Re: [PATCH] kvm/x86: Handle async PF in RCU read-side critical sections

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Sep 29, 2017 at 04:53:57PM +0200, Paolo Bonzini wrote:
> On 29/09/2017 13:01, Boqun Feng wrote:
> > Sasha Levin reported a WARNING:
> > 
> > | WARNING: CPU: 0 PID: 6974 at kernel/rcu/tree_plugin.h:329
> > | rcu_preempt_note_context_switch kernel/rcu/tree_plugin.h:329 [inline]
> > | WARNING: CPU: 0 PID: 6974 at kernel/rcu/tree_plugin.h:329
> > | rcu_note_context_switch+0x16c/0x2210 kernel/rcu/tree.c:458
> > ...
> > | CPU: 0 PID: 6974 Comm: syz-fuzzer Not tainted 4.13.0-next-20170908+ #246
> > | Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> > | 1.10.1-1ubuntu1 04/01/2014
> > | Call Trace:
> > ...
> > | RIP: 0010:rcu_preempt_note_context_switch kernel/rcu/tree_plugin.h:329 [inline]
> > | RIP: 0010:rcu_note_context_switch+0x16c/0x2210 kernel/rcu/tree.c:458
> > | RSP: 0018:ffff88003b2debc8 EFLAGS: 00010002
> > | RAX: 0000000000000001 RBX: 1ffff1000765bd85 RCX: 0000000000000000
> > | RDX: 1ffff100075d7882 RSI: ffffffffb5c7da20 RDI: ffff88003aebc410
> > | RBP: ffff88003b2def30 R08: dffffc0000000000 R09: 0000000000000001
> > | R10: 0000000000000000 R11: 0000000000000000 R12: ffff88003b2def08
> > | R13: 0000000000000000 R14: ffff88003aebc040 R15: ffff88003aebc040
> > | __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
> > | 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
> > | sprintf+0xbe/0xf0 lib/vsprintf.c:2386
> > | proc_self_get_link+0xfb/0x1c0 fs/proc/self.c:23
> > | get_link fs/namei.c:1047 [inline]
> > | link_path_walk+0x1041/0x1490 fs/namei.c:2127
> > ...
> > 
> > And this happened when we hit a page fault in an RCU read-side critical
> > section and then we tried to reschedule in kvm_async_pf_task_wait(),
> > this reschedule would hit the WARN in rcu_preempt_note_context_switch(),
> > and be treated as a sleep in RCU read-side critical section, which is
> > not allowed(even in preemptible RCU).
> 
> Just a small fix to the commit message:
> 
> This happened when the host hit a page fault, and delivered it as in an
> async page fault, while the guest was in an RCU read-side critical
> section.  The guest then tries to reschedule in kvm_async_pf_task_wait(),
> but rcu_preempt_note_context_switch() would treat the reschedule as a
> sleep in RCU read-side critical section, which is not allowed (even in
> preemptible RCU).  Thus the WARN.
> 
> Queued with that change, thanks.

Not to be repetitive, but if the schedule() is on the guest, this change
really does silently break up an RCU read-side critical section on
guests built with PREEMPT=n.  (Yes, they were already being broken,
but it would be good to avoid this breakage in PREEMPT=n as well as
in PREEMPT=y.)

							Thanx, Paul

> Paolo
> 
> > To cure this, make kvm_async_pf_task_wait() go to the halt path if the
> > PF happens in a RCU read-side critical section.
> > 
> > Reported-by: Sasha Levin <levinsasha928@gmail.com>
> > Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > ---
> >  arch/x86/kernel/kvm.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index aa60a08b65b1..e675704fa6f7 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> > @@ -140,7 +140,8 @@ 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);
> > 
>