new warning on sysrq kernel crash trigger

[Ani Sinha]

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)) {
  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.
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