* Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>:
> On Wed, 18 Mar 2009 16:40:06 -0600 Alex Chiang <achiang@xxxxxx> wrote:
>
> > This patch adds an attribute named "remove" to a PCI device's sysfs
> > directory. Writing a non-zero value to this attribute will remove the PCI
> > device and any children of it.
> >
> > Trent Piepho wrote the original implementation and documentation.
> >
> > Thanks to Vegard Nossum for testing under kmemcheck and finding locking
> > issues with the sysfs interface.
> >
> > ...
> >
> > --- a/drivers/pci/pci-sysfs.c
> > +++ b/drivers/pci/pci-sysfs.c
> > @@ -246,6 +246,47 @@ struct bus_attribute pci_bus_attrs[] = {
> > __ATTR(rescan, S_IWUSR, NULL, bus_rescan_store),
> > __ATTR_NULL
> > };
> > +
> > +static void remove_callback(struct device *dev)
> > +{
> > + int bridge = 0;
> > + struct pci_dev *pdev = to_pci_dev(dev);
> > +
> > + mutex_lock(&pci_remove_rescan_mutex);
> > +
> > + if (pdev->subordinate)
> > + bridge = 1;
> > +
> > + pci_remove_bus_device(pdev);
> > + if (bridge && list_empty(&pdev->bus->devices))
> > + pci_remove_bus(pdev->bus);
> > +
> > + mutex_unlock(&pci_remove_rescan_mutex);
> > +}
> > +
> > +static ssize_t
> > +remove_store(struct device *dev, struct device_attribute *dummy,
> > + const char *buf, size_t count)
> > +{
> > + int ret = 0;
> > + unsigned long val;
> > + struct pci_dev *pdev = to_pci_dev(dev);
> > +
> > + if (strict_strtoul(buf, 0, &val) < 0)
> > + return -EINVAL;
> > +
> > + if (!capable(CAP_SYS_ADMIN))
> > + return -EPERM;
> > +
> > + if (pdev->subordinate && pci_is_root_bus(pdev->bus))
> > + return -EBUSY;
> > +
> > + if (val)
> > + ret = device_schedule_callback(dev, remove_callback);
> > + if (ret)
> > + count = ret;
> > + return count;
> > +}
> > #endif
>
> It is very hard for the reader (this one at least) to work out why
> device_schedule_callback() is used here, instead of simply doing the work
> directly.
>
> The way to solve that problem is to add a code comment.
Hm, I thought it was well-known that a sysfs attribute cannot
remove itself without deadlocking. Thus, we need to use this
callback mechanism.
This thread has the most recent discussion:
http://thread.gmane.org/gmane.linux.kernel/805033
I will add a comment to the code.
> Given that we're in a sysfs write() handler where no relevant locks at all
> are held, it seems rather weird that we cannot perform this operation
> synchronously, but no doubt the comment will explain all of this.
>
> Do we need the CAP_SYS_ADMIN check if the sysfs file permissions are
> correct? (I keep on asking this then forgetting the answer).
I will remove this (as per Greg's advice in a later mail).
> The device_schedule_callback() thing exposes us to (I assume) a pile of
> races, the most obvious of which is "what locking or refcounting keeps
> *dev alive?". It would be nice to see an analysis/description of the
> lifetime issues here. Perhaps in the changelog, preferably in code
> comments.
No races; device_schedule_callback() takes a ref on dev, pinning
it until the callback handler returns, after which it releases
the ref. You can see this in sysfs_schedule_callback.
Thanks for the review.
/ac
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