linux-kernel - [PATCH] rcutorture: use monotonic timestamp for stall detection

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Date:   Mon, 18 Jun 2018 16:47:34 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Davidlohr Bueso <dave@...olabs.net>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Subject: [PATCH] rcutorture: use monotonic timestamp for stall detection

The get_seconds() call is deprecated because it overflows on 32-bit
architectures. The algorithm in rcu_torture_stall() can deal with
the overflow, but another problem here is that using a CLOCK_REALTIME
stamp can lead to a false-positive stall warning when a settimeofday()
happens concurrently.

Using ktime_get_seconds() instead avoids those issues and will never
overflow. The added cast to 'unsigned long' however is necessary to
make ULONG_CMP_LT() work correctly.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 kernel/rcu/rcutorture.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 42fcb7f05fac..f42cf69ef539 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1446,7 +1446,7 @@ static int rcu_torture_stall(void *args)
 		VERBOSE_TOROUT_STRING("rcu_torture_stall end holdoff");
 	}
 	if (!kthread_should_stop()) {
-		stop_at = get_seconds() + stall_cpu;
+		stop_at = ktime_get_seconds() + stall_cpu;
 		/* RCU CPU stall is expected behavior in following code. */
 		rcu_read_lock();
 		if (stall_cpu_irqsoff)
@@ -1455,7 +1455,8 @@ static int rcu_torture_stall(void *args)
 			preempt_disable();
 		pr_alert("rcu_torture_stall start on CPU %d.\n",
 			 smp_processor_id());
-		while (ULONG_CMP_LT(get_seconds(), stop_at))
+		while (ULONG_CMP_LT((unsigned long)ktime_get_seconds(),
+				    stop_at))
 			continue;  /* Induce RCU CPU stall warning. */
 		if (stall_cpu_irqsoff)
 			local_irq_enable();
-- 
2.9.0