Re: [PATCH] printk: Correctly handle preemption in console_unlock()

[Petr Mladek <pmladek@suse.com>]

On Fri 2017-01-13 11:05:42, Steven Rostedt wrote:
> On Fri, 13 Jan 2017 14:15:21 +0100
> Petr Mladek <pmladek@suse.com> wrote:
> 
> > ---
> > This is related to the thread
> > https://lkml.kernel.org/r/201612261954.FJE69201.OFLVtFJSQFOHMO@I-love.SAKURA.ne.jp
> > 
> >  kernel/printk/printk.c | 25 ++++++++++++-------------
> >  1 file changed, 12 insertions(+), 13 deletions(-)
> > 
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index 7180088cbb23..2ac54291230d 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -2150,7 +2150,7 @@ void console_unlock(void)
> >  	static u64 seen_seq;
> >  	unsigned long flags;
> >  	bool wake_klogd = false;
> > -	bool do_cond_resched, retry;
> > +	bool may_schedule_orig, retry;
> 
> <bike-shedding>
>  Hmm, I just hate the name of that variable.
>  console_may_schedule_orig, keep the full name?
> </bike-shedding>

No problem. I will use the full name in the next iteration
if there will be one.

> >  
> >  	if (console_suspended) {
> >  		up_console_sem();
> > @@ -2158,17 +2158,15 @@ void console_unlock(void)
> >  	}
> >  
> >  	/*
> > -	 * Console drivers are called under logbuf_lock, so
> > -	 * @console_may_schedule should be cleared before; however, we may
> > -	 * end up dumping a lot of lines, for example, if called from
> > -	 * console registration path, and should invoke cond_resched()
> > -	 * between lines if allowable.  Not doing so can cause a very long
> > -	 * scheduling stall on a slow console leading to RCU stall and
> > -	 * softlockup warnings which exacerbate the issue with more
> > -	 * messages practically incapacitating the system.
> > +	 * Console drivers are called with interrupts disabled, so
> > +	 * @console_may_schedule must be cleared before. The original
> > +	 * value must be restored so that we could schedule between lines.
> > +	 *
> > +	 * console_trylock() is not able to detect the preemptive context when
> > +	 * CONFIG_PREEMPT_COUNT is disabled. Therefore the value must be
> > +	 * stored before the "again" goto label.
> >  	 */
> > -	do_cond_resched = console_may_schedule;
> > -	console_may_schedule = 0;
> > +	may_schedule_orig = console_may_schedule;
> >  
> >  again:
> >  	/*
> > @@ -2235,12 +2233,13 @@ void console_unlock(void)
> >  		raw_spin_unlock(&logbuf_lock);
> >  
> >  		stop_critical_timings();	/* don't trace print latency */
> > +		console_may_schedule = 0;
> >  		call_console_drivers(ext_text, ext_len, text, len);
> > +		console_may_schedule = may_schedule_orig;
> >  		start_critical_timings();
> >  		printk_safe_exit_irqrestore(flags);
> >  
> > -		if (do_cond_resched)
> > -			cond_resched();
> > +		console_conditional_schedule();
> 
> Makes perfect sense to me. The only thing that worries me is that it
> does change the logic slightly, and I'm not sure if this will have any
> ramifications with it. That is, console_unlock() use to always leave
> with console_may_schedule equal to zero, where console_unlock() clears
> it. With this change, console_unlock() no longer clears that variable.
> Will that have any side effects that we are unaware of?

Good question!

If I get it correctly, the variable should never be used without the
console semaphore. IMHO, if it was used without the semaphore or if
it was not set correctly when the semaphore was taken, it would be a
bug. It means that leaving the variable set might actually help
to find a buggy usage if there is any.

My findings:

  + console_may_lock is set only by functions that get the console
    semaphore.

  + The function that takes the semaphore and does not set the
    variable is resume_console(). IMHO, it is a bug.

    We are on the safe side because the function is called from
    the same context as suspend_console() and it allows rescheduling.


  + I am not aware of any use of the variable without the
    semaphore. But it is not easy to prove just be reading
    the code.


Best Regards,
Petr