Re: new warning on sysrq kernel crash trigger

[Ani Sinha]

I backported your

ee376dbdf277 ("rcu: Consolidate rcu_synchronize and wakeme_after_rcu()" &
ec90a194ae2cb8b8e("rcu: Create a synchronize_rcu_mult()")

and tested this on our 3.18 kernel running on our board. The sysrq
kernel crash seems to have been fixed (behavior as per our old 3.4
kernel). I will send in a patch as per your former suggestion ...


On Fri, Dec 11, 2015 at 4:02 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Dec 11, 2015 at 03:41:04PM -0800, Ani Sinha wrote:
>> On Fri, Dec 11, 2015 at 2:27 PM, Paul E. McKenney
>> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>> > On Fri, Dec 11, 2015 at 05:10:43PM -0500, Rik van Riel wrote:
>> >> On 12/11/2015 03:44 PM, Ani Sinha wrote:
>> >> >
>> >> >
>> >> > On Thu, 10 Dec 2015, Paul E. McKenney wrote:
>> >> >
>> >> >> On Thu, Dec 10, 2015 at 03:57:09PM -0800, Ani Sinha wrote:
>> >> >>> Hi guys
>> >> >>>
>> >> >>> I am noticing a new warning in linux 3.18 which we did not see before
>> >> >>> in linux 3.4 :
>> >> >>>
>> >> >>> bash-4.1# echo c > /proc/sysrq-trigger
>> >> >>> [  978.807185] BUG: sleeping function called from invalid context at
>> >> >>> ../arch/x86/mm/fault.c:1187
>> >> >>> [  978.909816] in_atomic(): 0, irqs_disabled(): 0, pid: 4706, name: bash
>> >> >>> [  978.987358] Preemption disabled at:[<ffffffff81484339>] printk+0x48/0x4a
>> >> >>>
>> >> >>>
>> >> >>> I have bisected this to the following change :
>> >> >>>
>> >> >>> commit 984d74a72076a12b400339973e8c98fd2fcd90e5
>> >> >>> Author: Rik van Riel <riel@xxxxxxxxxx>
>> >> >>> Date:   Fri Jun 6 14:38:13 2014 -0700
>> >> >>>
>> >> >>>     sysrq: rcu-ify __handle_sysrq
>> >> >>>
>> >> >>>
>> >> >>> the rcu_read_lock() in handle_sysrq() bumps up
>> >> >>> current->rcu_read_lock_nesting. Hence, in __do_page_fault() when it
>> >> >>> calls might_sleep() in x86/mm/fault.c line 1191,
>> >> >>> preempt_count_equals(0) returns false and hence the warning is
>> >> >>> printed.
>> >> >>>
>> >> >>> One way to handle this would be to do something like this:
>> >> >>>
>> >> >>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
>> >> >>> index eef44d9..d4dbe22 100644
>> >> >>> --- a/arch/x86/mm/fault.c
>> >> >>> +++ b/arch/x86/mm/fault.c
>> >> >>> @@ -1132,7 +1132,7 @@ __do_page_fault(struct pt_regs *regs, unsigned
>> >> >>> long error_code,
>> >> >>>   * If we're in an interrupt, have no user context or are running
>> >> >>>   * in a region with pagefaults disabled then we must not take the fault
>> >> >>>   */
>> >> >>> - if (unlikely(faulthandler_disabled() || !mm)) {
>> >> >>> + if (unlikely(faulthandler_disabled() || rcu_preempt_depth() || !mm)) {
>> >> >>
>> >> >> This works if CONFIG_PREEMPT=y, but if CONFIG_PREEMPT=n, then
>> >> >> rcu_preempt_depth() unconditionally returns zero.  And if
>> >> >> CONFIG_PREEMPT_COUNT=y && CONFIG_PREEMPT=n, you would still see
>> >> >> the might_sleep() splat.
>> >> >>
>> >> >> Maybe use SRCU instead of RCU for this purpose?
>> >> >>
>> >> >
>> >> > From ae232ce3fb167b2ad363bfac7aab69001bc55a50 Mon Sep 17 00:00:00 2001
>> >> > From: Ani Sinha <ani@xxxxxxxxxx>
>> >> > Date: Fri, 11 Dec 2015 12:07:42 -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, replace RCU call in handle_sysrq() to use SRCU.
>> >>
>> >> The sysrq code can be called from irq context.
>> >>
>> >> Trying to use SRCU from an irq context sounds like it could
>> >> be a bad idea, though admittedly I do not know enough about
>> >> SRCU to know for sure :)
>> >
>> > Indeed, not the best idea!  ;-)
>> >
>> > I could imagine something like this:
>> >
>> >         if (in_irq())
>> >                 rcu_read_lock();
>> >         else
>> >                 idx = srcu_read_lock(&sysrq_rcu);
>> >
>> > And ditto for unlock.  Then, for the update:
>> >
>> >         synchronize_rcu_mult(call_rcu, call_sysrq_srcu);
>>
>> This won't work on 3.18 as this api was introduced in linux 4.3.
>
> Then do this:
>
>         synchronize_rcu();
>         synchronize_srcu(&sysrq_rcu);
>
>> > Where:
>> >
>> >         static void call_sysrq_srcu(struct rcu_head *head, rcu_callback_t func)
>> >         {
>> >                 call_srcu(&sysrq_rcu, head, func);
>> >         }
>> >
>> > Here I presume that the page-fault code avoids the might_sleep if invoked
>> > from irq context.
>>
>> Quick look at the code seems to indicate that this is true.
>
> Good!  ;-)
>
>                                                         Thanx, Paul
>
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