linux-kernel - Re: new warning on sysrq kernel crash trigger

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Date:	Wed, 16 Dec 2015 11:22:46 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Anirban Sinha <ani@...sta.com>
Cc:	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Richard Weinberger <richard@....at>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ivan Delalande <colona@...sta.com>,
	fruggeri <fruggeri@...sta.com>
Subject: Re: new warning on sysrq kernel crash trigger

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On 12/14/2015 07:14 PM, Anirban Sinha wrote:
> 
> 
> On Mon, 14 Dec 2015, Rik van Riel wrote:
> 
>> On 12/14/2015 11:24 AM, Ani Sinha wrote:
>>> Rik, any comments?
>> 
>> Another good option is to simply ignore this warning, or drop the
>> rcu_read_lock before doing the alt-syrsq-c action.
>> 
>> After all, alt-sysrq-c is "crash the system, take a crash dump", 
>> which is not an action the system ever returns from.
>> 
> 
> Yea I thought about this idea previously but then discarded it
> thinking it would be too hacky. Here's the cooked up patch. I hope
> this can be approved for mainline soon (I'm on vacation and working
> just on this issue remotely) :
> 
> From 105ff3ffce380650b3d58b3594a9be47bd604b28 Mon Sep 17 00:00:00
> 2001 From: Ani Sinha <ani@...sta.com> Date: Mon, 14 Dec 2015
> 14:55:08 -0800 Subject: [PATCH 1/1] Fix 'sleeping function called
> from invalid context' warning in sysrq generated crash.
> 
> Commit 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq") replaced
> spin_lock_irqsave() calls with rcu_read_lock() calls in sysrq.
> Since rcu_read_lock() does not disable preemption,
> faulthandler_disabled() in __do_page_fault() in x86/fault.c returns
> false. When the code later calls might_sleep() in the pagefault
> handler, we get the following warning:
> 
> BUG: sleeping function called from invalid context at
> ../arch/x86/mm/fault.c:1187 in_atomic(): 0, irqs_disabled(): 0,
> pid: 4706, name: bash Preemption disabled at:[<ffffffff81484339>]
> printk+0x48/0x4a
> 
> To fix this, we release the RCU read lock before we crash.
> 
> Tested this patch on linux 3.18 by booting off one of our boards.
> 
> Fixes: 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq")
> 
> Signed-off-by: Ani Sinha <ani@...sta.com>

Reviewed-by: Rik van Riel <riel@...hat.com>

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