Re: [PATCH 2/2] gpiolib: cdev: export the consumer's PID

[Kent Gibson <warthog618@gmail.com>]

On Tue, Sep 13, 2022 at 10:54:26AM +0200, Bartosz Golaszewski wrote:
> On Tue, Sep 13, 2022 at 4:12 AM Kent Gibson <warthog618@gmail.com> wrote:
> >
> > On Mon, Sep 12, 2022 at 11:56:17AM +0200, Bartosz Golaszewski wrote:
> > > On Mon, Sep 12, 2022 at 11:53 AM Kent Gibson <warthog618@gmail.com> wrote:
> > > >
> > >
> > > [snip]
> > >
> > > > >
> > > > > Using -1 sounds good but I've just realized there's a different
> > > > > problem. A process holding a file descriptor may fork and both the
> > > > > parent and the child will keep the same file descriptors open. Now
> > > > > we'll have two processes (with different PIDs) holding the same GPIO
> > > > > lines (specifically holding a file descriptor to the same anonymous
> > > > > inode).
> > > > >
> > > > > This already poses a problem for this patch as we'd need to return an
> > > > > array of PIDs which we don't have the space for but also is a
> > > > > situation which we haven't discussed previously IIRC - two processes
> > > > > keeping the same GPIO lines requested.
> > > > >
> > > > > I don't have any good idea on how to address this yet. One thing off
> > > > > the top of my head is: close the parent's file descriptor from kernel
> > > > > space (is it even possible?) on fork() (kind of like the close() on
> > > > > exec flag).
> > > > >
> > > > > I need to think about it more.
> > > > >
> > > >
> > > > I thought the O_CLOEXEC was set on the request fds exactly to prevent this
> > > > case - only one process can hold the request fd.
> > > >
> > >
> > > O_CLOEXEC means "close on exec" not "close on fork". When you fork,
> > > you inherit all file descriptors from your parent. Only once you call
> > > execve() are the fds with this flag closed *in the child*.
> > >
> >
> > Ah, ok.
> > You want to pass request fd ownership from parent to child??
> > Why not lock ownership to the parent, so O_CLOFORK, were that
> > available?
> >
> 
> Because what if we want to request a line and then daemonize i.e. fork
> and exit in parent? It makes much more sense to keep the lines
> requested in the child IMO.
> 

Then you are doing it backwards - daemonize first ;-).

Generally speaking, doesn't transfer of resource ownership to the forked
child create havoc in multi-threaded apps? i.e. one thread requests a
resource, another forks.  The parent thread unknowingly loses ownership,
and the forked child process only starts with a replica of the forking
thread.

> During the BoF at Linux Plumbers it was suggested to use
> /proc/$PID/fdinfo to expose the information about which lines are
> requested but I can't figure out a way to do it elegantly.
> 

Yeah, missed that :-(.

Makes sense.

As each request fd can contain multiple lines on a particular chip,
you would need to identify the gpiochip and the offsets for that request.
So two fields - the gpiochip path, and the list of offsets.

Is that already too clunky or am I missing something?

Cheers,
Kent.