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

[Andrew Morton <akpm@linux-foundation.org>]

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.

> http://marc.info/?l=linux-fsdevel&m=133024267507384

Why don't a billion blocking-procfs-read sites have this problem?