Re: [PATCH] PCI: Fix device attach failure handling

[Lukas Wunner <lukas@wunner.de>]

Hi Grygorii,

On Tue, Apr 05, 2016 at 02:29:38PM +0300, Grygorii Strashko wrote:
> On 03/31/2016 03:57 PM, Lukas Wunner wrote:
> > Linux 4.5 introduced a behavioral change in device probing during the
> > suspend process with commit 013c074f8642 ("PM / sleep: prohibit devices
> > probing during suspend/hibernation"): It defers device probing during
> > the entire suspend process, starting from the prepare phase and ending
> > with the complete phase. A rule existed before that "we rely on sub-
> > systems not to do any probing once a device is suspended" but it is
> > enforced only now (Alan Stern, https://lkml.org/lkml/2015/9/15/908).
> > 
> > This resulted in a WARN splat if a PCI device (e.g. Thunderbolt) is
> > plugged in while the system is asleep: Upon waking up, pciehp_resume()
> > discovers new devices in the resume phase and immediately tries to bind
> > them to a driver. Since probing is now deferred, device_attach() returns
> > -EPROBE_DEFER, which provoked a WARN in pci_bus_add_device().
> > 
> > Linux 4.6-rc1 aggravates the situation with commit ab1a187bba5c ("PCI:
> > Check device_attach() return value always"): pci_bus_add_device() no
> > longer sets dev->is_added = 1 if device_attach() returned a negative
> > value. This results in a BUG lockup in pci_bus_add_devices().
> > 
> > Fix the latter by not recursing to a child bus if device_attach() failed
> > for the bridge leading to it.
> > 
> > Fix the former by not interpreting -EPROBE_DEFER as failure. The device
> > will be probed eventually and there is proper locking in place to avoid
> > races (e.g. if devices are unplugged again und thus deleted from the
> > system before deferred probing happens, I have tested this). Also, those
> > functions which dereference dev->driver (e.g. pci_pm_*()) do contain
> > proper NULL pointer checks. So it seems safe to ignore -EPROBE_DEFER.
> > 
> > Note that even postponing the code in pciehp_resume() until the
> > complete phase wouldn't avoid these troubles because dpm_complete()
> > calls device_unblock_probing() only after ->complete has been
> > executed for all devices. We lack a pm hook from which it would
> > be safe to check a hotplug port and call device_attach() without
> > risking -EPROBE_DEFER.
> 
> Unfortunately, I can't say too much about pci in general.
> Regarding checking a hotplug port - Potentially,
> PM notifiers can be used PM_SUSPEND_PREPARE/PM_POST_SUSPEND.
> Smth. similar (more or less) was implemented for MMC
> 
> bbd4368 mmc: core: Signal wakeup event at card insert/removal
> 4c2ef25 mmc: fix all hangs related to mmc/sd card insert/removal during suspend/resume

Thank you, I hadn't thought of PM notifiers.

That would indeed be a viable alternative solution:
- Call pcie_disable_notification() on PM_SUSPEND_PREPARE /
  PM_HIBERNATION_PREPARE.
- Run the code in pciehp_resume() on PM_POST_SUSPEND / PM_POST_HIBERNATION.
  (This includes pcie_enable_notification(). The pciehp_suspend() and
  pciehp_resume() could be dropped completely.)

Bjorn, please let me know if you'd prefer that in favor of the patch below.
(The portion which replaces the BUG_ON would still be needed though.)

Thanks,

Lukas

> > 
> > Cc: Grygorii Strashko <grygorii.strashko@ti.com>
> > Cc: Alan Stern <stern@rowland.harvard.edu>
> > Cc: Rafael J. Wysocki <rafael@kernel.org>
> > Cc: Bjorn Helgaas <bhelgaas@google.com>
> > Signed-off-by: Lukas Wunner <lukas@wunner.de>
> > ---
> >   drivers/pci/bus.c | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/pci/bus.c b/drivers/pci/bus.c
> > index 6c9f546..dd7cdbe 100644
> > --- a/drivers/pci/bus.c
> > +++ b/drivers/pci/bus.c
> > @@ -294,7 +294,7 @@ void pci_bus_add_device(struct pci_dev *dev)
> >   
> >   	dev->match_driver = true;
> >   	retval = device_attach(&dev->dev);
> > -	if (retval < 0) {
> > +	if (retval < 0 && retval != -EPROBE_DEFER) {
> >   		dev_warn(&dev->dev, "device attach failed (%d)\n", retval);
> >   		pci_proc_detach_device(dev);
> >   		pci_remove_sysfs_dev_files(dev);
> > @@ -324,7 +324,9 @@ void pci_bus_add_devices(const struct pci_bus *bus)
> >   	}
> >   
> >   	list_for_each_entry(dev, &bus->devices, bus_list) {
> > -		BUG_ON(!dev->is_added);
> > +		/* Skip if device attach failed */
> > +		if (!dev->is_added)
> > +			continue;
> >   		child = dev->subordinate;
> >   		if (child)
> >   			pci_bus_add_devices(child);
> >