netdev - What protects rcu_dereference() in __in6_dev

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Message-ID: <20100114183215.GA16866@linux.vnet.ibm.com>
Date:	Thu, 14 Jan 2010 10:32:15 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	yoshfuji@...ux-ipv6.org
Cc:	netdev@...r.kernel.org, eric.dumazet@...il.com, mingo@...e.hu,
	akpm@...ux-foundation.org, peterz@...radead.org
Subject: What protects rcu_dereference() in __in6_dev_get()?

Hello, Yoshi,

Could you please tell me what protects the rcu_dereference() in
__in6_dev_get()?  I am adding lockdep-based checking to RCU, and
"git blame" said I should ask you about this one.

The current code, rcu_dereference(), assumes that this is protected only
by RCU.  My problem might be any of the following:

o	Some other flavor of RCU protects this, e.g., RCU-bh, which
	would require rcu_dereference_bh().

o	This is called from updates as well as from readers, and
	some lock protects the updates.

o	This is called during initialization, when this pointer is
	inaccessible to readers.

Please note that I can add a check to cover multiple possibilities.
For a real example in include/linux/fdtable.h:

	file = rcu_dereference_check(fdt->fd[fd],
				     rcu_read_lock_held() ||
				     lockdep_is_held(&files->file_lock) ||
				     atomic_read(&files->count) == 1);

The first argument is the pointer, and the second argument says that
this may be protected by either RCU (as opposed to RCU-bh, RCU-sched,
or SRCU), the files->file_lock as recorded by lockdep, or by being in
a single-threaded process as noted by the value of files->count.
(Please see http://lwn.net/Articles/368683/ for a recent patch, another
will go out soon.)

So, could you please tell me what protects the rcu_dereference()
in __in6_dev_get() so that I can craft the appropriate form of
rcu_dereference()?

							Thanx, Paul
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