linux-kernel - Re: [patch 31/32] genirq/msi: Simplify sysfs handling

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Date:   Sun, 28 Nov 2021 20:33:13 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Marc Zygnier <maz@...nel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Megha Dey <megha.dey@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-pci@...r.kernel.org,
        linux-s390@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Jon Mason <jdmason@...zu.us>,
        Dave Jiang <dave.jiang@...el.com>,
        Allen Hubbe <allenbh@...il.com>, linux-ntb@...glegroups.com,
        Neil Horman <nhorman@...driver.com>
Subject: Re: [patch 31/32] genirq/msi: Simplify sysfs handling

On Sun, Nov 28 2021 at 12:07, Greg Kroah-Hartman wrote:
> On Sat, Nov 27, 2021 at 08:31:37PM +0100, Thomas Gleixner wrote:
>> On Sat, Nov 27 2021 at 13:32, Greg Kroah-Hartman wrote:
>> > On Sat, Nov 27, 2021 at 02:23:15AM +0100, Thomas Gleixner wrote:
>> >> The sysfs handling for MSI is a convoluted maze and it is in the way of
>> >> supporting dynamic expansion of the MSI-X vectors because it only supports
>> >> a one off bulk population/free of the sysfs entries.
>> >> 
>> >> Change it to do:
>> >> 
>> >>    1) Creating an empty sysfs attribute group when msi_device_data is
>> >>       allocated
>> >> 
>> >>    2) Populate the entries when the MSI descriptor is initialized
>> >
>> > How much later does this happen?  Can it happen while the device has a
>> > driver bound to it?
>> 
>> That's not later than before. It's when the driver initializes the
>> MSI[X] interrupts, which usually happens in the probe() function.
>> 
>> The difference is that the group, (i.e.) directory is created slightly
>> earlier.
>
> Ok, but that still happens when probe() is called for the driver,
> right?

Yes.

>> >> +static inline int msi_sysfs_create_group(struct device *dev)
>> >> +{
>> >> +	return devm_device_add_group(dev, &msi_irqs_group);
>> >
>> > Much nicer, but you changed the lifetime rules of when these attributes
>> > will be removed, is that ok?
>> 
>> The msi entries are removed at the same place as they are removed in the
>> current mainline code, i.e. when the device driver shuts the device
>> down and disables MSI[X], which happens usually during remove()
>> 
>> What's different now is that the empty group stays around a bit
>> longer. I don't see how that matters.
>
> How much longer does it stick around?
>
> What happens if this sequence happens:
> 	- probe()
> 	- disconnect()
> 	- probe()
> with the same device (i.e. the device is not removed from the system)?
>
> Which can happen as userspace can trigger disconnect() or even worse, if
> the driver is unloaded and then loaded again?  Will the second call to
> create this directory fail as it is not cleaned up yet?
>
> I can never remember if devm_*() stuff sticks around for the device
> lifecycle, or for the driver/device lifecycle, which is one big reason
> why I don't like that api...

Driver lifecycle AFAICT.

>> > I still worry that these attributes show up "after" the device is
>> > registered with the driver core, but hey, it's no worse than it
>> > currently is, so that's not caused by this patch series...
>> 
>> Happens that register before or after driver->probe()?
>
> During probe is a bit too late, but we can handle that as we are used to
> it.  If it happens after probe() succeeds, based on something else being
> asked for in the driver (like the device being opened), then userspace
> has no chance of ever noticing these attributes being added.
>
> But again, this isn't new to your code series, so I wouldn't worry about
> it.  Obviously userspace tools do not care or really notice these
> attributes at all otherwise the authors of them would have complained
> a long time ago :)

I have no idea how these attributes are used at all. Neil should knows
as he added it in the first place.

> So again, no real objection from me here, just meta-comments, except for
> the above thing with the devm_* call to ensure that the
> probe/disconnect/probe sequence will still work just as well as it does
> today.  Should be easy enough to test out by just unloading a module and
> then loading it again with this patch series applied.

That works just fine. Tested that already before posting. After module
removal the directory is gone.

Thanks,

        tglx