linux-kernel - Re: [PATCH] netconsole: avoid a crash with multiple sysfs writers

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Date:	Thu, 8 Aug 2013 09:14:31 +0300
From:	Dan Aloni <alonid@...tram.com>
To:	Neil Horman <nhorman@...driver.com>
Cc:	linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] netconsole: avoid a crash with multiple sysfs writers

On Thu, Aug 8, 2013 at 8:50 AM, Neil Horman <nhorman@...driver.com> wrote:
>
> On Wed, Aug 07, 2013 at 12:02:44PM +0300, Dan Aloni wrote:
[..]
>
> > When my 'ifup eth' script was fired multiple times and ran concurrent o> @@ -682,7 +689,11 @@ restart:
> >                                * we might sleep in __netpoll_cleanup()
> >                                */
> >                               spin_unlock_irqrestore(&target_list_lock, flags);
> > +
> > +                             mutex_lock(&nt->mutex);
> >                               __netpoll_cleanup(&nt->np);
> > +                             mutex_unlock(&nt->mutex);
> > +
> NAK, you can't hold a mutex while calling __netpoll_cleanup.  __netpoll_cleanup
> may sleep and its illegal to hold a mutex while doing so.
> Neil
>

To my understanding, it mostly depends on locking order, and having
sleeplocks in the outer order and spinlocks in the inner order is
valid as long the locking order is not reversed.

Also, drivers/net/team/team.c - another netpoll user, already does the
same thing I intended in this patch - it locks the outer team->lock
mutex in team_uninit() while calling team_port_del() and then
team_port_disable_netpoll() calls __netpoll_cleanup().
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