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From:   "Joel Fernandes (Google)" <joel@joelfernandes.org>
To:     linux-kernel@vger.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Paul McKenney <paulmck@kernel.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Zhouyi Zhou <zhouzhouyi@gmail.com>, stable@vger.kernel.org,
        Davidlohr Bueso <dave@stgolabs.net>,
        Josh Triplett <josh@joshtriplett.org>
Subject: [PATCH] torture: Fix hang during kthread shutdown phase
Date:   Sun,  1 Jan 2023 06:15:55 +0000
Message-Id: <20230101061555.278129-1-joel@joelfernandes.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

During shutdown of rcutorture, the shutdown thread in
rcu_torture_cleanup() calls torture_cleanup_begin() which sets fullstop
to FULLSTOP_RMMOD. This is enough to cause the rcutorture threads for
readers and fakewriters to breakout of their main while loop and start
shutting down.

Once out of their main loop, they then call torture_kthread_stopping()
which in turn waits for kthread_stop() to be called, however
rcu_torture_cleanup() has not even called kthread_stop() on those
threads yet, it does that a bit later.  However, before it gets a chance
to do so, torture_kthread_stopping() calls
schedule_timeout_interruptible(1) in a tight loop. Tracing confirmed
this makes the timer softirq constantly execute timer callbacks, while
never returning back to the softirq exit path and is essentially "locked
up" because of that. If the softirq preempts the shutdown thread,
kthread_stop() may never be called.

This commit improves the situation dramatically, by increasing timeout
passed to schedule_timeout_interruptible() 1/20th of a second. This
causes the timer softirq to not lock up a CPU and everything works fine.
Testing has shown 100 runs of TREE07 passing reliably, which was not the
case before because of RCU stalls.

Cc: Paul McKenney <paulmck@kernel.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Zhouyi Zhou <zhouzhouyi@gmail.com>
Cc: <stable@vger.kernel.org> # 6.0.x
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/torture.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/torture.c b/kernel/torture.c
index 29afc62f2bfe..d024f3b7181f 100644
--- a/kernel/torture.c
+++ b/kernel/torture.c
@@ -915,7 +915,7 @@ void torture_kthread_stopping(char *title)
 	VERBOSE_TOROUT_STRING(buf);
 	while (!kthread_should_stop()) {
 		torture_shutdown_absorb(title);
-		schedule_timeout_uninterruptible(1);
+		schedule_timeout_uninterruptible(HZ/20);
 	}
 }
 EXPORT_SYMBOL_GPL(torture_kthread_stopping);
-- 
2.39.0.314.g84b9a713c41-goog