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).
|January 2017||Kernel Hackers Manual 4.8.|