Re: New RCU related warning due to rcu_preempt_depth() changes

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Apr 17, 2012 at 10:42:47AM +0200, Sasha Levin wrote:
> 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.

Ouch!!!

So it looks like I missed a place where I need to save and restore
the new per-CPU rcu_read_lock_nesting and rcu_read_unlock_special
variables.  My (probably hopelessly naive) guess is that I need to add
a rcu_switch_from() and rcu_switch_to() into schedule_tail(), but to
make rcu_switch_from() take the task_struct pointer as an argument,
passing in prev.

Does this make sense, or am I still missing something here?

							Thanx, Paul