Re: new warning on sysrq kernel crash trigger

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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@redhat.com>
> 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?

							Thanx, Paul

>   bad_area_nosemaphore(regs, error_code, address);
>   return;
>   }
> 
> I am wondering if this would be the right approach. I have tested that
> this patch does indeed suppress the warning. If you guys agree, I will
> send a patch. It's true that this is a trivial issue since we are
> intentionally crashing the kernel but in our case, this additional
> complaint from the kernel is confusing our test scripts and they are
> generating false positives.
>