KVM: async_pf: make rcu irq exit if not triggered from idle task

 WARNING: CPU: 5 PID: 1242 at kernel/rcu/tree_plugin.h:323 rcu_note_context_switch+0x207/0x6b0
 CPU: 5 PID: 1242 Comm: unity-settings- Not tainted 4.13.0-rc2+ #1
 RIP: 0010:rcu_note_context_switch+0x207/0x6b0
 Call Trace:
  __schedule+0xda/0xba0
  ? kvm_async_pf_task_wait+0x1b2/0x270
  schedule+0x40/0x90
  kvm_async_pf_task_wait+0x1cc/0x270
  ? prepare_to_swait+0x22/0x70
  do_async_page_fault+0x77/0xb0
  ? do_async_page_fault+0x77/0xb0
  async_page_fault+0x28/0x30
 RIP: 0010:__d_lookup_rcu+0x90/0x1e0

I encounter this when trying to stress the async page fault in L1 guest w/
L2 guests running.

Commit 9b132fbe (Add rcu user eqs exception hooks for async page
fault) adds rcu_irq_enter/exit() to kvm_async_pf_task_wait() to exit cpu
idle eqs when needed, to protect the code that needs use rcu.  However,
we need to call the pair even if the function calls schedule(), as seen
from the above backtrace.

This patch fixes it by informing the RCU subsystem exit/enter the irq
towards/away from idle for both n.halted and !n.halted.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kernel/kvm.c

@@ -151,6 +151,8 @@ void kvm_async_pf_task_wait(u32 token)
 		if (hlist_unhashed(&n.link))
 			break;
 
+		rcu_irq_exit();
+
 		if (!n.halted) {
 			local_irq_enable();
 			schedule();
@@ -159,11 +161,11 @@ void kvm_async_pf_task_wait(u32 token)
 			/*
 			 * We cannot reschedule. So halt.
 			 */
-			rcu_irq_exit();
 			native_safe_halt();
 			local_irq_disable();
-			rcu_irq_enter();
 		}
+
+		rcu_irq_enter();
 	}
 	if (!n.halted)
 		finish_swait(&n.wq, &wait);