Re: new warning on sysrq kernel crash trigger

[Ani Sinha]

On Fri, 11 Dec 2015, Paul E. McKenney 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);
> 
> Where:
> 
> 	static void call_sysrq_srcu(struct rcu_head *head, rcu_callback_t func)
> 	{
> 		call_srcu(&sysrq_rcu, head, func);
> 	}
>