[PATCH v4] KVM: async_pf: make rcu irq exit if not triggered from idle task

From: Wanpeng Li
Date: Tue Aug 01 2017 - 08:20:14 EST

From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>

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:
? kvm_async_pf_task_wait+0x1b2/0x270
? prepare_to_swait+0x22/0x70
? do_async_page_fault+0x77/0xb0
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 9b132fbe5419 (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. There is no need to call
this pairs if async page fault is not triggered from idle task.

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@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
v3 -> v4:
* pull rcu_irq_exit/enter outside the "if"
v2 -> v3:
* add the warning log
v1 -> v2:
* update patch description

arch/x86/kernel/kvm.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 71c17a5..d04e30e 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -151,6 +151,8 @@ void kvm_async_pf_task_wait(u32 token)
if (hlist_unhashed(&n.link))

+ rcu_irq_exit();
if (!n.halted) {
@@ -159,11 +161,11 @@ void kvm_async_pf_task_wait(u32 token)
* We cannot reschedule. So halt.
- rcu_irq_exit();
- rcu_irq_enter();
+ rcu_irq_enter();
if (!n.halted)
finish_swait(&n.wq, &wait);