netdev - Re: [BUG] NULL pointer dereference in skb

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Message-ID: <20080811232657.GQ6762@linux.vnet.ibm.com>
Date:	Mon, 11 Aug 2008 16:26:57 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jarek Poplawski <jarkao2@...il.com>
Cc:	David Miller <davem@...emloft.net>, emil.s.tantilov@...el.com,
	jeffrey.t.kirsher@...el.com, netdev@...r.kernel.org
Subject: Re: [BUG] NULL pointer dereference in skb_dequeue

On Mon, Aug 11, 2008 at 10:01:26AM +0000, Jarek Poplawski wrote:
> On 10-08-2008 21:04, Jarek Poplawski wrote:
> ...
> > Hmm.. Actually, it's completely unreasonable. Let's forget this.
> 
> But accidentally it might even sometimes work here...
> 
> Currently, the most suspicious place to me seems to be
> __netif_schedule(). Is it legal to store RCU protected pointers out of
> rcu_read_lock() sections?

Yes, but:

1.	You need to use one of the update-side primitives to do the
	store: rcu_assign_pointer(), list_add_rcu(), etc.

2.	There has to be some way for multiple updaters to coordinate,
	for example:

	a.	Only a single task is permitted to update.

	b.	Locking is used to coordinate among multiple updaters
		(so that only one such updater may proceed at a given
		time).

	c.	Atomic operations are used to coordinate multiple
		updaters.  Here be dragons, go for (a) or (b)
		instead unless you have an extremely good reason
		-and- you have both a proof of correctness and
		a totally brutal and malign test suite.

The main reason to update RCU-protected pointers within rcu_read_lock()
regions is when sharing code between RCU readers and updaters, or when
an RCU reader can see the need to do an update.

							Thanx, Paul
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