[PATCH] sched,kvm: Fix KVM preempt_notifier usage

[Peter Zijlstra <peterz@infradead.org>]

Subject: sched,kvm: Fix KVM preempt_notifier usage

The preempt-notifier API needs to sleep with the addition of the
static_key, we do however need to hold off preemption while modifying
the preempt notifier list, otherwise a preemption could observe an
inconsistent list state.

There is no reason to have preemption disabled in the caller,
registering a preempt notifier is a purely task local affair.

Therefore move the preempt_disable into the functions and change the
callers to a preemptible context.

Cc: Gleb Natapov <gleb@kernel.org>
Fixes: 1cde2930e154 ("sched/preempt: Add static_key() to preempt_notifiers")
Reported-and-Tested-by: Pontus Fuchs <pontus.fuchs@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/sched/core.c | 11 +++++++++++
 virt/kvm/kvm_main.c |  5 +++--
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b803e1b8ab0c..6169c167ac98 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2327,7 +2327,12 @@ static struct static_key preempt_notifier_key = STATIC_KEY_INIT_FALSE;
 void preempt_notifier_register(struct preempt_notifier *notifier)
 {
 	static_key_slow_inc(&preempt_notifier_key);
+	/*
+	 * Avoid preemption while changing the preempt notifier list.
+	 */
+	preempt_disable();
 	hlist_add_head(&notifier->link, &current->preempt_notifiers);
+	preempt_enable();
 }
 EXPORT_SYMBOL_GPL(preempt_notifier_register);
 
@@ -2339,7 +2344,13 @@ EXPORT_SYMBOL_GPL(preempt_notifier_register);
  */
 void preempt_notifier_unregister(struct preempt_notifier *notifier)
 {
+	/*
+	 * Avoid preemption while changing the preempt notifier list.
+	 */
+	preempt_disable();
 	hlist_del(&notifier->link);
+	preempt_enable();
+
 	static_key_slow_dec(&preempt_notifier_key);
 }
 EXPORT_SYMBOL_GPL(preempt_notifier_unregister);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 90977418aeb6..d7aafa0458a0 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -128,8 +128,9 @@ int vcpu_load(struct kvm_vcpu *vcpu)
 
 	if (mutex_lock_killable(&vcpu->mutex))
 		return -EINTR;
-	cpu = get_cpu();
 	preempt_notifier_register(&vcpu->preempt_notifier);
+
+	cpu = get_cpu();
 	kvm_arch_vcpu_load(vcpu, cpu);
 	put_cpu();
 	return 0;
@@ -139,8 +140,8 @@ void vcpu_put(struct kvm_vcpu *vcpu)
 {
 	preempt_disable();
 	kvm_arch_vcpu_put(vcpu);
-	preempt_notifier_unregister(&vcpu->preempt_notifier);
 	preempt_enable();
+	preempt_notifier_unregister(&vcpu->preempt_notifier);
 	mutex_unlock(&vcpu->mutex);
 }