linux-kernel - Re: [GIT PULL] printk: Support for full dynticks mode

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Message-ID: <1360032122.27007.4.camel@gandalf.local.home>
Date:	Mon, 04 Feb 2013 21:42:02 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [GIT PULL] printk: Support for full dynticks mode

On Mon, 2013-02-04 at 18:09 -0800, Andrew Morton wrote:

> I don't think so.  Conceptually printk() should be "inner" to the
> scheduler and shouldn't call into sched things at all.  The (afaik
> sole) exception to that was the klogd wakeup.
> 
> Traditionally the deadlock happened when calling printk() with
> tasklist_lock (now q->lock) held.  printk() would call wake_up(klogd)
> and wake_up() tries to take tasklist_lock and boom.  Moving the
> wake_up() out to the tick "thread" fixed that.
> 
> Maybe there were other deadlock scenarios, dunno.  That knowledge
> appears to be disappearing into the mists of time :(

Even without the printk irq_work the current printk method uses a
delayed wakeup anyway.

The wake_up_klogd() sets PRINTK_PENDING_WAKEUP, and the wakeup happens
at time of the tick. I don't see where there is a deadlock. I added a
printk in __sched_setscheduler() where the run queue lock is held, and
booted that with full lockdep debugging enabled. No deadlock is
detected.

Do we really even need that printk_sched()?

-- Steve


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