Re: [PATCH v2 04/10] PCI: Destroy pci dev only once

[Rafael J. Wysocki]

On Thursday, December 05, 2013 10:52:36 PM Yinghai Lu wrote:
> On Mon, Dec 2, 2013 at 6:49 AM, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> >
> > Scenario 5: pci_stop_and_remove_bus_device() is run concurrently
> >   for a device and its parent bridge via remove_callback().
> >
> >   In that case both code paths attempt to acquire
> >   pci_remove_rescan_mutex.  If the child device removal acquires
> >   it first, there will be no problems.  However, if the parent
> >   bridge removal acquires it first, it will eventually execute
> >   pci_destroy_dev() for the child device, but that device will
> >   not be freed yet due to the reference held by the concurrent
> >   child removal.  Consequently, both pci_stop_bus_device() and
> >   pci_remove_bus_device() will be executed for that device
> >   unnecessarily and pci_destroy_dev() will see a corrupted list
> >   head in that object.  Moreover, an excess put_device() will
> >   be executed for that device in that case which may lead to a
> >   use-after-free in the final kobject_put() done by
> >   sysfs_schedule_callback_work().
> >
> > Index: linux-pm/include/linux/pci.h
> > ===================================================================
> > --- linux-pm.orig/include/linux/pci.h
> > +++ linux-pm/include/linux/pci.h
> > @@ -321,6 +321,7 @@ struct pci_dev {
> >         unsigned int    multifunction:1;/* Part of multi-function device */
> >         /* keep track of device state */
> >         unsigned int    is_added:1;
> > +       unsigned int    is_gone:1;
> >         unsigned int    is_busmaster:1; /* device is busmaster */
> >         unsigned int    no_msi:1;       /* device may not use msi */
> >         unsigned int    block_cfg_access:1;     /* config space access is blocked */
> > Index: linux-pm/drivers/pci/remove.c
> > ===================================================================
> > --- linux-pm.orig/drivers/pci/remove.c
> > +++ linux-pm/drivers/pci/remove.c
> > @@ -34,6 +34,7 @@ static void pci_stop_dev(struct pci_dev
> >
> >  static void pci_destroy_dev(struct pci_dev *dev)
> >  {
> > +       dev->is_gone = 1;
> >         device_del(&dev->dev);
> >
> >         down_write(&pci_bus_sem);
> > @@ -109,8 +110,10 @@ static void pci_remove_bus_device(struct
> >   */
> >  void pci_stop_and_remove_bus_device(struct pci_dev *dev)
> >  {
> > -       pci_stop_bus_device(dev);
> > -       pci_remove_bus_device(dev);
> > +       if (!dev->is_gone) {
> > +               pci_stop_bus_device(dev);
> > +               pci_remove_bus_device(dev);
> > +       }
> >  }
> >  EXPORT_SYMBOL(pci_stop_and_remove_bus_device);
> >
> 
> Yes, above change should address sys double remove problem.

I've just realized that we don't need a new flag for that, though.

It looks like we only need to check dev->dev.kobj.parent and return if that is
NULL, because that means pci_destroy_dev() has run for that device already
(I'm wondering why device_del() doesn't clear dev->parent, BTW, it looks like
it should do that?).

Of course, that still is going to be racy if we don't hold
pci_remove_rescan_mutex around pci_stop_and_remove_bus_device() in every code
path using it (or use another similar synchronization mechanism).

Thanks,
Rafael

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