rcu: Fix long-grace-period race between forcing and initialization

Very long RCU read-side critical sections (50 milliseconds or
so) can cause a race between force_quiescent_state() and
rcu_start_gp() as follows on kernel builds with multi-level
rcu_node hierarchies:

1.	CPU 0 calls force_quiescent_state(), sees that there is a
	grace period in progress, and acquires ->fsqlock.

2.	CPU 1 detects the end of the grace period, and so
	cpu_quiet_msk_finish() sets rsp->completed to rsp->gpnum.
	This operation is carried out under the root rnp->lock,
	but CPU 0 has not yet acquired that lock.  Note that
	rsp->signaled is still RCU_SAVE_DYNTICK from the last
	grace period.

3.	CPU 1 calls rcu_start_gp(), but no one wants a new grace
	period, so it drops the root rnp->lock and returns.

4.	CPU 0 acquires the root rnp->lock and picks up rsp->completed
	and rsp->signaled, then drops rnp->lock.  It then enters the
	RCU_SAVE_DYNTICK leg of the switch statement.

5.	CPU 2 invokes call_rcu(), and now needs a new grace period.
	It calls rcu_start_gp(), which acquires the root rnp->lock, sets
	rsp->signaled to RCU_GP_INIT (too bad that CPU 0 is already in
	the RCU_SAVE_DYNTICK leg of the switch statement!)  and starts
	initializing the rcu_node hierarchy.  If there are multiple
	levels to the hierarchy, it will drop the root rnp->lock and
	initialize the lower levels of the hierarchy.

6.	CPU 0 notes that rsp->completed has not changed, which permits
        both CPU 2 and CPU 0 to try updating it concurrently.  If CPU 0's
	update prevails, later calls to force_quiescent_state() can
	count old quiescent states against the new grace period, which
	can in turn result in premature ending of grace periods.

	Not good.

This patch adds an RCU_GP_IDLE state for rsp->signaled that is
set initially at boot time and any time a grace period ends.
This prevents CPU 0 from getting into the workings of
force_quiescent_state() in step 4.  Additional locking and
checks prevent the concurrent update of rsp->signaled in step 6.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <1256742889199-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>