Re: new warning on sysrq kernel crash trigger

[Paul E. McKenney]

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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