KVM: Introduce kvm_get_kvm_safe()

Introduce this safe version of kvm_get_kvm() so that it can be called even
during vm destruction.  Use it in kvm_debugfs_open() and remove the verbose
comment.  Prepare to be used elsewhere.
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20210625153214.43106-3-peterx@redhat.com>
[Preserve the comment in kvm_debugfs_open. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/linux/kvm_host.h

@@ -755,6 +755,7 @@ int kvm_init(void *opaque, unsigned vcpu_size, unsigned vcpu_align,
 void kvm_exit(void);
 
 void kvm_get_kvm(struct kvm *kvm);
+bool kvm_get_kvm_safe(struct kvm *kvm);
 void kvm_put_kvm(struct kvm *kvm);
 bool file_is_kvm(struct file *file);
 void kvm_put_kvm_no_destroy(struct kvm *kvm);

virt/kvm/kvm_main.c

@@ -1120,6 +1120,16 @@ void kvm_get_kvm(struct kvm *kvm)
 }
 EXPORT_SYMBOL_GPL(kvm_get_kvm);
 
+/*
+ * Make sure the vm is not during destruction, which is a safe version of
+ * kvm_get_kvm().  Return true if kvm referenced successfully, false otherwise.
+ */
+bool kvm_get_kvm_safe(struct kvm *kvm)
+{
+	return refcount_inc_not_zero(&kvm->users_count);
+}
+EXPORT_SYMBOL_GPL(kvm_get_kvm_safe);
+
 void kvm_put_kvm(struct kvm *kvm)
 {
 	if (refcount_dec_and_test(&kvm->users_count))
@@ -4969,12 +4979,12 @@ static int kvm_debugfs_open(struct inode *inode, struct file *file,
 	struct kvm_stat_data *stat_data = (struct kvm_stat_data *)
 					  inode->i_private;
 
-	/* The debugfs files are a reference to the kvm struct which
-	 * is still valid when kvm_destroy_vm is called.
-	 * To avoid the race between open and the removal of the debugfs
-	 * directory we test against the users count.
+	/*
+	 * The debugfs files are a reference to the kvm struct which
+        * is still valid when kvm_destroy_vm is called.  kvm_get_kvm_safe
+        * avoids the race between open and the removal of the debugfs directory.
 	 */
-	if (!refcount_inc_not_zero(&stat_data->kvm->users_count))
+	if (!kvm_get_kvm_safe(stat_data->kvm))
 		return -ENOENT;
 
 	if (simple_attr_open(inode, file, get,