Large systems running RCU_FAST_NO_HZ kernels see extreme memory
contention on the rcu_state structure's ->fqslock field.  This
can be avoided by disabling RCU_FAST_NO_HZ, either at compile time
or at boot time (via the nohz kernel boot parameter), but large
systems will no doubt become sensitive to energy consumption.
This commit therefore uses a combining-tree approach to spread the
memory contention across new cache lines in the leaf rcu_node structures.
This can be thought of as a tournament lock that has only a try-lock
acquisition primitive.

The effect on small systems is minimal, because such systems have
an rcu_node "tree" consisting of a single node.  In addition, this
functionality is not used on fastpaths.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

RCU quiescent-state forcing is currently carried out without preemption
points, which can result in excessive latency spikes on large systems
(many hundreds or thousands of CPUs).  This patch therefore inserts
a voluntary preemption point into force_qs_rnp(), which should greatly
reduce the magnitude of these spikes.
Reported-by: NMike Galbraith <mgalbraith@suse.de>
Reported-by: NDimitri Sivanich <sivanich@sgi.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

As the first step towards allowing quiescent-state forcing to be
preemptible, this commit moves RCU quiescent-state forcing into the
same kthread that is now used to initialize and clean up after grace
periods.  This is yet another step towards keeping scheduling
latency down to a dull roar.

Updated to change from raw_spin_lock_irqsave() to raw_spin_lock_irq()
and to remove the now-unused rcu_state structure fields as suggested by
Peter Zijlstra.
Reported-by: NMike Galbraith <mgalbraith@suse.de>
Reported-by: NDimitri Sivanich <sivanich@sgi.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Earlier versions of RCU invoked the RCU core from the CPU_DYING notifier
in order to note a quiescent state for the outgoing CPU.  Because the
CPU is marked "offline" during the execution of the CPU_DYING notifiers,
the RCU core had to tolerate being invoked from an offline CPU.  However,
commit b1420f1c (Make rcu_barrier() less disruptive) left only tracing
code in the CPU_DYING notifier, so the RCU core need no longer execute
on offline CPUs.  This commit therefore enforces this restriction.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Then rcu_gp_kthread() function is too large and furthermore needs to
have the force_quiescent_state() code pulled in.  This commit therefore
breaks up rcu_gp_kthread() into rcu_gp_init() and rcu_gp_cleanup().
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

RCU grace-period cleanup is currently carried out with interrupts
disabled, which can result in excessive latency spikes on large systems
(many hundreds or thousands of CPUs).  This patch therefore makes the
RCU grace-period cleanup be preemptible, including voluntary preemption
points, which should eliminate those latency spikes.  Similar spikes from
forcing of quiescent states will be dealt with similarly by later patches.

Updated to replace uses of spin_lock_irqsave() with spin_lock_irq(), as
suggested by Peter Zijlstra.
Reported-by: NMike Galbraith <mgalbraith@suse.de>
Reported-by: NDimitri Sivanich <sivanich@sgi.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

As a first step towards allowing grace-period cleanup to be preemptible,
this commit moves the RCU grace-period cleanup into the same kthread
that is now used to initialize grace periods.  This is needed to keep
scheduling latency down to a dull roar.

[ paulmck: Get rid of stray spin_lock_irqsave() calls. ]
Reported-by: NMike Galbraith <mgalbraith@suse.de>
Reported-by: NDimitri Sivanich <sivanich@sgi.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

RCU grace-period initialization is currently carried out with interrupts
disabled, which can result in 200-microsecond latency spikes on systems
on which RCU has been configured for 4096 CPUs.  This patch therefore
makes the RCU grace-period initialization be preemptible, which should
eliminate those latency spikes.  Similar spikes from grace-period cleanup
and the forcing of quiescent states will be dealt with similarly by later
patches.
Reported-by: NMike Galbraith <mgalbraith@suse.de>
Reported-by: NDimitri Sivanich <sivanich@sgi.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The next step in reducing RCU's grace-period initialization latency on
large systems will make this initialization preemptible.  Unfortunately,
making the grace-period initialization subject to interrupts (let alone
preemption) exposes the following race on systems whose rcu_node tree
contains more than one node:

1.	CPU 31 starts initializing the grace period, including the
    	first leaf rcu_node structures, and is then preempted.

2.	CPU 0 refers to the first leaf rcu_node structure, and notes
    	that a new grace period has started.  It passes through a
    	quiescent state shortly thereafter, and informs the RCU core
    	of this rite of passage.

3.	CPU 0 enters an RCU read-side critical section, acquiring
    	a pointer to an RCU-protected data item.

4.	CPU 31 takes an interrupt whose handler removes the data item
	referenced by CPU 0 from the data structure, and registers an
	RCU callback in order to free it.

5.	CPU 31 resumes initializing the grace period, including its
    	own rcu_node structure.  In invokes rcu_start_gp_per_cpu(),
    	which advances all callbacks, including the one registered
    	in #4 above, to be handled by the current grace period.

6.	The remaining CPUs pass through quiescent states and inform
    	the RCU core, but CPU 0 remains in its RCU read-side critical
    	section, still referencing the now-removed data item.

7.	The grace period completes and all the callbacks are invoked,
    	including the one that frees the data item that CPU 0 is still
    	referencing.  Oops!!!

One way to avoid this race is to remove grace-period acceleration from
rcu_start_gp_per_cpu().  Now, the only reason for this acceleration was
to allow CPUs bringing RCU out of idle state to have their callbacks
invoked after only one grace period, rather than the two grace periods
that would otherwise be required.  But this acceleration does not
work when RCU grace-period initialization is moved to a kthread because
the CPU posting the callback is no longer necessarily the CPU that is
initializing the resulting grace period.

This commit therefore removes this now-pointless (and soon to be dangerous)
grace-period acceleration, thus avoiding the above race.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

As the first step towards allowing grace-period initialization to be
preemptible, this commit moves the RCU grace-period initialization
into its own kthread.  This is needed to keep large-system scheduling
latency at reasonable levels.

Also change raw_spin_lock_irqsave() to raw_spin_lock_irq() as suggested
by Peter Zijlstra in review comments.
Reported-by: NMike Galbraith <mgalbraith@suse.de>
Reported-by: NDimitri Sivanich <sivanich@sgi.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Each grace period is supposed to have at least one callback waiting
for that grace period to complete.  However, if CONFIG_NO_HZ=n, an
extra callback-free grace period is no big problem -- it will chew up
a tiny bit of CPU time, but it will complete normally.  In contrast,
CONFIG_NO_HZ=y kernels have the potential for all the CPUs to go to
sleep indefinitely, in turn indefinitely delaying completion of the
callback-free grace period.  Given that nothing is waiting on this grace
period, this is also not a problem.

That is, unless RCU CPU stall warnings are also enabled, as they are
in recent kernels.  In this case, if a CPU wakes up after at least one
minute of inactivity, an RCU CPU stall warning will result.  The reason
that no one noticed until quite recently is that most systems have enough
OS noise that they will never remain absolutely idle for a full minute.
But there are some embedded systems with cut-down userspace configurations
that consistently get into this situation.

All this begs the question of exactly how a callback-free grace period
gets started in the first place.  This can happen due to the fact that
CPUs do not necessarily agree on which grace period is in progress.
If a CPU still believes that the grace period that just completed is
still ongoing, it will believe that it has callbacks that need to wait for
another grace period, never mind the fact that the grace period that they
were waiting for just completed.  This CPU can therefore erroneously
decide to start a new grace period.  Note that this can happen in
TREE_RCU and TREE_PREEMPT_RCU even on a single-CPU system:  Deadlock
considerations mean that the CPU that detected the end of the grace
period is not necessarily officially informed of this fact for some time.

Once this CPU notices that the earlier grace period completed, it will
invoke its callbacks.  It then won't have any callbacks left.  If no
other CPU has any callbacks, we now have a callback-free grace period.

This commit therefore makes CPUs check more carefully before starting a
new grace period.  This new check relies on an array of tail pointers
into each CPU's list of callbacks.  If the CPU is up to date on which
grace periods have completed, it checks to see if any callbacks follow
the RCU_DONE_TAIL segment, otherwise it checks to see if any callbacks
follow the RCU_WAIT_TAIL segment.  The reason that this works is that
the RCU_WAIT_TAIL segment will be promoted to the RCU_DONE_TAIL segment
as soon as the CPU is officially notified that the old grace period
has ended.

This change is to cpu_needs_another_gp(), which is called in a number
of places.  The only one that really matters is in rcu_start_gp(), where
the root rcu_node structure's ->lock is held, which prevents any
other CPU from starting or completing a grace period, so that the
comparison that determines whether the CPU is missing the completion
of a grace period is stable.
Reported-by: NBecky Bruce <bgillbruce@gmail.com>
Reported-by: NSubodh Nijsure <snijsure@grid-net.com>
Reported-by: NPaul Walmsley <paul@pwsan.com>
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Paul Walmsley <paul@pwsan.com>  # OMAP3730, OMAP4430
Cc: stable@vger.kernel.org

The Linux kernel coding style says that single-statement blocks should
omit curly braces unless the other leg of the "if" statement has
multiple statements, in which case the curly braces should be included.
This commit fixes RCU's violations of this rule.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The recent bug that introduced the RCU callback list/count mismatch
showed the need for a diagnostic to check for this, which this commit
adds.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Problems in RCU idle entry and exit are almost always confined to the
offending CPU.  This commit therefore switches ftrace_dump() from
DUMP_ALL to DUMP_ORIG.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Tested-by: NPascal Chapperon <pascal.chapperon@wanadoo.fr>

If a CPU goes offline with callbacks queued, those callbacks might be
indefinitely postponed, which can result in a system hang.  This commit
therefore inserts warnings for this condition.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

It is time to optimize CONFIG_TREE_PREEMPT_RCU's synchronize_rcu()
for uniprocessor optimization, which means that rcu_blocking_is_gp()
can no longer rely on RCU read-side critical sections having disabled
preemption.  This commit therefore disables preemption across
rcu_blocking_is_gp()'s scan of the cpu_online_mask.

(Updated from previous version to fix embarrassing bug spotted by
Wu Fengguang.)
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The __call_rcu() function is a bit overweight, so this commit splits
it into actual enqueuing of and accounting for the callback (__call_rcu())
and associated RCU-core processing (__call_rcu_core()).
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The __call_rcu() function will invoke the RCU core, for example, if
it detects that the current CPU has too many callbacks.  However, this
can happen on an offline CPU that is on its way to the idle loop, in
which case it is an error to invoke the RCU core, and the excess callbacks
will be adopted in any case.  This commit therefore adds checks to
__call_rcu() for running on an offline CPU, refraining from invoking
the RCU core in this case.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Although __call_rcu() is handled correctly when called from a momentary
non-idle period, if it is called on a CPU that RCU believes to be idle
on RCU_FAST_NO_HZ kernels, the callback might be indefinitely postponed.
This commit therefore ensures that RCU is aware of the new callback and
has a chance to force the CPU out of dyntick-idle mode when a new callback
is posted.
Reported-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The _rcu_barrier() function accesses other CPUs' rcu_data structure's
->qlen field without benefit of locking.  This commit therefore adds
the required ACCESS_ONCE() wrappers around accesses and updates that
need it.

ACCESS_ONCE() is not needed when a CPU accesses its own ->qlen, or
in code that cannot run while _rcu_barrier() is sampling ->qlen fields.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

There are a couple of open-coded initializations of the rcu_data
structure's RCU callback list.  This commit therefore consolidates
them into a new init_callback_list() function.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The code that attempts to identify stalls that end just as we detect
them is broken by both flavors of initialization failure.  This commit
therefore properly initializes and computes the count of the number
of reasons why the RCU grace period is stalled.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Before RCU had unified idle, the RCU_SAVE_DYNTICK leg of the switch
statement in force_quiescent_state() was dead code for CONFIG_NO_HZ=n
kernel builds.  With unified idle, the code is never dead.  This commit
therefore removes the "if" statement designed to make gcc aware of when
the code was and was not dead.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The arrival of TREE_PREEMPT_RCU some years back included some ugly
code involving either #ifdef or #ifdef'ed wrapper functions to iterate
over all non-SRCU flavors of RCU.  This commit therefore introduces
a for_each_rcu_flavor() iterator over the rcu_state structures for each
flavor of RCU to clean up a bit of the ugliness.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

With the advent of __this_cpu_ptr(), it is no longer necessary to pass
both the rcu_state and rcu_data structures into __rcu_process_callbacks().
This commit therefore computes the rcu_data pointer from the rcu_state
pointer within __rcu_process_callbacks() so that callers can pass in
only the pointer to the rcu_state structure.  This paves the way for
linking the rcu_state structures together and iterating over them.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

This commit adds event tracing for _rcu_barrier() execution.  This
is defined only if RCU_TRACE=y.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The traditional rcu_barrier() implementation has serialized all requests,
regardless of RCU flavor, and also does not coalesce concurrent requests.
In the past, this has been good and sufficient.

However, systems are getting larger and use of rcu_barrier() has been
increasing.  This commit therefore introduces a counter-based scheme
that allows _rcu_barrier() calls for the same flavor of RCU to take
advantage of each others' work.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

For global variables, C defaults all fields to zero.  The initialization
of the rcu_state structure's ->n_force_qs and ->n_force_qs_ngp fields
is therefore redundant, so this commit removes these initializations.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

In order to allow each RCU flavor to concurrently execute its
rcu_barrier() function, it is necessary to move the relevant
state to the rcu_state structure.  This commit therefore moves the
rcu_barrier_mutex global variable to a new ->barrier_mutex field
in the rcu_state structure.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

In order to allow each RCU flavor to concurrently execute its
rcu_barrier() function, it is necessary to move the relevant
state to the rcu_state structure.  This commit therefore moves the
rcu_barrier_completion global variable to a new ->barrier_completion
field in the rcu_state structure.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

In order to allow each RCU flavor to concurrently execute its rcu_barrier()
function, it is necessary to move the relevant state to the rcu_state
structure.  This commit therefore moves the rcu_barrier_cpu_count global
variable to a new ->barrier_cpu_count field in the rcu_state structure.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

In order for multiple flavors of RCU to each concurrently run one
rcu_barrier(), each flavor needs its own per-CPU set of rcu_head
structures.  This commit therefore moves _rcu_barrier()'s set of
per-CPU rcu_head structures from per-CPU variables to the existing
per-CPU and per-RCU-flavor rcu_data structures.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

This is a preparatory commit for increasing rcu_barrier()'s concurrency.
It adds a pointer in the rcu_data structure to the corresponding call_rcu()
function.  This allows a pointer to the rcu_data structure to imply the
function pointer, which allows _rcu_barrier() state to be placed in the
rcu_state structure.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Upcoming rcu_barrier() concurrency commits will result in line lengths
greater than 80 characters in the RCU_STATE_INITIALIZER(), so this commit
shortens the name of the macro's argument to prevent this.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The rcu_node tree array is sized based on compile-time constants,
including NR_CPUS.  Although this approach has worked well in the past,
the recent trend by many distros to define NR_CPUS=4096 results in
excessive grace-period-initialization latencies.

This commit therefore substitutes the run-time computed nr_cpu_ids for
the compile-time NR_CPUS when building the tree.  This can result in
much of the compile-time-allocated rcu_node array being unused.  If
this is a major problem, you are in a specialized situation anyway,
so you can manually adjust the NR_CPUS, RCU_FANOUT, and RCU_FANOUT_LEAF
kernel config parameters.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Although making RCU_FANOUT_LEAF a kernel configuration parameter rather
than a fixed constant makes it easier for people to decrease cache-miss
overhead for large systems, it is of little help for people who must
run a single pre-built kernel binary.

This commit therefore allows the value of RCU_FANOUT_LEAF to be
increased (but not decreased!) via a boot-time parameter named
rcutree.rcu_fanout_leaf.
Reported-by: NMike Galbraith <efault@gmx.de>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This reverts commit 616c310e.
(Move PREEMPT_RCU preemption to switch_to() invocation).
Testing by Sasha Levin <levinsasha928@gmail.com> showed that this
can result in deadlock due to invoking the scheduler when one of
the runqueue locks is held.  Because this commit was simply a
performance optimization, revert it.
Reported-by: NSasha Levin <levinsasha928@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NSasha Levin <levinsasha928@gmail.com>

Commit b1420f1c (Make rcu_barrier() less disruptive) rearranged the
code in rcu_do_batch(), moving the ->qlen manipulation to follow
the requeueing of the callbacks.  Unfortunately, this rearrangement
clobbered the value of the "count" local variable before the value
of rdp->qlen was adjusted, resulting in the value of rdp->qlen being
inaccurate.  This commit therefore introduces an index variable "i",
avoiding the inadvertent multiplexing.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

In the present implementations of CPU hotplug, the outgoing CPU is
guaranteed to run its stop-machine process on the way out, which
will guarantee that RCU_FAST_NO_HZ forces the CPU out of dyntick-idle
mode.

However, new versions of CPU hotplug might not work this way.  This
commit therefore removes this design constraint by explicitly notifying
CPUs when they adopt non-lazy RCU callbacks.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Tested-by: NPascal Chapperon <pascal.chapperon@wanadoo.fr>

The rcu_barrier() primitive interrupts each and every CPU, registering
a callback on every CPU.  Once all of these callbacks have been invoked,
rcu_barrier() knows that every callback that was registered before
the call to rcu_barrier() has also been invoked.

However, there is no point in registering a callback on a CPU that
currently has no callbacks, most especially if that CPU is in a
deep idle state.  This commit therefore makes rcu_barrier() avoid
interrupting CPUs that have no callbacks.  Doing this requires reworking
the handling of orphaned callbacks, otherwise callbacks could slip through
rcu_barrier()'s net by being orphaned from a CPU that rcu_barrier() had
not yet interrupted to a CPU that rcu_barrier() had already interrupted.
This reworking was needed anyway to take a first step towards weaning
RCU from the CPU_DYING notifier's use of stop_cpu().
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>