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 19:55:32 +0200
From:	Aaro Koskinen <aaro.koskinen@....fi>
To:	James Hogan <james.hogan@...tec.com>
Cc:	"Maciej W. Rozycki" <macro@...ux-mips.org>,
	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

Hi,

On Fri, Jan 30, 2015 at 02:59:57PM +0000, James Hogan wrote:
> 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.

It does not set the flag. But flags are kept in module's static data,
so the original problem stays.

A.
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