Re: md:When opened /proc/mdstat, increase the refcount of

[NeilBrown <neilb@suse.de>]

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On Sun, 26 Feb 2012 16:33:25 +0800 "majianpeng" <majianpeng@gmail.com> wrote:

> I wrote a letter to maintainer of procfs. He's answer is follow:
> >No.  Read remove_proc_entry(), especially the part under ->pde_unload_lock.
> >The whole damn point of that stuff is that opened file on procfs does *not*
> >pin the module down; IO in progress does, but that's it.
> >You can't deadlock rmmod foobar </proc/crap/foobar; with your patch we'll
> >be back to a pile of deadlocks in there.
> >IOW, NAK.

Hmmm... that sounds like Al Viro.

I see the point of that code now.  And your patch would be open to the same
problem wouldn't it. i.e.
    rmmod md_mod < /proc/mdstat

would deadlock??

So we still need to find a fix that actually works correctly.

That probably means moving the call to 'poll_wait' into procfs code, and using
a wait_queue_head which is managed by procfs.

So maybe:
 - put a wait_queue_head in 'struct proc_dir_entry'
 - have proc_reg_poll call poll_wait passing that wait_queue_head
 - write a function e.g. proc_poll_wake which calls wake_up on that
   wait_queue_head
 - have md.c save the return value from proc_create, and call the above
   function instead of calling "wake_up(&md_event_waiters)"

That should work.

NeilBrown

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