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


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