linux-kernel - Re: [PATCH 3/3] printk: Avoid softlockups in console

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Message-Id: <20130215142219.01915825.akpm@linux-foundation.org>
Date:	Fri, 15 Feb 2013 14:22:19 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>, jslaby@...e.cz,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"kay.sievers" <kay.sievers@...y.org>
Subject: Re: [PATCH 3/3] printk: Avoid softlockups in console_unlock()

On Fri, 15 Feb 2013 17:57:10 +0100
Jan Kara <jack@...e.cz> wrote:

> A CPU can be caught in console_unlock() for a long time (tens of seconds are
> reported by our customers) when other CPUs are using printk heavily and serial
> console makes printing slow. Despite serial console drivers are calling
> touch_nmi_watchdog() this triggers softlockup warnings because
> interrupts are disabled for the whole time console_unlock() runs (e.g.
> vprintk() calls console_unlock() with interrupts disabled). Thus IPIs
> cannot be processed and other CPUs get stuck spinning in calls like
> smp_call_function_many(). Also RCU eventually starts reporting lockups.
> 
> In my artifical testing I also managed to trigger a situation when disk
> disappeared from the system apparently because commands to / from it
> could not be delivered for long enough. This is why just silencing
> watchdogs isn't a reliable solution to the problem and we simply have to
> avoid spending too long in console_unlock().
> 
> We fix the issue by limiting the time we spend in console_unlock() to
> watchdog_thresh() / 4 (unless we are in an early boot stage or oops is
> happening). The rest of the buffer will be printed either by further
> callers to printk() or during next timer tick.
> 

It still gives me tummy ache :(

The patch adds additional tests of oops_in_progress.  Some description
of your thinking on that matter would be appropriate?

> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -1990,17 +1990,31 @@ int is_console_locked(void)
>  #define PRINTK_PENDING_OUTPUT	2
>  
>  static unsigned long printk_pending;
> +static int last_printing_cpu = -1;
> +
> +static bool __console_unlock(void);
>  
>  void printk_tick(void)

printk_tick() no longer exists in linux-next.

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