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

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Date:	Thu, 28 May 2015 09:54:14 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Petr Mladek <pmladek@...e.cz>,
	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>, 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 Wed, 27 May 2015, Andrew Morton wrote:

> > +static int try_logbuf_lock_in_nmi(void)
> > +{
> > +	u64 start_time, current_time;
> > +	int this_cpu = smp_processor_id();
> > +
> > +	/* no way if we are already locked on this CPU */
> > +	if (logbuf_cpu == this_cpu)
> > +		return 0;
> > +
> > +	/* 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);
> > +	}
> 
> (Looks at the read_seqcount_retry() in
> kernel/time/sched_clock.c:sched_clock())
> 
> Running cpu_clock() in NMI context seems a generally bad idea.  

Umm, why? Requirement for it to be NMI-safe is explicitly documented. See 
the corresponding section in Documentation/timers/timekeeping.txt.

-- 
Jiri Kosina
SUSE Labs
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