linux-kernel - Re: [PATCH 02/10] printk: Try harder to get logbuf

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Date:	Fri, 29 May 2015 13:46:13 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Petr Mladek <pmladek@...e.cz>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Dave Anderson <anderson@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Kay Sievers <kay@...y.org>, Jiri Kosina <jkosina@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Jan Kara <jack@...e.cz>,
	linux-kernel@...r.kernel.org, Wang Long <long.wanglong@...wei.com>,
	peifeiyue@...wei.com, dzickus@...hat.com, morgan.wang@...wei.com,
	sasha.levin@...cle.com
Subject: Re: [PATCH 02/10] printk: Try harder to get logbuf_lock on NMI

On Fri, 29 May 2015 12:56:07 +0200 Petr Mladek <pmladek@...e.cz> wrote:

> > > +	/* try hard to get the lock but do not wait forever */
> > > +	start_time = cpu_clock(this_cpu);
> > > +	current_time = start_time;
> > > +	while (current_time - start_time < TRY_LOCKBUF_LOCK_MAX_DELAY_NS) {
> > > +		if (raw_spin_trylock(&logbuf_lock))
> > > +			return 1;
> > > +		cpu_relax();
> > > +		current_time = cpu_clock(this_cpu);
> > > +	}
> > > +
> > > +	return 0;
> > > +}
> > 
> > That CPU is now going to spin around for 100us and then time out.
> 
> Yes, there was a deadlock without the patch. So, limited spinning is
> still a win.
> 
> Or would you like to detect the deadlock immediately in all cases?
> I mean to add the proposed wrapper around take/release lock calls
> and set/test some cpu-specific variable there?

Yes.  Pointlessly spinning in NMI for 100us is bad.

> It sounds interesting. Well, the detection will not be 100% correct
> because there is a small race window between taking @logbuf_lock
> and setting @lockbuf_cpu. I wonder if it is worth doing. But I will
> do it if you want.

You might be able to do something with checking logbuf_cpu within the
loop to avoid the worst-case scenarios.

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