New RCU related warning due to rcu_preempt_depth() changes

From: Sasha Levin
Date: Tue Apr 17 2012 - 04:43:16 EST

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
[ 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.

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