linux-kernel - [PATCH tip/core/rcu 18/20] torture: Make stutter less vulnerable to compilers and races

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Date:   Fri,  1 Dec 2017 12:09:03 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 18/20] torture: Make stutter less vulnerable to compilers and races

The stutter_wait() function repeatedly fetched stutter_pause_test, and
should really just fetch it once on each pass.  The races should be
harmless, but why have the races?  Also, the whole point of the value
"2" for stutter_pause_test is to get everyone to start at very nearly
the same time, but the value "2" was the first jiffy of the stutter
rather than the last jiffy of the stutter.

This commit rearranges the code to be more sensible.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/torture.c | 33 +++++++++++++++++----------------
 1 file changed, 17 insertions(+), 16 deletions(-)

diff --git a/kernel/torture.c b/kernel/torture.c
index 52781e838541..3bcbd4fbfe18 100644
--- a/kernel/torture.c
+++ b/kernel/torture.c
@@ -573,18 +573,21 @@ static int stutter;
  */
 void stutter_wait(const char *title)
 {
+	int spt;
+
 	cond_resched_rcu_qs();
-	while (READ_ONCE(stutter_pause_test) ||
-	       (torture_runnable && !READ_ONCE(*torture_runnable))) {
-		if (stutter_pause_test)
-			if (READ_ONCE(stutter_pause_test) == 1)
-				schedule_timeout_interruptible(1);
-			else
-				while (READ_ONCE(stutter_pause_test))
-					cond_resched();
-		else
+	spt = READ_ONCE(stutter_pause_test);
+	while (spt || (torture_runnable && !READ_ONCE(*torture_runnable))) {
+		if (spt == 1) {
+			schedule_timeout_interruptible(1);
+		} else if (spt == 2) {
+			while (READ_ONCE(stutter_pause_test))
+				cond_resched();
+		} else {
 			schedule_timeout_interruptible(round_jiffies_relative(HZ));
+		}
 		torture_shutdown_absorb(title);
+		spt = READ_ONCE(stutter_pause_test);
 	}
 }
 EXPORT_SYMBOL_GPL(stutter_wait);
@@ -597,17 +600,15 @@ static int torture_stutter(void *arg)
 {
 	VERBOSE_TOROUT_STRING("torture_stutter task started");
 	do {
-		if (!torture_must_stop()) {
-			if (stutter > 1) {
-				schedule_timeout_interruptible(stutter - 1);
-				WRITE_ONCE(stutter_pause_test, 2);
-			}
-			schedule_timeout_interruptible(1);
+		if (!torture_must_stop() && stutter > 1) {
 			WRITE_ONCE(stutter_pause_test, 1);
+			schedule_timeout_interruptible(stutter - 1);
+			WRITE_ONCE(stutter_pause_test, 2);
+			schedule_timeout_interruptible(1);
 		}
+		WRITE_ONCE(stutter_pause_test, 0);
 		if (!torture_must_stop())
 			schedule_timeout_interruptible(stutter);
-		WRITE_ONCE(stutter_pause_test, 0);
 		torture_shutdown_absorb("torture_stutter");
 	} while (!torture_must_stop());
 	torture_kthread_stopping("torture_stutter");
-- 
2.5.2