summaryrefslogtreecommitdiffstats
path: root/kernel/rcutree.c
diff options
context:
space:
mode:
authorIngo Molnar <mingo@kernel.org>2012-05-14 08:41:20 +0200
committerIngo Molnar <mingo@kernel.org>2012-05-14 08:41:46 +0200
commit2d84e023cb5ec00403ff5d447533c6fd58fcc7ff (patch)
treecb10d9a568ebb4be8593821a6f205efedf2f4ddd /kernel/rcutree.c
parent9ff00d58a915b6747ba2e843ab2d04c712b4dc32 (diff)
parentdc36be4419311fd57becdf54bfeef6bd04a6741d (diff)
Merge branch 'rcu/next' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu into core/rcu
Pull the v3.5 RCU tree from Paul E. McKenney: 1) A set of improvements and fixes to the RCU_FAST_NO_HZ feature (with more on the way for 3.6). Posted to LKML: https://lkml.org/lkml/2012/4/23/324 (commits 1-3 and 5), https://lkml.org/lkml/2012/4/16/611 (commit 4), https://lkml.org/lkml/2012/4/30/390 (commit 6), and https://lkml.org/lkml/2012/5/4/410 (commit 7, combined with the other commits for the convenience of the tester). 2) Changes to make rcu_barrier() avoid disrupting execution of CPUs that have no RCU callbacks. Posted to LKML: https://lkml.org/lkml/2012/4/23/322. 3) A couple of commits that improve the efficiency of the interaction between preemptible RCU and the scheduler, these two being all that survived an abortive attempt to allow preemptible RCU's __rcu_read_lock() to be inlined. The full set was posted to LKML at https://lkml.org/lkml/2012/4/14/143, and the first and third patches of that set remain. 4) Lai Jiangshan's algorithmic implementation of SRCU, which includes call_srcu() and srcu_barrier(). A major feature of this new implementation is that synchronize_srcu() no longer disturbs the execution of other CPUs. This work is based on earlier implementations by Peter Zijlstra and Paul E. McKenney. Posted to LKML: https://lkml.org/lkml/2012/2/22/82. 5) A number of miscellaneous bug fixes and improvements which were posted to LKML at: https://lkml.org/lkml/2012/4/23/353 with subsequent updates posted to LKML. Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/rcutree.c')
-rw-r--r--kernel/rcutree.c332
1 files changed, 244 insertions, 88 deletions
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index d0c5baf1ab1..0da7b88d92d 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -75,6 +75,8 @@ static struct lock_class_key rcu_node_class[NUM_RCU_LVLS];
.gpnum = -300, \
.completed = -300, \
.onofflock = __RAW_SPIN_LOCK_UNLOCKED(&structname##_state.onofflock), \
+ .orphan_nxttail = &structname##_state.orphan_nxtlist, \
+ .orphan_donetail = &structname##_state.orphan_donelist, \
.fqslock = __RAW_SPIN_LOCK_UNLOCKED(&structname##_state.fqslock), \
.n_force_qs = 0, \
.n_force_qs_ngp = 0, \
@@ -145,6 +147,13 @@ static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp);
unsigned long rcutorture_testseq;
unsigned long rcutorture_vernum;
+/* State information for rcu_barrier() and friends. */
+
+static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
+static atomic_t rcu_barrier_cpu_count;
+static DEFINE_MUTEX(rcu_barrier_mutex);
+static struct completion rcu_barrier_completion;
+
/*
* Return true if an RCU grace period is in progress. The ACCESS_ONCE()s
* permit this function to be invoked without holding the root rcu_node
@@ -192,7 +201,6 @@ void rcu_note_context_switch(int cpu)
{
trace_rcu_utilization("Start context switch");
rcu_sched_qs(cpu);
- rcu_preempt_note_context_switch(cpu);
trace_rcu_utilization("End context switch");
}
EXPORT_SYMBOL_GPL(rcu_note_context_switch);
@@ -1311,95 +1319,133 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
#ifdef CONFIG_HOTPLUG_CPU
/*
- * Move a dying CPU's RCU callbacks to online CPU's callback list.
- * Also record a quiescent state for this CPU for the current grace period.
- * Synchronization and interrupt disabling are not required because
- * this function executes in stop_machine() context. Therefore, cleanup
- * operations that might block must be done later from the CPU_DEAD
- * notifier.
- *
- * Note that the outgoing CPU's bit has already been cleared in the
- * cpu_online_mask. This allows us to randomly pick a callback
- * destination from the bits set in that mask.
+ * Send the specified CPU's RCU callbacks to the orphanage. The
+ * specified CPU must be offline, and the caller must hold the
+ * ->onofflock.
*/
-static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
+static void
+rcu_send_cbs_to_orphanage(int cpu, struct rcu_state *rsp,
+ struct rcu_node *rnp, struct rcu_data *rdp)
{
int i;
- unsigned long mask;
- int receive_cpu = cpumask_any(cpu_online_mask);
- struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
- struct rcu_data *receive_rdp = per_cpu_ptr(rsp->rda, receive_cpu);
- RCU_TRACE(struct rcu_node *rnp = rdp->mynode); /* For dying CPU. */
- /* First, adjust the counts. */
+ /*
+ * Orphan the callbacks. First adjust the counts. This is safe
+ * because ->onofflock excludes _rcu_barrier()'s adoption of
+ * the callbacks, thus no memory barrier is required.
+ */
if (rdp->nxtlist != NULL) {
- receive_rdp->qlen_lazy += rdp->qlen_lazy;
- receive_rdp->qlen += rdp->qlen;
+ rsp->qlen_lazy += rdp->qlen_lazy;
+ rsp->qlen += rdp->qlen;
+ rdp->n_cbs_orphaned += rdp->qlen;
rdp->qlen_lazy = 0;
rdp->qlen = 0;
}
/*
- * Next, move ready-to-invoke callbacks to be invoked on some
- * other CPU. These will not be required to pass through another
- * grace period: They are done, regardless of CPU.
+ * Next, move those callbacks still needing a grace period to
+ * the orphanage, where some other CPU will pick them up.
+ * Some of the callbacks might have gone partway through a grace
+ * period, but that is too bad. They get to start over because we
+ * cannot assume that grace periods are synchronized across CPUs.
+ * We don't bother updating the ->nxttail[] array yet, instead
+ * we just reset the whole thing later on.
*/
- if (rdp->nxtlist != NULL &&
- rdp->nxttail[RCU_DONE_TAIL] != &rdp->nxtlist) {
- struct rcu_head *oldhead;
- struct rcu_head **oldtail;
- struct rcu_head **newtail;
-
- oldhead = rdp->nxtlist;
- oldtail = receive_rdp->nxttail[RCU_DONE_TAIL];
- rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];
- *rdp->nxttail[RCU_DONE_TAIL] = *oldtail;
- *receive_rdp->nxttail[RCU_DONE_TAIL] = oldhead;
- newtail = rdp->nxttail[RCU_DONE_TAIL];
- for (i = RCU_DONE_TAIL; i < RCU_NEXT_SIZE; i++) {
- if (receive_rdp->nxttail[i] == oldtail)
- receive_rdp->nxttail[i] = newtail;
- if (rdp->nxttail[i] == newtail)
- rdp->nxttail[i] = &rdp->nxtlist;
- }
+ if (*rdp->nxttail[RCU_DONE_TAIL] != NULL) {
+ *rsp->orphan_nxttail = *rdp->nxttail[RCU_DONE_TAIL];
+ rsp->orphan_nxttail = rdp->nxttail[RCU_NEXT_TAIL];
+ *rdp->nxttail[RCU_DONE_TAIL] = NULL;
}
/*
- * Finally, put the rest of the callbacks at the end of the list.
- * The ones that made it partway through get to start over: We
- * cannot assume that grace periods are synchronized across CPUs.
- * (We could splice RCU_WAIT_TAIL into RCU_NEXT_READY_TAIL, but
- * this does not seem compelling. Not yet, anyway.)
+ * Then move the ready-to-invoke callbacks to the orphanage,
+ * where some other CPU will pick them up. These will not be
+ * required to pass though another grace period: They are done.
*/
if (rdp->nxtlist != NULL) {
- *receive_rdp->nxttail[RCU_NEXT_TAIL] = rdp->nxtlist;
- receive_rdp->nxttail[RCU_NEXT_TAIL] =
- rdp->nxttail[RCU_NEXT_TAIL];
- receive_rdp->n_cbs_adopted += rdp->qlen;
- rdp->n_cbs_orphaned += rdp->qlen;
-
- rdp->nxtlist = NULL;
- for (i = 0; i < RCU_NEXT_SIZE; i++)
- rdp->nxttail[i] = &rdp->nxtlist;
+ *rsp->orphan_donetail = rdp->nxtlist;
+ rsp->orphan_donetail = rdp->nxttail[RCU_DONE_TAIL];
}
+ /* Finally, initialize the rcu_data structure's list to empty. */
+ rdp->nxtlist = NULL;
+ for (i = 0; i < RCU_NEXT_SIZE; i++)
+ rdp->nxttail[i] = &rdp->nxtlist;
+}
+
+/*
+ * Adopt the RCU callbacks from the specified rcu_state structure's
+ * orphanage. The caller must hold the ->onofflock.
+ */
+static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
+{
+ int i;
+ struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);
+
/*
- * Record a quiescent state for the dying CPU. This is safe
- * only because we have already cleared out the callbacks.
- * (Otherwise, the RCU core might try to schedule the invocation
- * of callbacks on this now-offline CPU, which would be bad.)
+ * If there is an rcu_barrier() operation in progress, then
+ * only the task doing that operation is permitted to adopt
+ * callbacks. To do otherwise breaks rcu_barrier() and friends
+ * by causing them to fail to wait for the callbacks in the
+ * orphanage.
*/
- mask = rdp->grpmask; /* rnp->grplo is constant. */
+ if (rsp->rcu_barrier_in_progress &&
+ rsp->rcu_barrier_in_progress != current)
+ return;
+
+ /* Do the accounting first. */
+ rdp->qlen_lazy += rsp->qlen_lazy;
+ rdp->qlen += rsp->qlen;
+ rdp->n_cbs_adopted += rsp->qlen;
+ rsp->qlen_lazy = 0;
+ rsp->qlen = 0;
+
+ /*
+ * We do not need a memory barrier here because the only way we
+ * can get here if there is an rcu_barrier() in flight is if
+ * we are the task doing the rcu_barrier().
+ */
+
+ /* First adopt the ready-to-invoke callbacks. */
+ if (rsp->orphan_donelist != NULL) {
+ *rsp->orphan_donetail = *rdp->nxttail[RCU_DONE_TAIL];
+ *rdp->nxttail[RCU_DONE_TAIL] = rsp->orphan_donelist;
+ for (i = RCU_NEXT_SIZE - 1; i >= RCU_DONE_TAIL; i--)
+ if (rdp->nxttail[i] == rdp->nxttail[RCU_DONE_TAIL])
+ rdp->nxttail[i] = rsp->orphan_donetail;
+ rsp->orphan_donelist = NULL;
+ rsp->orphan_donetail = &rsp->orphan_donelist;
+ }
+
+ /* And then adopt the callbacks that still need a grace period. */
+ if (rsp->orphan_nxtlist != NULL) {
+ *rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxtlist;
+ rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxttail;
+ rsp->orphan_nxtlist = NULL;
+ rsp->orphan_nxttail = &rsp->orphan_nxtlist;
+ }
+}
+
+/*
+ * Trace the fact that this CPU is going offline.
+ */
+static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
+{
+ RCU_TRACE(unsigned long mask);
+ RCU_TRACE(struct rcu_data *rdp = this_cpu_ptr(rsp->rda));
+ RCU_TRACE(struct rcu_node *rnp = rdp->mynode);
+
+ RCU_TRACE(mask = rdp->grpmask);
trace_rcu_grace_period(rsp->name,
rnp->gpnum + 1 - !!(rnp->qsmask & mask),
"cpuofl");
- rcu_report_qs_rdp(smp_processor_id(), rsp, rdp, rsp->gpnum);
- /* Note that rcu_report_qs_rdp() might call trace_rcu_grace_period(). */
}
/*
* The CPU has been completely removed, and some other CPU is reporting
- * this fact from process context. Do the remainder of the cleanup.
+ * this fact from process context. Do the remainder of the cleanup,
+ * including orphaning the outgoing CPU's RCU callbacks, and also
+ * adopting them, if there is no _rcu_barrier() instance running.
* There can only be one CPU hotplug operation at a time, so no other
* CPU can be attempting to update rcu_cpu_kthread_task.
*/
@@ -1409,17 +1455,21 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
unsigned long mask;
int need_report = 0;
struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
- struct rcu_node *rnp = rdp->mynode; /* Outgoing CPU's rnp. */
+ struct rcu_node *rnp = rdp->mynode; /* Outgoing CPU's rdp & rnp. */
/* Adjust any no-longer-needed kthreads. */
rcu_stop_cpu_kthread(cpu);
rcu_node_kthread_setaffinity(rnp, -1);
- /* Remove the dying CPU from the bitmasks in the rcu_node hierarchy. */
+ /* Remove the dead CPU from the bitmasks in the rcu_node hierarchy. */
/* Exclude any attempts to start a new grace period. */
raw_spin_lock_irqsave(&rsp->onofflock, flags);
+ /* Orphan the dead CPU's callbacks, and adopt them if appropriate. */
+ rcu_send_cbs_to_orphanage(cpu, rsp, rnp, rdp);
+ rcu_adopt_orphan_cbs(rsp);
+
/* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */
mask = rdp->grpmask; /* rnp->grplo is constant. */
do {
@@ -1456,6 +1506,10 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
#else /* #ifdef CONFIG_HOTPLUG_CPU */
+static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
+{
+}
+
static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
{
}
@@ -1524,9 +1578,6 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
rcu_is_callbacks_kthread());
/* Update count, and requeue any remaining callbacks. */
- rdp->qlen_lazy -= count_lazy;
- rdp->qlen -= count;
- rdp->n_cbs_invoked += count;
if (list != NULL) {
*tail = rdp->nxtlist;
rdp->nxtlist = list;
@@ -1536,6 +1587,10 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
else
break;
}
+ smp_mb(); /* List handling before counting for rcu_barrier(). */
+ rdp->qlen_lazy -= count_lazy;
+ rdp->qlen -= count;
+ rdp->n_cbs_invoked += count;
/* Reinstate batch limit if we have worked down the excess. */
if (rdp->blimit == LONG_MAX && rdp->qlen <= qlowmark)
@@ -1823,11 +1878,14 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
rdp = this_cpu_ptr(rsp->rda);
/* Add the callback to our list. */
- *rdp->nxttail[RCU_NEXT_TAIL] = head;
- rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
rdp->qlen++;
if (lazy)
rdp->qlen_lazy++;
+ else
+ rcu_idle_count_callbacks_posted();
+ smp_mb(); /* Count before adding callback for rcu_barrier(). */
+ *rdp->nxttail[RCU_NEXT_TAIL] = head;
+ rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
if (__is_kfree_rcu_offset((unsigned long)func))
trace_rcu_kfree_callback(rsp->name, head, (unsigned long)func,
@@ -1893,6 +1951,38 @@ void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
}
EXPORT_SYMBOL_GPL(call_rcu_bh);
+/*
+ * Because a context switch is a grace period for RCU-sched and RCU-bh,
+ * any blocking grace-period wait automatically implies a grace period
+ * if there is only one CPU online at any point time during execution
+ * of either synchronize_sched() or synchronize_rcu_bh(). It is OK to
+ * occasionally incorrectly indicate that there are multiple CPUs online
+ * when there was in fact only one the whole time, as this just adds
+ * some overhead: RCU still operates correctly.
+ *
+ * Of course, sampling num_online_cpus() with preemption enabled can
+ * give erroneous results if there are concurrent CPU-hotplug operations.
+ * For example, given a demonic sequence of preemptions in num_online_cpus()
+ * and CPU-hotplug operations, there could be two or more CPUs online at
+ * all times, but num_online_cpus() might well return one (or even zero).
+ *
+ * However, all such demonic sequences require at least one CPU-offline
+ * operation. Furthermore, rcu_blocking_is_gp() giving the wrong answer
+ * is only a problem if there is an RCU read-side critical section executing
+ * throughout. But RCU-sched and RCU-bh read-side critical sections
+ * disable either preemption or bh, which prevents a CPU from going offline.
+ * Therefore, the only way that rcu_blocking_is_gp() can incorrectly return
+ * that there is only one CPU when in fact there was more than one throughout
+ * is when there were no RCU readers in the system. If there are no
+ * RCU readers, the grace period by definition can be of zero length,
+ * regardless of the number of online CPUs.
+ */
+static inline int rcu_blocking_is_gp(void)
+{
+ might_sleep(); /* Check for RCU read-side critical section. */
+ return num_online_cpus() <= 1;
+}
+
/**
* synchronize_sched - wait until an rcu-sched grace period has elapsed.
*
@@ -2166,11 +2256,10 @@ static int rcu_cpu_has_callbacks(int cpu)
rcu_preempt_cpu_has_callbacks(cpu);
}
-static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
-static atomic_t rcu_barrier_cpu_count;
-static DEFINE_MUTEX(rcu_barrier_mutex);
-static struct completion rcu_barrier_completion;
-
+/*
+ * RCU callback function for _rcu_barrier(). If we are last, wake
+ * up the task executing _rcu_barrier().
+ */
static void rcu_barrier_callback(struct rcu_head *notused)
{
if (atomic_dec_and_test(&rcu_barrier_cpu_count))
@@ -2200,27 +2289,94 @@ static void _rcu_barrier(struct rcu_state *rsp,
void (*call_rcu_func)(struct rcu_head *head,
void (*func)(struct rcu_head *head)))
{
- BUG_ON(in_interrupt());
+ int cpu;
+ unsigned long flags;
+ struct rcu_data *rdp;
+ struct rcu_head rh;
+
+ init_rcu_head_on_stack(&rh);
+
/* Take mutex to serialize concurrent rcu_barrier() requests. */
mutex_lock(&rcu_barrier_mutex);
- init_completion(&rcu_barrier_completion);
+
+ smp_mb(); /* Prevent any prior operations from leaking in. */
+
/*
- * Initialize rcu_barrier_cpu_count to 1, then invoke
- * rcu_barrier_func() on each CPU, so that each CPU also has
- * incremented rcu_barrier_cpu_count. Only then is it safe to
- * decrement rcu_barrier_cpu_count -- otherwise the first CPU
- * might complete its grace period before all of the other CPUs
- * did their increment, causing this function to return too
- * early. Note that on_each_cpu() disables irqs, which prevents
- * any CPUs from coming online or going offline until each online
- * CPU has queued its RCU-barrier callback.
+ * Initialize the count to one rather than to zero in order to
+ * avoid a too-soon return to zero in case of a short grace period
+ * (or preemption of this task). Also flag this task as doing
+ * an rcu_barrier(). This will prevent anyone else from adopting
+ * orphaned callbacks, which could cause otherwise failure if a
+ * CPU went offline and quickly came back online. To see this,
+ * consider the following sequence of events:
+ *
+ * 1. We cause CPU 0 to post an rcu_barrier_callback() callback.
+ * 2. CPU 1 goes offline, orphaning its callbacks.
+ * 3. CPU 0 adopts CPU 1's orphaned callbacks.
+ * 4. CPU 1 comes back online.
+ * 5. We cause CPU 1 to post an rcu_barrier_callback() callback.
+ * 6. Both rcu_barrier_callback() callbacks are invoked, awakening
+ * us -- but before CPU 1's orphaned callbacks are invoked!!!
*/
+ init_completion(&rcu_barrier_completion);
atomic_set(&rcu_barrier_cpu_count, 1);
- on_each_cpu(rcu_barrier_func, (void *)call_rcu_func, 1);
+ raw_spin_lock_irqsave(&rsp->onofflock, flags);
+ rsp->rcu_barrier_in_progress = current;
+ raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
+
+ /*
+ * Force every CPU with callbacks to register a new callback
+ * that will tell us when all the preceding callbacks have
+ * been invoked. If an offline CPU has callbacks, wait for
+ * it to either come back online or to finish orphaning those
+ * callbacks.
+ */
+ for_each_possible_cpu(cpu) {
+ preempt_disable();
+ rdp = per_cpu_ptr(rsp->rda, cpu);
+ if (cpu_is_offline(cpu)) {
+ preempt_enable();
+ while (cpu_is_offline(cpu) && ACCESS_ONCE(rdp->qlen))
+ schedule_timeout_interruptible(1);
+ } else if (ACCESS_ONCE(rdp->qlen)) {
+ smp_call_function_single(cpu, rcu_barrier_func,
+ (void *)call_rcu_func, 1);
+ preempt_enable();
+ } else {
+ preempt_enable();
+ }
+ }
+
+ /*
+ * Now that all online CPUs have rcu_barrier_callback() callbacks
+ * posted, we can adopt all of the orphaned callbacks and place
+ * an rcu_barrier_callback() callback after them. When that is done,
+ * we are guaranteed to have an rcu_barrier_callback() callback
+ * following every callback that could possibly have been
+ * registered before _rcu_barrier() was called.
+ */
+ raw_spin_lock_irqsave(&rsp->onofflock, flags);
+ rcu_adopt_orphan_cbs(rsp);
+ rsp->rcu_barrier_in_progress = NULL;
+ raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
+ atomic_inc(&rcu_barrier_cpu_count);
+ smp_mb__after_atomic_inc(); /* Ensure atomic_inc() before callback. */
+ call_rcu_func(&rh, rcu_barrier_callback);
+
+ /*
+ * Now that we have an rcu_barrier_callback() callback on each
+ * CPU, and thus each counted, remove the initial count.
+ */
if (atomic_dec_and_test(&rcu_barrier_cpu_count))
complete(&rcu_barrier_completion);
+
+ /* Wait for all rcu_barrier_callback() callbacks to be invoked. */
wait_for_completion(&rcu_barrier_completion);
+
+ /* Other rcu_barrier() invocations can now safely proceed. */
mutex_unlock(&rcu_barrier_mutex);
+
+ destroy_rcu_head_on_stack(&rh);
}
/**
@@ -2417,7 +2573,7 @@ static void __init rcu_init_levelspread(struct rcu_state *rsp)
for (i = NUM_RCU_LVLS - 1; i > 0; i--)
rsp->levelspread[i] = CONFIG_RCU_FANOUT;
- rsp->levelspread[0] = RCU_FANOUT_LEAF;
+ rsp->levelspread[0] = CONFIG_RCU_FANOUT_LEAF;
}
#else /* #ifdef CONFIG_RCU_FANOUT_EXACT */
static void __init rcu_init_levelspread(struct rcu_state *rsp)