FAILED: patch "[PATCH] rcutorture: Fix stuttering races and other issues" failed to apply to 6.1-stable tree

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The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@xxxxxxxxxxxxxxx>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x cca42bd8eb1b54a4c9bbf48c79d120e66619a3e4
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@xxxxxxxxxxxxxxx>' --in-reply-to '2023112203-roping-darling-f64a@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:

cca42bd8eb1b ("rcutorture: Fix stuttering races and other issues")
10af43671e8b ("torture: Move stutter_wait() timeouts to hrtimers")
5d248bb39fe1 ("torture: Add lock_torture writer_fifo module parameter")
f8619c300f49 ("locktorture: Add long_hold to adjust lock-hold delays")
45bcf0bd8cbe ("locktorture: With nested locks, occasionally skip main lock")
b63343207da2 ("locktorture: Add nested_[un]lock() hooks and nlocks parameter")
c24501b24074 ("locktorture: Make the rt_boost factor a tunable")
e01f3a1a589e ("locktorture: Allow non-rtmutex lock types to be boosted")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From cca42bd8eb1b54a4c9bbf48c79d120e66619a3e4 Mon Sep 17 00:00:00 2001
From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>
Date: Sat, 29 Jul 2023 14:27:31 +0000
Subject: [PATCH] rcutorture: Fix stuttering races and other issues

The stuttering code isn't functioning as expected. Ideally, it should
pause the torture threads for a designated period before resuming. Yet,
it fails to halt the test for the correct duration. Additionally, a race
condition exists, potentially causing the stuttering code to pause for
an extended period if the 'spt' variable is non-zero due to the stutter
orchestration thread's inadequate CPU time.

Moreover, over-stuttering can hinder RCU's progress on TREE07 kernels.
This happens as the stuttering code may run within a softirq due to RCU
callbacks. Consequently, ksoftirqd keeps a CPU busy for several seconds,
thus obstructing RCU's progress. This situation triggers a warning
message in the logs:

[ 2169.481783] rcu_torture_writer: rtort_pipe_count: 9

This warning suggests that an RCU torture object, although invisible to
RCU readers, couldn't make it past the pipe array and be freed -- a
strong indication that there weren't enough grace periods during the
stutter interval.

To address these issues, this patch sets the "stutter end" time to an
absolute point in the future set by the main stutter thread. This is
then used for waiting in stutter_wait(). While the stutter thread still
defines this absolute time, the waiters' waiting logic doesn't rely on
the stutter thread receiving sufficient CPU time to halt the stuttering
as the halting is now self-controlled.

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

diff --git a/kernel/torture.c b/kernel/torture.c
index 6ba62e5993e7..fd353f98162f 100644
--- a/kernel/torture.c
+++ b/kernel/torture.c
@@ -720,7 +720,7 @@ static void torture_shutdown_cleanup(void)
  * suddenly applied to or removed from the system.
  */
 static struct task_struct *stutter_task;
-static int stutter_pause_test;
+static ktime_t stutter_till_abs_time;
 static int stutter;
 static int stutter_gap;
 
@@ -730,30 +730,16 @@ static int stutter_gap;
  */
 bool stutter_wait(const char *title)
 {
-	unsigned int i = 0;
 	bool ret = false;
-	int spt;
+	ktime_t till_ns;
 
 	cond_resched_tasks_rcu_qs();
-	spt = READ_ONCE(stutter_pause_test);
-	for (; spt; spt = READ_ONCE(stutter_pause_test)) {
-		if (!ret && !rt_task(current)) {
-			sched_set_normal(current, MAX_NICE);
-			ret = true;
-		}
-		if (spt == 1) {
-			torture_hrtimeout_jiffies(1, NULL);
-		} else if (spt == 2) {
-			while (READ_ONCE(stutter_pause_test)) {
-				if (!(i++ & 0xffff))
-					torture_hrtimeout_us(10, 0, NULL);
-				cond_resched();
-			}
-		} else {
-			torture_hrtimeout_jiffies(round_jiffies_relative(HZ), NULL);
-		}
-		torture_shutdown_absorb(title);
+	till_ns = READ_ONCE(stutter_till_abs_time);
+	if (till_ns && ktime_before(ktime_get(), till_ns)) {
+		torture_hrtimeout_ns(till_ns, 0, HRTIMER_MODE_ABS, NULL);
+		ret = true;
 	}
+	torture_shutdown_absorb(title);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(stutter_wait);
@@ -764,23 +750,16 @@ EXPORT_SYMBOL_GPL(stutter_wait);
  */
 static int torture_stutter(void *arg)
 {
-	DEFINE_TORTURE_RANDOM(rand);
-	int wtime;
+	ktime_t till_ns;
 
 	VERBOSE_TOROUT_STRING("torture_stutter task started");
 	do {
 		if (!torture_must_stop() && stutter > 1) {
-			wtime = stutter;
-			if (stutter > 2) {
-				WRITE_ONCE(stutter_pause_test, 1);
-				wtime = stutter - 3;
-				torture_hrtimeout_jiffies(wtime, &rand);
-				wtime = 2;
-			}
-			WRITE_ONCE(stutter_pause_test, 2);
-			torture_hrtimeout_jiffies(wtime, NULL);
+			till_ns = ktime_add_ns(ktime_get(),
+					       jiffies_to_nsecs(stutter));
+			WRITE_ONCE(stutter_till_abs_time, till_ns);
+			torture_hrtimeout_jiffies(stutter - 1, NULL);
 		}
-		WRITE_ONCE(stutter_pause_test, 0);
 		if (!torture_must_stop())
 			torture_hrtimeout_jiffies(stutter_gap, NULL);
 		torture_shutdown_absorb("torture_stutter");




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