[Devel] Re: [PATCH] ia64 sn xpc: Convert to use kthread API.

[Dean Nelson]

On Wed, May 02, 2007 at 09:44:11AM -0600, Eric W. Biederman wrote:
> Dean Nelson <dcn at sgi.com> writes:
> > On Thu, Apr 26, 2007 at 01:11:15PM -0600, Eric W. Biederman wrote:
> >>
> >> Ok.    Because of the module unloading issue, and because we don't have
> >> a lot of these threads running around, the current plan is to fix
> >> thread_create and kthread_stop so that they must always be paired,
> >> and so that kthread_stop will work correctly if the task has already
> >> exited.
> >>
> >> Basically that just involves calling get_task_struct in kthread_create
> >> and put_task_struct in kthread_stop.
> >
> > Okay, so I need to expand upon Christoph Hellwig's patch so that all
> > the kthread_create()'d threads are kthread_stop()'d.
> >
> > This is easy to do for the XPC thread that exists for the lifetime of XPC,
> > as well as for the threads created to manage the SGI system partitions.
> >
> > XPC has the one discovery thread that is created when XPC is first started
> > and exits as soon as it has finished discovering all existing SGI system
> > partitions. With your forthcoming change to kthread_stop() that will allow
> > it to be called after the thread has exited, doing this one is also easy.
> > Note that the kthread_stop() for this discovery thread won't occur until
> > XPC is rmmod'd. This means that its task_struct will not be freed for
> > possibly a very long time (i.e., weeks). Is that a problem?
> 
> As long as there is only one, not really.  It would be good if we could
> get rid of it though.
> 
> The practical problem is the race with rmmod, in particular if someone
> calls rmmod while this thread is still running.

I guess I'm not seeing the race with rmmod that you're talking
about? In XPC's case, rmmod calls xpc_exit() which currently does a
wait_for_completion() on the discovery thread and on the other thread
mentioned above. These will be changed to kthread_stop() calls.  And if
the discovery thread has already exited the kthread_stop() will return
immediately and if not it will wait until the discovery thread has
exited. rmmod won't return from xpc_exit() until both threads have exited.

Any thought as to when the changes to kthread_stop() that allow it to be
called for a kthread that has already exited will get into the -mm tree?

> > Is there any way to have a version of kthread_create() that doesn't
> > require a matching kthread_stop()? Or add a kthread_not_stopping()
> > that does the put_task_struct() call, so as to eliminate the need for
> > calling kthread_stop()?
> 
> Yes.  I was thinking calling it kthread_orphan or something like that.
> We can't make anything like that the default, because of the modular
> remove problem, but it's not to hard.

Again, when xpc_exit() is called by rmmod it waits for XPC's pool of
threads to exit before it returns, so not a problem.

Any thought as to when kthread_orphan() will get into the -mm tree? Once
kthread_stop() is changed and kthread_orphan() added I can proceed with
a patch to change XPC to use the kthread API.

> > Or should we reconsider the kthread pool approach
> > (and get XPC out of the thread management business altogether)? Robin
> > Holt is putting together a proposal for how one could do a kthread pool,
> > it should provide a bit more justification for going down that road.

Robin has changed his mind about tieing in the management of a pool
of threads with the kthread API, so there won't be the fore mentioned
proposal.

Thanks,
Dean
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