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

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Date:   Thu, 23 Mar 2017 17:26:40 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        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 Wed 2017-01-25 16:02:36, Petr Mladek wrote:
> On Sat 2017-01-21 19:47:29, Sergey Senozhatsky wrote:
> > There is no need to always call blocking console_lock() in
> > console_cpu_notify(), it's quite possible that console_sem can
> > be locked by other CPU on the system, either already printing
> > or soon to begin printing the messages. console_lock() in this
> > case can simply block CPU hotplug for unknown period of time
> > (console_unlock() is time unbound). Not that hotplug is very
> > fast, but still, with other CPUs being online and doing
> > printk() console_cpu_notify() can stuck.
> > 
> > Use console_trylock() instead and opt-out if console_sem is
> > already acquired from another CPU, since that CPU will do
> > the printing for us.
> > 
> > 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;
> >  }
> 
> All in all. The change looks fine to me because it most likely
> improves the performance[1] and does not affect the reliability
> that much.
> 
> [1] https://lkml.kernel.org/r/20170119120744.GB435@tigerII.localdomain

Acked-by: Petr Mladek <pmladek@...e.com>

I am going to add this patch into printk.git for 4.12.

Best Regards,
Petr