Re: new warning on sysrq kernel crash trigger

[Ani Sinha]

Rik, any comments?

On Sat, Dec 12, 2015 at 6:33 AM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Dec 11, 2015 at 04:16:37PM -0800, Ani Sinha wrote:
>> 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);
>> >     }
>> >
>>
>> >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
>> in non-irq context.
>>
>> Tested this patch on linux 3.18 by booting off one of our boards.
>>
>> Fixes: 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq")
>>
>> Signed-off-by: Ani Sinha <ani@xxxxxxxxxx>
>
> From an RCU perspective:
>
> Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>
> But I must defer to Rik from an sysrq perspective.
>
>                                                         Thanx, Paul
>
>> ---
>> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
>> index 5381a72..df7d747 100644
>> --- a/drivers/tty/sysrq.c
>> +++ b/drivers/tty/sysrq.c
>> @@ -54,6 +54,7 @@
>>  /* Whether we react on sysrq keys or just ignore them */
>>  static int __read_mostly sysrq_enabled = CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE;
>>  static bool __read_mostly sysrq_always_enabled;
>> +DEFINE_STATIC_SRCU(sysrq_rcu);
>>
>>  static bool sysrq_on(void)
>>  {
>> @@ -519,10 +520,13 @@ void __handle_sysrq(int key, bool check_mask)
>>  {
>>       struct sysrq_key_op *op_p;
>>       int orig_log_level;
>> -     int i;
>> +     int i, idx;
>>
>>       rcu_sysrq_start();
>> -     rcu_read_lock();
>> +     if (in_irq())
>> +             rcu_read_lock();
>> +     else
>> +             idx = srcu_read_lock(&sysrq_rcu);
>>       /*
>>        * Raise the apparent loglevel to maximum so that the sysrq header
>>        * is shown to provide the user with positive feedback.  We do not
>> @@ -564,7 +568,10 @@ void __handle_sysrq(int key, bool check_mask)
>>               pr_cont("\n");
>>               console_loglevel = orig_log_level;
>>       }
>> -     rcu_read_unlock();
>> +     if (in_irq())
>> +             rcu_read_unlock();
>> +     else
>> +             srcu_read_unlock(&sysrq_rcu, idx);
>>       rcu_sysrq_end();
>>  }
>>
>> @@ -1040,6 +1047,11 @@ int sysrq_toggle_support(int enable_mask)
>>       return 0;
>>  }
>>
>> +static void call_sysrq_srcu(struct rcu_head *head, rcu_callback_t func)
>> +{
>> +     call_srcu(&sysrq_rcu, head, func);
>> +}
>> +
>>  static int __sysrq_swap_key_ops(int key, struct sysrq_key_op *insert_op_p,
>>                                  struct sysrq_key_op *remove_op_p)
>>  {
>> @@ -1059,7 +1071,7 @@ static int __sysrq_swap_key_ops(int key, struct sysrq_key_op *insert_op_p,
>>        * Wait for it to go away before returning, so the code for an old
>>        * op is not freed (eg. on module unload) while it is in use.
>>        */
>> -     synchronize_rcu();
>> +     synchronize_rcu_mult(call_rcu, call_sysrq_srcu);
>>
>>       return retval;
>>  }
>>
>
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