Date: Fri, 28 Feb 2014 08:34:43 +1100
From: NeilBrown <neilb@suse.de>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>, linux-fsdevel@vger.kernel.org,
        linux RAID <linux-raid@vger.kernel.org>,
        "majianpeng" <majianpeng@gmail.com>,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] md / procfs: avoid Oops if md-mod removed while
 /proc/mdstat is being polled.
Message-ID: <20140228083443.64c92382@notabene.brown>
In-Reply-To: <20140227125807.d034c50c7fe2e1a9f3c38ec1@linux-foundation.org>
References: <20140227172445.13644477@notabene.brown>
	<20140227125807.d034c50c7fe2e1a9f3c38ec1@linux-foundation.org>
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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:
>=20
> > If poll or select is waiting on /proc/mdstat when md-mod is unloaded
> > an oops will ensure when the poll/select completes.
> >=20
> > 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).
> >=20
> > 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.
> >=20
> > 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_pol=
l_wait
> >=20
> > and make use of all that in md.c
>=20
> 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.

http://marc.info/?l=3Dlinux-fsdevel&m=3D133024267507384

Thanks,
NeilBrown

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