Re: new warning on sysrq kernel crash trigger

[Ani Sinha]

Well I can certainly send a patch but I wonder if simply using SRCU
for this one instance in Rik's original patch will not break anything
else. Rik, please provide your thoughts.

On Thu, Dec 10, 2015 at 9:26 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> 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?
>
>                                                         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.
>>
>
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