Re: new warning on sysrq kernel crash trigger

[Ani Sinha]

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?
>