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

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Message-Id: <20180618153027.GU3593@linux.vnet.ibm.com>
Date:   Mon, 18 Jun 2018 08:30:27 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Davidlohr Bueso <dave@...olabs.net>,
        Josh Triplett <josh@...htriplett.org>,
        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: Re: [PATCH] rcutorture: use monotonic timestamp for stall detection

On Mon, Jun 18, 2018 at 04:47:34PM +0200, Arnd Bergmann wrote:
> 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>

Queued for testing and review, thank you!  If things go well, I
expect this to be ready for the v4.19 merge window.

							Thanx, Paul

> ---
>  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
>