Re: [PATCH] md / procfs: avoid Oops if md-mod removed while /proc/mdstat is being polled.

[NeilBrown <neilb@suse.de>]

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On Thu, 27 Feb 2014 15:32:42 -0800 Andrew Morton <akpm@linux-foundation.org>
wrote:

> On Fri, 28 Feb 2014 10:07:57 +1100 NeilBrown <neilb@suse.de> wrote:
> 
> > On Thu, 27 Feb 2014 13:51:25 -0800 Andrew Morton <akpm@linux-foundation.org>
> > wrote:
> > 
> > > On Fri, 28 Feb 2014 08:34:43 +1100 NeilBrown <neilb@suse.de> wrote:
> > > 
> > > > On Thu, 27 Feb 2014 12:58:07 -0800 Andrew Morton <akpm@linux-foundation.org>
> > > > wrote:
> > > > 
> > > > > On Thu, 27 Feb 2014 17:24:45 +1100 NeilBrown <neilb@suse.de> wrote:
> > > > > 
> > > > > > If poll or select is waiting on /proc/mdstat when md-mod is unloaded
> > > > > > an oops will ensure when the poll/select completes.
> > > > > > 
> > > > > > This is because the wait_queue_head which is registered with poll_wait()
> > > > > > is local to the module and no longer exists when the poll completes and
> > > > > > detaches that wait_queue_head (in poll_free_wait -> remove_wait_queue).
> > > > > > 
> > > > > > To fix this we need the wait_queue_head to have (at least) the same life
> > > > > > time as the proc_dir_entry.  So this patch places it in that structure.
> > > > > > 
> > > > > > We:
> > > > > >   - add pde_poll_wait to struct proc_dir_entry
> > > > > >   - call poll_wait() passing this when poll() is called on the proc file
> > > > > >   - export a function proc_wake_up which will call wake_up() on pde_poll_wait
> > > > > > 
> > > > > > and make use of all that in md.c
> > > > > 
> > > > > This sounds wrong.  If a userspace process is waiting on
> > > > > md_event_waiters then the md module is "busy" and the rmmod attempt
> > > > > should fail?
> > > > 
> > > > Al Viro says "no" quite firmly.
> > > > 
> > > > I think the core argument is that
> > > > 
> > > >   rmmod md-mod < /proc/mdstat
> > > > 
> > > > would deadlock.
> > > 
> > > Well, only if the rmmod hangs around waiting for the module to go idle.
> > > I'm thinking rmmod should fail.  EBUSY.
> 
> This?  What happens if we just fail the rmmod when the module is busy
> (which it is).

I was hoping that Al would step in with an answer, but I got impatient had
had a look around myself.

I found

commit 99b76233803beab302123d243eea9e41149804f3
Author: Alexey Dobriyan <adobriyan@gmail.com>
Date:   Wed Mar 25 22:48:06 2009 +0300

    proc 2/2: remove struct proc_dir_entry::owner
    


which references

https://bugzilla.kernel.org/show_bug.cgi?id=12454

which seems to suggest that we used to try to do as you suggest but it was
racy and would be a lot of work to "fix". The chosen solution was to only
'get' the module during read/write, not just while the /proc file is open.

NeilBrown

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