As noted in earlier commit logs, CPU hotplug operations running
concurrently with grace-period initialization can result in a given
leaf rcu_node structure having all CPUs offline and no blocked readers,
but with this rcu_node structure nevertheless blocking the current
grace period.  Therefore, the quiescent-state forcing code now checks
for this situation and repairs it.

Unfortunately, this checking can result in false positives, for example,
when the last task has just removed itself from this leaf rcu_node
structure, but has not yet started clearing the ->qsmask bits further
up the structure.  This means that the grace-period kthread (which
forces quiescent states) and some other task might be attempting to
concurrently clear these ->qsmask bits.  This is usually not a problem:
One of these tasks will be the first to acquire the upper-level rcu_node
structure's lock and with therefore clear the bit, and the other task,
seeing the bit already cleared, will stop trying to clear bits.

Sadly, this means that the following unusual sequence of events -can-
result in a problem:

1.	The grace-period kthread wins, and clears the ->qsmask bits.

2.	This is the last thing blocking the current grace period, so
	that the grace-period kthread clears ->qsmask bits all the way
	to the root and finds that the root ->qsmask field is now zero.

3.	Another grace period is required, so that the grace period kthread
	initializes it, including setting all the needed qsmask bits.

4.	The leaf rcu_node structure (the one that started this whole
	mess) is blocking this new grace period, either because it
	has at least one online CPU or because there is at least one
	task that had blocked within an RCU read-side critical section
	while running on one of this leaf rcu_node structure's CPUs.
	(And yes, that CPU might well have gone offline before the
	grace period in step (3) above started, which can mean that
	there is a task on the leaf rcu_node structure's ->blkd_tasks
	list, but ->qsmask equal to zero.)

5.	The other kthread didn't get around to trying to clear the upper
	level ->qsmask bits until all the above had happened.  This means
	that it now sees bits set in the upper-level ->qsmask field, so it
	proceeds to clear them.  Too bad that it is doing so on behalf of
	a quiescent state that does not apply to the current grace period!

This sequence of events can result in the new grace period being too
short.  It can also result in the new grace period ending before the
leaf rcu_node structure's ->qsmask bits have been cleared, which will
result in splats during initialization of the next grace period.  In
addition, it can result in tasks blocking the new grace period still
being queued at the start of the next grace period, which will result
in other splats.  Sasha's testing turned up another of these splats,
as did rcutorture testing.  (And yes, rcutorture is being adjusted to
make these splats show up more quickly.  Which probably is having the
undesirable side effect of making other problems show up less quickly.
Can't have everything!)
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: <stable@vger.kernel.org> # 4.0.x
Tested-by: Sasha Levin <sasha.levin@oracle.com>