Re: new warning on sysrq kernel crash trigger

[Paul E. McKenney]

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);

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.

Thoughts?

							Thanx, Paul

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