Date:   Tue, 29 Nov 2022 08:30:34 +0900
From:   Vincent MAILHOL <mailhol.vincent@...adoo.fr>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Andrew Lunn <andrew@...n.ch>, linux-can@...r.kernel.org,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        netdev@...r.kernel.org, linux-usb@...r.kernel.org,
        Saeed Mahameed <saeed@...nel.org>,
        Jiri Pirko <jiri@...dia.com>,
        Lukas Magel <lukas.magel@...teo.net>
Subject: Re: [PATCH v4 2/6] can: etas_es58x: add devlink support

On Tue. 29 Nov. 2022 at 00:50, Alan Stern <stern@...land.harvard.edu> wrote:
> On Mon, Nov 28, 2022 at 02:32:23PM +0900, Vincent MAILHOL wrote:
> > On Mon. 28 Nov. 2022 at 10:34, Vincent MAILHOL
> > <mailhol.vincent@...adoo.fr> wrote:
> > > On Mon. 28 Nov. 2022 at 00:41, Alan Stern <stern@...land.harvard.edu> wrote:
> > > > On Sun, Nov 27, 2022 at 02:10:32PM +0900, Vincent MAILHOL wrote:
> > > > > > Should devlink_free() be after usb_set_inftdata()?
> > > > >
> > > > > A look at
> > > > >   $ git grep -W "usb_set_intfdata(.*NULL)"
> > > > >
> > > > > shows that the two patterns (freeing before or after
> > > > > usb_set_intfdata()) coexist.
> > > > >
> > > > > You are raising an important question here. usb_set_intfdata() does
> > > > > not have documentation that freeing before it is risky. And the
> > > > > documentation of usb_driver::disconnect says that:
> > > > >   "@disconnect: Called when the interface is no longer accessible,
> > > > >    usually because its device has been (or is being) disconnected
> > > > >    or the driver module is being unloaded."
> > > > >   Ref: https://elixir.bootlin.com/linux/v6.1-rc6/source/include/linux/usb.h#L1130
> > > > >
> > > > > So the interface no longer being accessible makes me assume that the
> > > > > order does not matter. If it indeed matters, then this is a foot gun
> > > > > and there is some clean-up work waiting for us on many drivers.
> > > > >
> > > > > @Greg, any thoughts on whether or not the order of usb_set_intfdata()
> > > > > and resource freeing matters or not?
> > > >
> > > > In fact, drivers don't have to call usb_set_intfdata(NULL) at all; the
> > > > USB core does it for them after the ->disconnect() callback returns.
> > >
> > > Interesting. This fact is widely unknown, cf:
> > >   $ git grep "usb_set_intfdata(.*NULL)" | wc -l
> > >   215
> > >
> > > I will do some clean-up later on, at least for the CAN USB drivers.
> > >
> > > > But if a driver does make the call, it should be careful to ensure that
> > > > the call happens _after_ the driver is finished using the interface-data
> > > > pointer.  For example, after all outstanding URBs have completed, if the
> > > > completion handlers will need to call usb_get_intfdata().
> > >
> > > ACK. I understand that it should be called *after* the completion of
> > > any ongoing task.
> > >
> > > My question was more on:
> > >
> > >         devlink_free(priv_to_devlink(es58x_dev));
> > >         usb_set_intfdata(intf, NULL);
> > >
> > > VS.
> > >
> > >         usb_set_intfdata(intf, NULL);
> > >         devlink_free(priv_to_devlink(es58x_dev));
> > >
> > > From your comments, I understand that both are fine.
> >
> > Do we agree that the usb-skeleton is doing it wrong?
> >   https://elixir.bootlin.com/linux/latest/source/drivers/usb/usb-skeleton.c#L567
> > usb_set_intfdata(interface, NULL) is called before deregistering the
> > interface and terminating the outstanding URBs!
>
> Going through the usb-skeleton.c source code, you will find that
> usb_get_intfdata() is called from only a few routines:
>
>         skel_open()
>         skel_disconnect()
>         skel_suspend()
>         skel_pre_reset()
>         skel_post_reset()
>
> Of those, all but the first are called only by the USB core and they are
> mutually exclusive with disconnect processing (except for
> skel_disconnect() itself, of course).  So they don't matter.
>
> The first, skel_open(), can be called as a result of actions by the
> user, so the driver needs to ensure that this can't happen after it
> clears the interface-data pointer.  The user can open the device file at
> any time before the minor number is given back, so it is not proper to
> call usb_set_intfdata(interface, NULL) before usb_deregister_dev() --
> but the driver does exactly this!
>
> (Well, it's not quite that bad.  skel_open() does check whether the
> interface-data pointer value it gets from usb_get_intfdata() is NULL.
> But it's still a race.)
>
> So yes, the current code is wrong.  And in fact, it will still be wrong
> even after the usb_set_intfdata(interface, NULL) line is removed,
> because there is no synchronization between skel_open() and
> skel_disconnect().

ACK. I did not look outside of skel_disconnect(). Regardless, I think
that removing the usb_set_intdata(interface, NULL) is still one step
in the good direction despite the other synchronisation issues. I sent
a patch for that which Greg already pick-up:
  https://git.kernel.org/gregkh/usb/c/c568f8bb41a4

>It is possible for skel_disconnect() to run to
> completion and the USB core to clear the interface-data pointer all
> while skel_open() is running.  The driver needs a static private mutex
> to synchronize opens with unregistrations.  (This is a general
> phenomenon, true of all drivers that have a user interface such as a
> device file.)
>
> The driver _does_ have a per-instance mutex, dev->io_mutex, to
> synchronize I/O with disconnects.  But that's separate from
> synchronizing opens with unregistrations, because at open time the
> driver doesn't yet know the address of the private data structure or
> even if the structure is still allocated.  So obviously it can't use a
> mutex that is embedded within the private data structure for this
> purpose.

ACK. However, I have other priorities, I will not invest more time to
dig in the usb-skeleton.c

Thank you for the answer! That was a long but interesting diversion
from the initial topic :)


Yours sincerely,
Vincent Mailhol

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