[PATCH RFC tip/core/rcu 3/9] rcu: Add synchronous grace-period waiting for RCU-tasks
From: Paul E. McKenney
Date: Mon Jul 28 2014 - 18:56:44 EST
From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
It turns out to be easier to add the synchronous grace-period waiting
functions to RCU-tasks than to work around their absense in rcutorture,
so this commit adds them. The key point is that the existence of
call_rcu_tasks() means that rcutorture needs an rcu_barrier_tasks().
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
include/linux/rcupdate.h | 2 ++
kernel/rcu/update.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 57 insertions(+)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 3299ff98ad03..17c7e25c38be 100644
@@ -216,6 +216,8 @@ void synchronize_sched(void);
* memory ordering guarantees.
void call_rcu_tasks(struct rcu_head *head, void (*func)(struct rcu_head *head));
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 2bcb1e611460..310ecf4f8e74 100644
@@ -387,6 +387,61 @@ void call_rcu_tasks(struct rcu_head *rhp, void (*func)(struct rcu_head *rhp))
+ * synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed.
+ * Control will return to the caller some time after a full rcu-tasks
+ * grace period has elapsed, in other words after all currently
+ * executing rcu-tasks read-side critical sections have elapsed. These
+ * read-side critical sections are delimited by calls to schedule(),
+ * cond_resched_rcu_qs(), idle execution, userspace execution, calls
+ * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched().
+ * This is a very specialized primitive, intended only for a few uses in
+ * tracing and other situations requiring manipulation of function
+ * preambles and profiling hooks. The synchronize_rcu_tasks() function
+ * is not (yet) intended for heavy use from multiple CPUs.
+ * Note that this guarantee implies further memory-ordering guarantees.
+ * On systems with more than one CPU, when synchronize_rcu_tasks() returns,
+ * each CPU is guaranteed to have executed a full memory barrier since the
+ * end of its last RCU-tasks read-side critical section whose beginning
+ * preceded the call to synchronize_rcu_tasks(). In addition, each CPU
+ * having an RCU-tasks read-side critical section that extends beyond
+ * the return from synchronize_rcu_tasks() is guaranteed to have executed
+ * a full memory barrier after the beginning of synchronize_rcu_tasks()
+ * and before the beginning of that RCU-tasks read-side critical section.
+ * Note that these guarantees include CPUs that are offline, idle, or
+ * executing in user mode, as well as CPUs that are executing in the kernel.
+ * Furthermore, if CPU A invoked synchronize_rcu_tasks(), which returned
+ * to its caller on CPU B, then both CPU A and CPU B are guaranteed
+ * to have executed a full memory barrier during the execution of
+ * synchronize_rcu_tasks() -- even if CPU A and CPU B are the same CPU
+ * (but again only if the system has more than one CPU).
+ /* Complain if the scheduler has not started. */
+ "synchronize_rcu_tasks called too soon");
+ /* Wait for the grace period. */
+ * rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks.
+ * Although the current implementation is guaranteed to wait, it is not
+ * obligated to, for example, if there are no pending callbacks.
+ /* There is only one callback queue, so this is easy. ;-) */
/* RCU-tasks kthread that detects grace periods and invokes callbacks. */
static int __noreturn rcu_tasks_kthread(void *arg)
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