[PATCH 2/9] KVM: Introduce kvm_get_kvm_safe()

[Peter Xu <peterx@redhat.com>]

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>
---
 include/linux/kvm_host.h |  1 +
 virt/kvm/kvm_main.c      | 17 +++++++++++------
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 2f34487e21f2..53d7d09eebd7 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -698,6 +698,7 @@ 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);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d170f65e15b0..0b4f55370b18 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1049,6 +1049,16 @@ 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))
@@ -4713,12 +4723,7 @@ 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.
-	 */
-	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,
-- 
2.31.1