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authorPeter Zijlstra <peterz@infradead.org>2015-07-03 18:53:58 +0200
committerPaolo Bonzini <pbonzini@redhat.com>2015-07-03 18:55:00 +0200
commit2ecd9d29abb171d6e97a4f3eb29d7456a11401b7 (patch)
tree2c31785e4dff264db31ee0dea5f74c5d29905898
parent9bdc771f2c29a11920f477fba05a58e23ee42554 (diff)
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>
-rw-r--r--include/linux/preempt.h2
-rw-r--r--kernel/sched/core.c17
-rw-r--r--virt/kvm/kvm_main.c3
3 files changed, 20 insertions, 2 deletions
diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index 0f1534acaf60..84991f185173 100644
--- 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 b803e1b8ab0c..552710ab19e0 100644
--- 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 848af90b8091..8b8a44453670 100644
--- 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);
}