Re: [PATCH RT] printk: Shortcut out of waiter spinning on PREEMPT_RT

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 15 Jan 2019 19:54:33 +0100
Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:

> On 2019-01-15 12:39:10 [-0500], Steven Rostedt wrote:
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -1742,6 +1742,13 @@ static int console_trylock_spinning(void)
> >  	if (console_trylock())
> >  		return 1;
> >  
> > +	/*
> > +	 * The consoles are preemptable in PREEMPT_RT, which can cause
> > +	 * spinning to deadlock.
> > +	 */
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL))
> > +		return 0;
> > +
> >  	printk_safe_enter_irqsave(flags);
> >  
> >  	raw_spin_lock(&console_owner_lock);  
> 
> So my ("printk: Make rt aware") patch does:
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 6553508ff3889..d983c509f74a2 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1617,6 +1617,7 @@ SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
>         return do_syslog(type, buf, len, SYSLOG_FROM_READER);
>  }
>  
> +#ifndef CONFIG_PREEMPT_RT_FULL
>  /*
>   * Special console_lock variants that help to reduce the risk of soft-lockups.
>   * They allow to pass console_lock to another printk() call using a busy wait.
> @@ -1757,6 +1758,15 @@ static int console_trylock_spinning(void)
>         return 1;
>  }
>  
> +#else
> +
> +static int console_trylock_spinning(void)
> +{
> +       return console_trylock();
> +}
> +
> +#endif
> +
>  /*
>   * Call the console drivers, asking them to write out
>   * log_buf[start] to log_buf[end - 1].
> 
> 
> So it never gets into the codepath where you try to avoid spinning.
> Right?
>

You right! Which appears to be missing from my 4.14, and I thought I
looked for that in 4.19-rt too, but I guess I was still looking at
4.14 :-/. OK, ignore. Sorry for the noise.

Well, it wasn't a total waste. I now know what to backport to 4.14-rt
;-)

-- Steve