linux-kernel - Re: [PATCH RT 4/2] hrtimer: Don't lose state in cpu

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Message-ID: <1551105815.6234.14.camel@gmx.de>
Date:   Mon, 25 Feb 2019 15:43:35 +0100
From:   Mike Galbraith <efault@....de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-rt-users@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH RT 4/2] hrtimer: Don't lose state in cpu_chill()

Hi Sebastian,

My box claims that this patch is busted.  It argues its case by IO
deadlocking any kernel this patch is applied to when spinning rust is
flogged, including virgin 4.19-rt14, said kernel becoming stable again
when I whack the accused.

On Tue, 2019-02-19 at 17:08 +0100, Sebastian Andrzej Siewior wrote:
> In cpu_chill() the state is set to TASK_UNINTERRUPTIBLE and a timer is
> programmed. On return the state is always TASK_RUNNING which means we
> lose the state if it was something other than RUNNING. Also
> set_current_state() sets ->task_state_change to within cpu_chill() which
> is not expected.
> 
> Save the task state on entry and restore it on return. Simply set the
> state in order to avoid updating ->task_state_change.
> 
> Cc: stable-rt@...r.kernel.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  kernel/time/hrtimer.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 851b2134e77f4..6f2736ec4b8ef 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -1902,15 +1902,18 @@ void cpu_chill(void)
>  {
>  	ktime_t chill_time;
>  	unsigned int freeze_flag = current->flags & PF_NOFREEZE;
> +	long saved_state;
>  
> +	saved_state = current->state;
>  	chill_time = ktime_set(0, NSEC_PER_MSEC);
> -	set_current_state(TASK_UNINTERRUPTIBLE);
> +	__set_current_state_no_track(TASK_UNINTERRUPTIBLE);
>  	current->flags |= PF_NOFREEZE;
>  	sleeping_lock_inc();
>  	schedule_hrtimeout(&chill_time, HRTIMER_MODE_REL_HARD);
>  	sleeping_lock_dec();
>  	if (!freeze_flag)
>  		current->flags &= ~PF_NOFREEZE;
> +	__set_current_state_no_track(saved_state);
>  }
>  EXPORT_SYMBOL(cpu_chill);
>  #endif