New RCU related warning due to rcu_preempt_depth() changes

[Sasha Levin]

Hi Paul,

It looks like commit 7298b03 ("rcu: Move __rcu_read_lock() and
__rcu_read_unlock() to per-CPU variables") is causing the following
warning (I've added the extra fields on the second line):

[   77.330920] BUG: sleeping function called from invalid context at
mm/memory.c:3933
[   77.336571] in_atomic(): 0, irqs_disabled(): 0, preempt count: 0,
preempt offset: 0, rcu depth: 1, pid: 5669, name: trinity
[   77.344135] no locks held by trinity/5669.
[   77.349644] Pid: 5669, comm: trinity Tainted: G        W
3.4.0-rc3-next-20120417-sasha-dirty #83
[   77.354401] Call Trace:
[   77.355956]  [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210
[   77.358811]  [<ffffffff81198eaf>] might_fault+0x2f/0xa0
[   77.361997]  [<ffffffff810e3228>] schedule_tail+0x88/0xb0
[   77.364671]  [<ffffffff826a01d3>] ret_from_fork+0x13/0x80

As you can see, rcu_preempt_depth() returns 1 when running in that
context, which looks pretty odd.

Thanks.
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