BACKPORT: rcu: Merge bug fix patch from mainline

rcu: Allow only one expedited GP to run concurrently with wakeups

The current expedited RCU grace-period code expects that a task
requesting an expedited grace period cannot awaken until that grace
period has reached the wakeup phase.  However, it is possible for a long
preemption to result in the waiting task never sleeping.  For example,
consider the following sequence of events:

1.	Task A starts an expedited grace period by invoking
	synchronize_rcu_expedited().  It proceeds normally up to the
	wait_event() near the end of that function, and is then preempted
	(or interrupted or whatever).

2.	The expedited grace period completes, and a kworker task starts
	the awaken phase, having incremented the counter and acquired
	the rcu_state structures .exp_wake_mutex.  This kworker task
	is then preempted or interrupted or whatever.

3.	Task A resumes and enters wait_event(), which notes that the
	expedited grace period has completed, and thus doesnt sleep.

4.	Task B starts an expedited grace period exactly as did Task A,
	complete with the preemption (or whatever delay) just before
	the call to wait_event().

5.	The expedited grace period completes, and another kworker
	task starts the awaken phase, having incremented the counter.
	However, it blocks when attempting to acquire the rcu_state
	structures .exp_wake_mutex because step 2s kworker task has
	not yet released it.

6.	Steps 4 and 5 repeat, resulting in overflow of the rcu_node
	structures ->exp_wq[] array.

In theory, this is harmless.  Tasks waiting on the various ->exp_wq[]
array will just be spuriously awakened, but they will just sleep again
on noting that the rcu_state structures ->expedited_sequence value has
not advanced far enough.

In practice, this wastes CPU time and is an accident waiting to happen.
This commit therefore moves the rcu_exp_gp_seq_end() call that officially
ends the expedited grace period (along with associate tracing) until
after the ->exp_wake_mutex has been acquired.  This prevents Task A from
awakening prematurely, thus preventing more than one expedited grace
period from being in flight during a previous expedited grace periods
wakeup phase.

Fixes: 3b5f668e ("rcu: Overlap wakeups with next expedited grace period")
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
[ paulmck: Added updated comment. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

conflicts:
Fix the difference between kernel mainline and kernel-4.9

MTK-Commit-Id: 388c28d890dfab61050facef32beab61d4034f9b

Change-Id: I5644df96f1fdbcaff10eb010c035c1f8669d0e7c
Signed-off-by: Cheng Jui Wang <cheng-jui.wang@mediatek.com>
CR-Id: ALPS05359266
Feature: [Module]Official Kernel Patch
(cherry picked from commit 7e75e64d285096a02848822066244b1050086d8f)