linux-kernel - Re: [PATCH] printk: use console_trylock() in console_cpu

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Date:   Thu, 2 Feb 2017 17:56:41 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] printk: use console_trylock() in console_cpu_notify()

On Thu 2017-02-02 12:53:47, Sergey Senozhatsky wrote:
> Hello,
> 
> thanks for taking a look, Petr.
> 
> On (01/25/17 16:02), Petr Mladek wrote:
> [..]
> > > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> > > ---
> > >  kernel/printk/printk.c | 11 ++++++-----
> > >  1 file changed, 6 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > > index 7180088cbb23..772eb16436ce 100644
> > > --- a/kernel/printk/printk.c
> > > +++ b/kernel/printk/printk.c
> > > @@ -2028,15 +2028,16 @@ void resume_console(void)
> > >   * @cpu: unused
> > >   *
> > >   * If printk() is called from a CPU that is not online yet, the messages
> > > - * will be spooled but will not show up on the console.  This function is
> > > - * called when a new CPU comes online (or fails to come up), and ensures
> > > - * that any such output gets printed.
> > > + * will be printed on the console only if there are CON_ANYTIME consoles.
> > > + * This function is called when a new CPU comes online (or fails to come
> > > + * up) or goes offline.
> > >   */
> > >  static int console_cpu_notify(unsigned int cpu)
> > >  {
> > >  	if (!cpuhp_tasks_frozen) {
> > > -		console_lock();
> > > -		console_unlock();
> > > +		/* If trylock fails, someone else is doing the printing */
> > > +		if (console_trylock())
> > > +			console_unlock();
> > >  	}
> > >  	return 0;
> > >  }
> > 
> > Rather theoretically, the other owner of console_sem might be
> > on a CPU that is going online/offline and will refuse to print
> > the messages.
> 
> hm, if console_sem is locked by CPU that is going offline
> then console_cpu_notify() will deadlock in console_lock().
> if I understood your point correctly.

No, it will not deadlock. I guess that it will just do nothing.

console_unlock() would probably return at the beginning because
can_use_console() would return false because cpu_online()
would return false.

I am not sure about it. It is not easy for me to find
in which cpu_online() state the notifiers are called.

If the console really is not usable, we should rather
defer the console work to another CPU.

Best Regards,
Petr