diff options
| -rw-r--r-- | include/linux/rcutree.h | 7 | ||||
| -rw-r--r-- | kernel/rcutree.c | 32 |
2 files changed, 32 insertions, 7 deletions
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h index e8ee5dd0854..b06363055ef 100644 --- a/include/linux/rcutree.h +++ b/include/linux/rcutree.h @@ -98,13 +98,6 @@ extern void rcu_force_quiescent_state(void); extern void rcu_bh_force_quiescent_state(void); extern void rcu_sched_force_quiescent_state(void); -/* A context switch is a grace period for RCU-sched and RCU-bh. */ -static inline int rcu_blocking_is_gp(void) -{ - might_sleep(); /* Check for RCU read-side critical section. */ - return num_online_cpus() == 1; -} - extern void rcu_scheduler_starting(void); extern int rcu_scheduler_active __read_mostly; diff --git a/kernel/rcutree.c b/kernel/rcutree.c index 780acf8e15e..8f6a344306e 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -1894,6 +1894,38 @@ 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. * |