@@ -180,6 +180,20 @@ over a rather long period of time, but improvements are always welcome!
operations that would not normally be undertaken while a real-time
workload is running.
+ In particular, if you find yourself invoking one of the expedited
+ primitives repeatedly in a loop, please do everyone a favor:
+ Restructure your code so that it batches the updates, allowing
+ a single non-expedited primitive to cover the entire batch.
+ This will very likely be faster than the loop containing the
+ expedited primitive, and will be much much easier on the rest
+ of the system, especially to real-time workloads running on
+ the rest of the system.
+
+ In addition, it is illegal to call the expedited forms from
+ a CPU-hotplug notifier, or while holding a lock that is acquired
+ by a CPU-hotplug notifier. Failing to observe this restriction
+ will result in deadlock.
+
7. If the updater uses call_rcu() or synchronize_rcu(), then the
corresponding readers must use rcu_read_lock() and
rcu_read_unlock(). If the updater uses call_rcu_bh() or
@@ -63,6 +63,22 @@ extern void synchronize_rcu_expedited(void);
void kfree_call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu));
+/**
+ * synchronize_rcu_bh_expedited - Brute-force RCU-bh grace period
+ *
+ * Wait for an RCU-bh grace period to elapse, but use a "big hammer"
+ * approach to force the grace period to end quickly. This consumes
+ * significant time on all CPUs and is unfriendly to real-time workloads,
+ * so is thus not recommended for any sort of common-case code. In fact,
+ * if you are using synchronize_rcu_bh_expedited() in a loop, please
+ * restructure your code to batch your updates, and then use a single
+ * synchronize_rcu_bh() instead.
+ *
+ * Note that it is illegal to call this function while holding any lock
+ * that is acquired by a CPU-hotplug notifier. And yes, it is also illegal
+ * to call this function from a CPU-hotplug notifier. Failing to observe
+ * these restriction will result in deadlock.
+ */
static inline void synchronize_rcu_bh_expedited(void)
{
synchronize_sched_expedited();
@@ -1962,15 +1962,21 @@ static int synchronize_sched_expedited_cpu_stop(void *data)
return 0;
}
-/*
- * Wait for an rcu-sched grace period to elapse, but use "big hammer"
- * approach to force grace period to end quickly. This consumes
- * significant time on all CPUs, and is thus not recommended for
- * any sort of common-case code.
+/**
+ * synchronize_sched_expedited - Brute-force RCU-sched grace period
+ *
+ * Wait for an RCU-sched grace period to elapse, but use a "big hammer"
+ * approach to force the grace period to end quickly. This consumes
+ * significant time on all CPUs and is unfriendly to real-time workloads,
+ * so is thus not recommended for any sort of common-case code. In fact,
+ * if you are using synchronize_sched_expedited() in a loop, please
+ * restructure your code to batch your updates, and then use a single
+ * synchronize_sched() instead.
*
- * Note that it is illegal to call this function while holding any
- * lock that is acquired by a CPU-hotplug notifier. Failing to
- * observe this restriction will result in deadlock.
+ * Note that it is illegal to call this function while holding any lock
+ * that is acquired by a CPU-hotplug notifier. And yes, it is also illegal
+ * to call this function from a CPU-hotplug notifier. Failing to observe
+ * these restriction will result in deadlock.
*
* This implementation can be thought of as an application of ticket
* locking to RCU, with sync_sched_expedited_started and
@@ -834,10 +834,22 @@ sync_rcu_preempt_exp_init(struct rcu_state *rsp, struct rcu_node *rnp)
rcu_report_exp_rnp(rsp, rnp, false); /* Don't wake self. */
}
-/*
- * Wait for an rcu-preempt grace period, but expedite it. The basic idea
- * is to invoke synchronize_sched_expedited() to push all the tasks to
- * the ->blkd_tasks lists and wait for this list to drain.
+/**
+ * synchronize_rcu_expedited - Brute-force RCU grace period
+ *
+ * Wait for an RCU-preempt grace period, but expedite it. The basic
+ * idea is to invoke synchronize_sched_expedited() to push all the tasks to
+ * the ->blkd_tasks lists and wait for this list to drain. This consumes
+ * significant time on all CPUs and is unfriendly to real-time workloads,
+ * so is thus not recommended for any sort of common-case code.
+ * In fact, if you are using synchronize_rcu_expedited() in a loop,
+ * please restructure your code to batch your updates, and then Use a
+ * single synchronize_rcu() instead.
+ *
+ * Note that it is illegal to call this function while holding any lock
+ * that is acquired by a CPU-hotplug notifier. And yes, it is also illegal
+ * to call this function from a CPU-hotplug notifier. Failing to observe
+ * these restriction will result in deadlock.
*/
void synchronize_rcu_expedited(void)
{
@@ -284,19 +284,26 @@ void synchronize_srcu(struct srcu_struct *sp)
EXPORT_SYMBOL_GPL(synchronize_srcu);
/**
- * synchronize_srcu_expedited - like synchronize_srcu, but less patient
+ * synchronize_srcu_expedited - Brute-force SRCU grace period
* @sp: srcu_struct with which to synchronize.
*
- * Flip the completed counter, and wait for the old count to drain to zero.
- * As with classic RCU, the updater must use some separate means of
- * synchronizing concurrent updates. Can block; must be called from
- * process context.
+ * Wait for an SRCU grace period to elapse, but use a "big hammer"
+ * approach to force the grace period to end quickly. This consumes
+ * significant time on all CPUs and is unfriendly to real-time workloads,
+ * so is thus not recommended for any sort of common-case code. In fact,
+ * if you are using synchronize_srcu_expedited() in a loop, please
+ * restructure your code to batch your updates, and then use a single
+ * synchronize_srcu() instead.
*
- * Note that it is illegal to call synchronize_srcu_expedited()
- * from the corresponding SRCU read-side critical section; doing so
- * will result in deadlock. However, it is perfectly legal to call
- * synchronize_srcu_expedited() on one srcu_struct from some other
- * srcu_struct's read-side critical section.
+ * Note that it is illegal to call this function while holding any lock
+ * that is acquired by a CPU-hotplug notifier. And yes, it is also illegal
+ * to call this function from a CPU-hotplug notifier. Failing to observe
+ * these restriction will result in deadlock. It is also illegal to call
+ * synchronize_srcu_expedited() from the corresponding SRCU read-side
+ * critical section; doing so will result in deadlock. However, it is
+ * perfectly legal to call synchronize_srcu_expedited() on one srcu_struct
+ * from some other srcu_struct's read-side critical section, as long as
+ * the resulting graph of srcu_structs is acyclic.
*/
void synchronize_srcu_expedited(struct srcu_struct *sp)
{