Re: new warning on sysrq kernel crash trigger

[Ani Sinha <ani@arista.com>]

Rik, should I send a separate email with the patch or you are OK with
what I sent in the email? Are you queueing up my patch for applying
upstream?

On Tue, Dec 15, 2015 at 5:44 AM, Anirban Sinha <ani@arista.com> 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@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
>