linux-kernel - Re: [PATCH v2 2/2] MIPS: fix kernel lockup or crash after CPU offline/online

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Date:	Fri, 30 Jan 2015 14:59:57 +0000
From:	James Hogan <james.hogan@...tec.com>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>,
	Aaro Koskinen <aaro.koskinen@....fi>
CC:	Ralf Baechle <ralf@...ux-mips.org>,
	David Daney <david.daney@...ium.com>,
	<linux-mips@...ux-mips.org>, <linux-kernel@...r.kernel.org>,
	Hemmo Nieminen <hemmo.nieminen@....fi>,
	<stable@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] MIPS: fix kernel lockup or crash after CPU offline/online

On 30/01/15 12:47, Maciej W. Rozycki wrote:
> On Fri, 30 Jan 2015, James Hogan wrote:
> 
>>>  Hmm, why can a call to `printk' cause a TLB miss, what's so special about 
>>> this function?  Does it use kernel mapped addresses for any purpose such 
>>> as `vmalloc'?
>>
>> It would be the fact netconsole (or whatever other console is in use) is
>> built as a kernel module, memory for which is allocated from the vmalloc
>> area.
> 
>  Ah, I see, thanks for enlightening me.  But in that case wouldn't it be 
> possible to postpone console output from `printk' until it is safe to 
> access the device?  In a manner similar to how for example we handle calls 
> to `printk' made from the hardirq context.  That would make things less 
> fragile.

Hmm, kernel/printk/printk.c does have:

static inline int can_use_console(unsigned int cpu)
{
	return cpu_online(cpu) || have_callable_console();
}

which should prevent it dumping printk buffer to console. CPU shouldn't
be marked online that early, which suggests that the console has the
CON_ANYTIME flag set, which it probably shouldn't if it depends on
module code. call_console_drivers() seems to ensure the CPU is online or
has CON_ANYTIME before calling the console write callback.

A quick glance and I can't see any evidence of netconsole being able to
get CON_ANYTIME.

serial8250_console does appear to set that flag *and* is tristate
though, which is slightly worrying.

Aaro, what is the content of your /proc/consoles?

Cheers
James

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