Re: [PATCH v3 tip/core/rcu 40/40] rcu: Make non-preemptive schedule be Tasks RCU quiescent state

[Paolo Bonzini <pbonzini@redhat.com>]

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On 29/09/2017 11:30, Boqun Feng wrote:
> On Thu, Sep 28, 2017 at 04:05:14PM +0000, Paul E. McKenney wrote:
> [...]
>>>  __schedule+0x201/0x2240 kernel/sched/core.c:3292
>>>  schedule+0x113/0x460 kernel/sched/core.c:3421
>>>  kvm_async_pf_task_wait+0x43f/0x940 arch/x86/kernel/kvm.c:158
>>
>> It is kvm_async_pf_task_wait() that calls schedule(), but it carefully
>> sets state to make that legal.  Except...
>>
>>>  do_async_page_fault+0x72/0x90 arch/x86/kernel/kvm.c:271
>>>  async_page_fault+0x22/0x30 arch/x86/entry/entry_64.S:1069
>>> RIP: 0010:format_decode+0x240/0x830 lib/vsprintf.c:1996
>>> RSP: 0018:ffff88003b2df520 EFLAGS: 00010283
>>> RAX: 000000000000003f RBX: ffffffffb5d1e141 RCX: ffff88003b2df670
>>> RDX: 0000000000000001 RSI: dffffc0000000000 RDI: ffffffffb5d1e140
>>> RBP: ffff88003b2df560 R08: dffffc0000000000 R09: 0000000000000000
>>> R10: ffff88003b2df718 R11: 0000000000000000 R12: ffff88003b2df5d8
>>> R13: 0000000000000064 R14: ffffffffb5d1e140 R15: 0000000000000000
>>>  vsnprintf+0x173/0x1700 lib/vsprintf.c:2136
>>
>> We took a page fault in vsnprintf() while doing link_path_walk(),
>> which looks to be within an RCU read-side critical section.
>>
>> Maybe the page fault confused lockdep?
>>
>> Sigh.  It is going to be a real pain if all printk()s need to be
>> outside of RCU read-side critical sections due to the possibility of
>> page faults...
>>
> 
> Does this mean whenever we get a page fault in a RCU read-side critical
> section, we may hit this?
> 
> Could we simply avoid to schedule() in kvm_async_pf_task_wait() if the
> fault process is in a RCU read-side critical section as follow?
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index aa60a08b65b1..291ea13b23d2 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -140,7 +140,7 @@ void kvm_async_pf_task_wait(u32 token)
>  
>  	n.token = token;
>  	n.cpu = smp_processor_id();
> -	n.halted = is_idle_task(current) || preempt_count() > 1;
> +	n.halted = is_idle_task(current) || preempt_count() > 1 || rcu_preempt_depth();
>  	init_swait_queue_head(&n.wq);
>  	hlist_add_head(&n.link, &b->list);
>  	raw_spin_unlock(&b->lock);
> 
> (Add KVM folks and list Cced)

Yes, that would work.  Mind to send it as a proper patch?

Thanks,

Paolo

> Regards,
> Boqun
> 
>> 							Thanx, Paul
> [...]
> 



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