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    SubjectRe: [RFC] PCI device list locking - take 2
    * Greg KH (greg@kroah.com) wrote:
    > static void *pci_seq_start(struct seq_file *m, loff_t *pos)
    > {
    > - struct list_head *p = &pci_devices;
    > + struct pci_dev *dev = NULL;
    > loff_t n = *pos;
    >
    > - /* XXX: surely we need some locking for traversing the list? */
    > + dev = pci_get_device(PCI_ANY_ID, PCI_ANY_ID, dev);
    > while (n--) {
    > - p = p->next;
    > - if (p == &pci_devices)
    > - return NULL;
    > + dev = pci_get_device(PCI_ANY_ID, PCI_ANY_ID, dev);
    > + if (dev == NULL)
    > + goto exit;

    I think this still has the same problem. pci_get_device grabs lock,
    walks list, gets ref, and drops lock. But the ref doesn't hold it on the
    list, right?. So some pci_remove_* could do list_del(&dev->global_list),
    poison the prev/next pointers. Subsequent pci_get_device would do ->next
    and oops. It seems the lock needs to be held for entire start/next/stop
    sequence, or the ref needs to keep it on list.

    > +struct pci_dev *
    > +pci_get_subsys(unsigned int vendor, unsigned int device,
    <snip>
    > +exit:
    > + if (from)
    > + pci_put_dev(from);
    > + if (dev)
    > + pci_get_dev(dev);

    Heh, the hch in me notes that pci_{put,get}_dev already check NULL device ;-)

    thanks,
    -chris
    --
    Linux Security Modules http://lsm.immunix.org http://lsm.bkbits.net
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