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

[Boqun Feng]

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)

Regards,
Boqun

> 							Thanx, Paul
[...]

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