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SubjectRe: new warning on sysrq kernel crash trigger


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@arista.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@arista.com>
---
drivers/tty/sysrq.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 5381a72..08987ad 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -133,6 +133,12 @@ static void sysrq_handle_crash(int key)
{
char *killer = NULL;

+ /* we need to release the RCU read lock here,
+ otherwise we get an annoying
+ 'BUG: sleeping function called from invalid context'
+ complaint from the kernel before the panic.
+ */
+ rcu_read_unlock();
panic_on_oops = 1; /* force panic */
wmb();
*killer = 1;
--
1.8.1.4


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