. While it's probably not a big deal to do
it twice almost back to back, it does make the macro a bit messy.

Simplify the implementation to evaluate the timeout at the
very start, then follow up with 'op'/'cond', and finally
check if the timeout did in fact happen or not.

For good measure throw in a compiler barrier between the timeout
and 'op'/'cond' evaluations to make sure the compiler can't reoder
the operations (which could cause false positive timeouts).
The similar i915 __wait_for() macro already has the barrier, though
there it is between the 'op' and 'cond' evaluations, which seems
like it could still allow 'op' and the timeout evaluations to get
reordered incorrectly. I suppose the ktime_get() might itself act
as a sufficient barrier here, but better safe than sorry I guess.

Cc: Jani Nikula <jani.nikula at intel.com>
Cc: Lucas De Marchi <lucas.demarchi at intel.com>
Cc: Dibin Moolakadan Subrahmanian <dibin.moolakadan.subrahmanian at intel.com>
Cc: Imre Deak <imre.deak at intel.com>
Cc: David Laight <david.laight.linux at gmail.com>
Cc: Geert Uytterhoeven <geert+renesas at glider.be>
Cc: Matt Wagantall <mattw at codeaurora.org>
Cc: Dejin Zheng <zhengdejin5 at gmail.com>
Cc: intel-gfx at lists.freedesktop.org
Cc: intel-xe at lists.freedesktop.org
Cc: linux-kernel at vger.kernel.org
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
 include/linux/iopoll.h | 24 +++++++++++-------------
 1 file changed, 11 insertions(+), 13 deletions(-)

diff --git a/include/linux/iopoll.h b/include/linux/iopoll.h
index 69296e6adbf3..0e0940a60fdb 100644
--- a/include/linux/iopoll.h
+++ b/include/linux/iopoll.h
@@ -41,18 +41,17 @@
 	if ((sleep_before_op) && __sleep_us) \
 		usleep_range((__sleep_us >> 2) + 1, __sleep_us); \
 	for (;;) { \
+		bool __expired = __timeout_us && \
+			ktime_compare(ktime_get(), __timeout) > 0; \
+		/* guarantee 'op' and 'cond' are evaluated after timeout expired */ \
+		barrier(); \
 		op; \
 		if (cond) { \
 			___ret = 0; \
 			break; \
 		} \
-		if (__timeout_us && \
-		    ktime_compare(ktime_get(), __timeout) > 0) { \
-			op; \
-			if (cond) \
-				___ret = 0; \
-			else \
-				___ret = -ETIMEDOUT; \
+		if (__expired) { \
+			___ret = -ETIMEDOUT; \
 			break; \
 		} \
 		if (__sleep_us) \
@@ -97,17 +96,16 @@
 			__left_ns -= __delay_ns; \
 	} \
 	for (;;) { \
+		bool __expired = __timeout_us && __left_ns < 0; \
+		/* guarantee 'op' and 'cond' are evaluated after timeout expired */ \
+		barrier(); \
 		op; \
 		if (cond) { \
 			___ret = 0; \
 			break; \
 		} \
-		if (__timeout_us && __left_ns < 0) { \
-			op; \
-			if (cond) \
-				___ret = 0; \
-			else \
-				___ret = -ETIMEDOUT; \
+		if (__expired) { \
+			___ret = -ETIMEDOUT; \
 			break; \
 		} \
 		if (__delay_us) { \
-- 
2.49.0