Patch "KVM: async_pf: make rcu irq exit if not triggered from idle task" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-async_pf-make-rcu-irq-exit-if-not-triggered-from-idle-task.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Wanpeng Li <[email protected]>
Date: Tue, 1 Aug 2017 05:20:03 -0700
Subject: KVM: async_pf: make rcu irq exit if not triggered from idle task
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From: Wanpeng Li <[email protected]>

commit 337c017ccdf2653d0040099433fc1a2b1beb5926 upstream.

 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 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. 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.

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

@@ -152,6 +152,8 @@ void kvm_async_pf_task_wait(u32 token)
 		if (hlist_unhashed(&n.link))
 			break;
 
+		rcu_irq_exit();
+
 		if (!n.halted) {
 			local_irq_enable();
 			schedule();

@@ -160,11 +162,11 @@ 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);

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/kvm-async_pf-make-rcu-irq-exit-if-not-triggered-from-idle-task.patch