Re: [RFC PATCH 2/6] pci: Set pci_dev->is_added before calling device_add

From: Bjorn Helgaas
Date: Fri Aug 17 2018 - 12:25:39 EST


On Fri, Aug 17, 2018 at 02:48:58PM +1000, Benjamin Herrenschmidt wrote:
> This re-fixes the bug reported by Hari Vyas <hari.vyas@xxxxxxxxxxxx>
> after my revert of his commit but in a much simpler way.
>
> The main issues is that is_added was being set after the driver
> got bound and started, and thus setting it could race with other
> changes to struct pci_dev.

The "bind driver, then set dev->added = 1" order seems to have been
there since the beginning of dev->is_added:

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=8a1bc9013a03

This patch seems reasonable, but I'm a little dubious about the
existence of "is_added" in the first place. As far as I can tell, the
only other buses with something similar are the MEN Chameleon bus and
the Intel Management Engine Interface.

The PCI uses of "is_added" don't seem *that* critical or unique to
PCI, so I'm not 100% convinced we need it at all. But I haven't
looked into it enough to be able to propose an alternative.

> This fixes it by setting the flag first, which also has the
> advantage of matching the fact that we are clearing it *after*
> unbinding in the remove path, thus the flag is now symtetric
> and always set while the driver code is running.
>
> Signed-off-by: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> ---
> drivers/pci/bus.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/pci/bus.c b/drivers/pci/bus.c
> index 35b7fc87eac5..48ae63673aa8 100644
> --- a/drivers/pci/bus.c
> +++ b/drivers/pci/bus.c
> @@ -321,16 +321,16 @@ void pci_bus_add_device(struct pci_dev *dev)
> pci_proc_attach_device(dev);
> pci_bridge_d3_update(dev);
>
> + dev->is_added = 1;
> dev->match_driver = true;
> retval = device_attach(&dev->dev);
> if (retval < 0 && retval != -EPROBE_DEFER) {
> + dev->is_added = 0;
> pci_warn(dev, "device attach failed (%d)\n", retval);
> pci_proc_detach_device(dev);
> pci_remove_sysfs_dev_files(dev);
> return;
> }
> -
> - dev->is_added = 1;
> }
> EXPORT_SYMBOL_GPL(pci_bus_add_device);
>
> --
> 2.17.1
>