Re: [PATCH/RFC 1/2] anon-inodes: Remove fd_install() from anon_inode_getfd()

Christoph Hellwig
2008-03-06T15:15:56Z

On Wed, Feb 27, 2008 at 03:42:42PM -0800, Roland Dreier wrote:
> > http://git.kernel.org/?p=linux/kernel/git/viro/vfs-2.6.git;a=commit;h=49be4f8114e6ff0efdab10ebba2493fb67bc3034
>
> Actually, looking closer at the kvm changes here, I think that
> create_vcpu_fd() needs the same treatment as kvm_dev_ioctl_create_vm()
> gets in the patch because of the race I mentioned in the changelog
> for my patch: otherwise kvm_vcpu_release() could drop the last
> reference to vcpu->kvm->filp before the get_file() gets an extra
> reference.

Actually using the file pointer for reference counting in kvm is quite
stupid and risky, it should use a normal reference count instead.
See untested patch below.

> I'm beginning to think that moving the fd_install() out of
> anon_inode_getfd() really is worth it to make a safer interface.

No, the best interface is one where the driver doesn't even see
inode or file. Of course that's not actually possible in a few
nasty cases like the infiniband code, and for those it might be
better to do the fd_isntall themselves.

Index: linux-2.6/include/linux/kvm_host.h
===================================================================
--- linux-2.6.orig/include/linux/kvm_host.h 2008-02-23 20:28:14.000000000 +0100
+++ linux-2.6/include/linux/kvm_host.h 2008-02-23 20:36:20.000000000 +0100
@@ -113,7 +113,7 @@ struct kvm {
KVM_PRIVATE_MEM_SLOTS];
struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
struct list_head vm_list;
- struct file *filp;
+ int refcount;
struct kvm_io_bus mmio_bus;
struct kvm_io_bus pio_bus;
struct kvm_vm_stat stat;
Index: linux-2.6/virt/kvm/kvm_main.c
===================================================================
--- linux-2.6.orig/virt/kvm/kvm_main.c 2008-02-23 20:29:12.000000000 +0100
+++ linux-2.6/virt/kvm/kvm_main.c 2008-02-24 02:34:44.000000000 +0100
@@ -169,6 +169,8 @@ static struct kvm *kvm_create_vm(void)
kvm_io_bus_init(&kvm->pio_bus);
mutex_init(&kvm->lock);
kvm_io_bus_init(&kvm->mmio_bus);
+ kvm->refcount = 1;
+
spin_lock(&kvm_lock);
list_add(&kvm->vm_list, &vm_list);
spin_unlock(&kvm_lock);
@@ -201,11 +203,16 @@ void kvm_free_physmem(struct kvm *kvm)
kvm_free_physmem_slot(&kvm->memslots[i], NULL);
}

-static void kvm_destroy_vm(struct kvm *kvm)
+static void kvm_put_vm(struct kvm *kvm)
{
struct mm_struct *mm = kvm->mm;

spin_lock(&kvm_lock);
+ if (--kvm->refcount) {
+ spin_lock(&kvm_lock);
+ return;
+ }
+
list_del(&kvm->vm_list);
spin_unlock(&kvm_lock);
kvm_io_bus_destroy(&kvm->pio_bus);
@@ -216,9 +223,7 @@ static void kvm_destroy_vm(struct kvm *k

static int kvm_vm_release(struct inode *inode, struct file *filp)
{
- struct kvm *kvm = filp->private_data;
-
- kvm_destroy_vm(kvm);
+ kvm_put_vm(filp->private_data);
return 0;
}

@@ -700,7 +705,7 @@ static int kvm_vcpu_release(struct inode
{
struct kvm_vcpu *vcpu = filp->private_data;

- fput(vcpu->kvm->filp);
+ kvm_put_vm(vcpu->kvm);
return 0;
}

@@ -724,7 +729,16 @@ static int create_vcpu_fd(struct kvm_vcp
"kvm-vcpu", &kvm_vcpu_fops, vcpu);
if (r)
return r;
- atomic_inc(&vcpu->kvm->filp->f_count);
+
+ /*
+ * It is guaranteed that the refcount is non-zero here because
+ * this function is only called as ioctl method on an open
+ * file that has a reference to it.
+ */
+ spin_lock(&kvm_lock);
+ vcpu->kvm->refcount++;
+ spin_unlock(&kvm_lock);
+
return fd;
}

@@ -1023,12 +1037,10 @@ static int kvm_dev_ioctl_create_vm(void)
return PTR_ERR(kvm);
r = anon_inode_getfd(&fd, &inode, &file, "kvm-vm", &kvm_vm_fops, kvm);
if (r) {
- kvm_destroy_vm(kvm);
+ kvm_put_vm(kvm);
return r;
}

- kvm->filp = file;
-
return fd;
}