sched, preempt_notifier: separate notifier registration from static_key inc/dec

Commit 1cde2930e154 ("sched/preempt: Add static_key() to preempt_notifiers")
had two problems.  First, 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.  KVM correctly registers and
unregisters preempt notifiers with preemption disabled, so the sleep
caused dmesg splats.

Second, KVM registers and unregisters preemption notifiers very often
(in vcpu_load/vcpu_put).  With a single uniprocessor guest the static key
would move between 0 and 1 continuously, hitting the slow path on every
userspace exit.

To fix this, wrap the static_key inc/dec in a new API, and call it from
KVM.

Fixes: 1cde2930e154 ("sched/preempt: Add static_key() to preempt_notifiers")
Reported-by: Pontus Fuchs <pontus.fuchs@gmail.com>
Reported-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index 0f1534acaf60983da1cc84548659ccc1398d79e9..84991f1851739490774436726e49b5c43f16db75 100644 (file)
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -293,6 +293,8 @@ struct preempt_notifier {
        struct preempt_ops *ops;
 };
 
+void preempt_notifier_inc(void);
+void preempt_notifier_dec(void);
 void preempt_notifier_register(struct preempt_notifier *notifier);
 void preempt_notifier_unregister(struct preempt_notifier *notifier);
 

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b803e1b8ab0cf7eb381600a32855b822be3c8c89..552710ab19e0ccb6d726e85880d443d84f9d45ef 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2320,13 +2320,27 @@ void wake_up_new_task(struct task_struct *p)
 
 static struct static_key preempt_notifier_key = STATIC_KEY_INIT_FALSE;
 
+void preempt_notifier_inc(void)
+{
+       static_key_slow_inc(&preempt_notifier_key);
+}
+EXPORT_SYMBOL_GPL(preempt_notifier_inc);
+
+void preempt_notifier_dec(void)
+{
+       static_key_slow_dec(&preempt_notifier_key);
+}
+EXPORT_SYMBOL_GPL(preempt_notifier_dec);
+
 /**
  * preempt_notifier_register - tell me when current is being preempted & rescheduled
  * @notifier: notifier struct to register
  */
 void preempt_notifier_register(struct preempt_notifier *notifier)
 {
-       static_key_slow_inc(&preempt_notifier_key);
+       if (!static_key_false(&preempt_notifier_key))
+               WARN(1, "registering preempt_notifier while notifiers disabled\n");
+
        hlist_add_head(&notifier->link, &current->preempt_notifiers);
 }
 EXPORT_SYMBOL_GPL(preempt_notifier_register);
@@ -2340,7 +2354,6 @@ EXPORT_SYMBOL_GPL(preempt_notifier_register);
 void preempt_notifier_unregister(struct preempt_notifier *notifier)
 {
        hlist_del(&notifier->link);
-       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 848af90b8091a9a3bd8ed7e18d6ace2692210c36..8b8a4445367011b31afdb06f5363a120ea1db019 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -553,6 +553,8 @@ static struct kvm *kvm_create_vm(unsigned long type)
        list_add(&kvm->vm_list, &vm_list);
        spin_unlock(&kvm_lock);
 
+       preempt_notifier_inc();
+
        return kvm;
 
 out_err:
@@ -620,6 +622,7 @@ static void kvm_destroy_vm(struct kvm *kvm)
        cleanup_srcu_struct(&kvm->irq_srcu);
        cleanup_srcu_struct(&kvm->srcu);
        kvm_arch_free_vm(kvm);
+       preempt_notifier_dec();
        hardware_disable_all();
        mmdrop(mm);
 }