Re: [PATCH v5 0/4] New Microsemi PCI Switch Management Driver

[Logan Gunthorpe <logang@deltatee.com>]

On 01/03/17 02:41 PM, Bjorn Helgaas wrote:
> I don't think this is indicating a bug in the PCI core (although I do
> think a BUG_ON() here is an excessive response).  I think it's an
> indication that the driver didn't disconnect its ISR.  Without more
> details of the failure it's hard to tell if the BUG_ON is a symptom of
> a problem in the driver or what.

Yes, my assumption was that when you force an unbind on the PCI core,
it's designed to stop using the PCI device right away even if there are
users using it. Thus it becomes the drivers responsibility to handle
this situation.

> An "alive" flag feels racy, and I can't tell if it's really the best
> way to deal with this, or if it's just avoiding the issue.  There must
> be other drivers with the same cleanup issue -- do they handle it the
> same way?

I haven't done a comprehensive search, but it's very common for people
to use (and this is what I've adopted again in v5):

devm_request_irq(&pdev->dev, ...)

In this way, the IRQs are released with the pci_dev (or often platform)
and thus the BUG_ON never hits. However, it means any user space program
waiting on an IRQ (like via a cdev call) will hang unless handled with
other means. Exactly what those means are seems driver specific and not
always obvious. I wouldn't be surprised if a lot of drivers get this
aspect wrong.

A couple examples I've looked at:

1) drivers/dax/dax.c uses an alive flag without any mutexes, atomics or
anything. So I don't know if it's racy or perhaps correct for other reasons.

2) drivers/char/hw_random has a drop_current_rng that looks like it
could easily be racy with the get_current_rng in the userspace flow.

3) A couple of drivers drivers/char/tpm doesn't seem to have any
protection at all and appears like they would continue to use io
operations even after the they may get unmapped because the char device
persists.

So I'm not sure where you'd find a driver that does it correctly and in
a simpler way..

Another thing: based on comments in [1], a lot of people don't seem to
realize that cdev instances can persist long after cdev_del so it's
probably very common for drivers to get this wrong.

Logan

[1] https://lists.01.org/pipermail/linux-nvdimm/2017-February/009001.html



>>   To solve this, we've moved the pci release code back into the
>>   unregister function and reintroduced an alive flag. This time,
>>   however, the alive flag is protected by mrpc_mutex and we're very
>>   careful about what happens to devices still in use (they should
>>   all be released through the timeout path and an ENODEV error
>>   returned to userspace; while new commands are blocked with the
>>   same error).