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SubjectRe: sysfs methods can race with ->remove
Hello, Alan.

On Thu, Jan 15, 2015 at 01:22:03PM -0500, Alan Stern wrote:
> > It has a reference to keep it from beeing freed, but so far I can't find
> > anything that prevents ->remove from beeing called while we are in or
> > just before a method call.
>
> There are two types of methods to think about: Those registered by the
> subsystem and those registered by the driver.
>
> If a method is registered by the driver, then the driver will
> unregister it when the ->remove routine runs. I don't know for
> certain, but I would expect that the sysfs/kernfs core will make sure
> that any existing method calls complete before unregister returns.
> This would prevent races.

Yes, attribute deletions are blocked till the on-going sysfs
read/write operations are finished and further rw accesses are failed.

> If a method is registered by the subsystem, and if the method runs
> entirely within the subsystem's code, then ->remove doesn't matter.
> The driver could be unbound while the method is running and it would be
> okay.
>
> The only time we have a problem is when the method is registered by the
> subsystem and the method calls into the driver. (Note that this is
> exactly what happens with scsi_rescan_device.)
>
> > > > But this seems like a more generic problem, and at least a quick glance at
> > > > the pci_driver methods seems like others don't have a good
> > > > synchroniation of ->remove against random driver methods.
> > >
> > > Can you give one or two examples?
> >
> > I look at the sriov_configure PCI method, or the various sub-methods
> > under pci_driver.err_handler.
>
> The sriov_numvfs_store method does have the same problem, and so does
> the reset_store method (by way of pci_reset_function ->
> pci_dev_save_and_disable -> pci_reset_notify).
>
> Tejun, is my analysis correct? How should we fix these races?

I'm not really following what the actual problem case is, so SCSI
subsystem store methods are derefing dev->driver without synchronizing
against detach events? If that's the case, the solution would be
synchronizing against attach/detach events? Sorry if I'm being
totally idiotic. I'm having a bit of hard time jumping right in. :)

Thanks.

--
tejun


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