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From: Ani Sinha <ani@arista.com>
Date: Fri, 18 Dec 2015 07:33:36 -0800
Message-ID: <CAOxq_8NbcGdXAfPnS1a283JPkfhrpKuER3eJoq9gb21GkJC9Ug@mail.gmail.com>
Subject: Re: [PATCH 1/1] Fix 'sleeping function called from invalid context'
 warning in sysrq generated crash.
To: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ani Sinha <ani@anirban.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jslaby@suse.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Rik van Riel <riel@redhat.com>, Ivan Delalande <colona@arista.com>,
        Francesco Ruggeri <fruggeri@arista.com>
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On Fri, Dec 18, 2015 at 5:01 AM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> On Thu, Dec 17, 2015 at 05:15:10PM -0800, Ani Sinha wrote:
>> 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>
>
> I queued this with Rik's Signed-off-by, and fixed some checkpatch.pl
> errors.  Please run scripts/checkpatch.pl on your patches in the future.
>
> Please see below for the result.

Yes, sorry. I forgot to run the script before sending Thanks for
fixing my comment and the trailing whitespace.

>
> Rik, did you test this as well?  If so, may I also have your Tested-by?
>
>                                                         Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit c95a158356397844a5a6deb0bd58758084f891df
> Author: Ani Sinha <ani@arista.com>
> Date:   Thu Dec 17 17:15:10 2015 -0800
>
>     sysrq: Fix 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>
>     Reviewed-by: Rik van Riel <riel@redhat.com>
>     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> index 5381a728d23e..e5139402e7f8 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;
>