linux-kernel - Re: [PATCH] Avoid null-pointer access in w1/slaves/w1

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Message-ID: <20150319000908.GG21067@spacedout.fries.net>
Date:	Wed, 18 Mar 2015 19:09:08 -0500
From:	David Fries <david@...es.net>
To:	Evgeniy Polyakov <zbr@...emap.net>
Cc:	Thorsten Bschorr <thorsten@...horr.de>,
	Jonathan ALIBERT <jonathan.alibert@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Avoid null-pointer access in w1/slaves/w1_therm

On Wed, Mar 18, 2015 at 06:18:53PM +0300, Evgeniy Polyakov wrote:
> Hi
> 
> 18.03.2015, 07:20, "David Fries" <david@...es.net>:
> >  static void w1_therm_remove_slave(struct w1_slave *sl)
> >  {
> > + int refcnt = atomic_sub_return(1, THERM_REFCNT(sl->family_data));
> > + while(refcnt) {
> > + msleep(1000);
> > + refcnt = atomic_read(THERM_REFCNT(sl->family_data));
> > + }
> >          kfree(sl->family_data);
> >          sl->family_data = NULL;
> >  }
> 
> Can we replace this whole atomic manipulations with kref_t and free family data in the place
> which actually drops reference counter to zero?
> 
> I.e. we return from remove_slave() function potentially leaving family data floating around, it will be freed
> when the last user drops the reference. There is still a race between increasing reference when starting
> reading and removing slave device, i.e. one starts reading, while attached slave device is being removed,
> but that's a different problem.

With the two while loops I posted, I see with two clients reading
w1_slave, the other command to remove a slave gets permanently stuck
in w1_therm_remove_slave, which keeps the slave around while the
clients continue to read.  I wouldn't predict things going better by
keeping family_data around longer, the slave data would still go away
with readers around.

-- 
David Fries <david@...es.net>    PGP pub CB1EE8F0
http://fries.net/~david/
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