path: root/kernel/rcupdate.c
diff options
authorLai Jiangshan <laijs@cn.fujitsu.com>2009-03-20 17:40:06 +0800
committerIngo Molnar <mingo@elte.hu>2009-03-31 00:09:37 +0200
commitf69b17d7e745d8edd7c0d90390cbaa77e63c5ea3 (patch)
treed903954135e29cd882220180d39cef1df30795b8 /kernel/rcupdate.c
parentdfbbe89e197a77f2c8046a51c74e33e35f878080 (diff)
rcu: rcu_barrier VS cpu_hotplug: Ensure callbacks in dead cpu are migrated to online cpu
cpu hotplug may happen asynchronously, some rcu callbacks are maybe still on dead cpu, rcu_barrier() also needs to wait for these rcu callbacks to complete, so we must ensure callbacks in dead cpu are migrated to online cpu. Paul E. McKenney's review: Good stuff, Lai!!! Simpler than any of the approaches that I was considering, and, better yet, independent of the underlying RCU implementation!!! I was initially worried that wake_up() might wake only one of two possible wait_event()s, namely rcu_barrier() and the CPU_POST_DEAD code, but the fact that wait_event() clears WQ_FLAG_EXCLUSIVE avoids that issue. I was also worried about the fact that different RCU implementations have different mappings of call_rcu(), call_rcu_bh(), and call_rcu_sched(), but this is OK as well because we just get an extra (harmless) callback in the case that they map together (for example, Classic RCU has call_rcu_sched() mapping to call_rcu()). Overlap of CPU-hotplug operations is prevented by cpu_add_remove_lock, and any stray callbacks that arrive (for example, from irq handlers running on the dying CPU) either are ahead of the CPU_DYING callbacks on the one hand (and thus accounted for), or happened after the rcu_barrier() started on the other (and thus don't need to be accounted for). Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> LKML-Reference: <49C36476.1010400@cn.fujitsu.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'kernel/rcupdate.c')
1 files changed, 44 insertions, 0 deletions
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index cae8a059cf4..2c7b8457d0d 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -122,6 +122,8 @@ static void rcu_barrier_func(void *type)
+static inline void wait_migrated_callbacks(void);
* Orchestrate the specified type of RCU barrier, waiting for all
* RCU callbacks of the specified type to complete.
@@ -147,6 +149,7 @@ static void _rcu_barrier(enum rcu_barrier type)
+ wait_migrated_callbacks();
@@ -176,9 +179,50 @@ void rcu_barrier_sched(void)
+static atomic_t rcu_migrate_type_count = ATOMIC_INIT(0);
+static struct rcu_head rcu_migrate_head[3];
+static DECLARE_WAIT_QUEUE_HEAD(rcu_migrate_wq);
+static void rcu_migrate_callback(struct rcu_head *notused)
+ if (atomic_dec_and_test(&rcu_migrate_type_count))
+ wake_up(&rcu_migrate_wq);
+static inline void wait_migrated_callbacks(void)
+ wait_event(rcu_migrate_wq, !atomic_read(&rcu_migrate_type_count));
+static int __cpuinit rcu_barrier_cpu_hotplug(struct notifier_block *self,
+ unsigned long action, void *hcpu)
+ if (action == CPU_DYING) {
+ /*
+ * preempt_disable() in on_each_cpu() prevents stop_machine(),
+ * so when "on_each_cpu(rcu_barrier_func, (void *)type, 1);"
+ * returns, all online cpus have queued rcu_barrier_func(),
+ * and the dead cpu(if it exist) queues rcu_migrate_callback()s.
+ *
+ * These callbacks ensure _rcu_barrier() waits for all
+ * RCU callbacks of the specified type to complete.
+ */
+ atomic_set(&rcu_migrate_type_count, 3);
+ call_rcu_bh(rcu_migrate_head, rcu_migrate_callback);
+ call_rcu_sched(rcu_migrate_head + 1, rcu_migrate_callback);
+ call_rcu(rcu_migrate_head + 2, rcu_migrate_callback);
+ } else if (action == CPU_POST_DEAD) {
+ /* rcu_migrate_head is protected by cpu_add_remove_lock */
+ wait_migrated_callbacks();
+ }
+ return NOTIFY_OK;
void __init rcu_init(void)
+ hotcpu_notifier(rcu_barrier_cpu_hotplug, 0);
void rcu_scheduler_starting(void)