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synchronize_sched - wait until an rcu-sched grace period has elapsed.


void synchronize_sched(void);



no arguments


Control will return to the caller some time after a full rcu-sched grace period has elapsed, in other words after all currently executing rcu-sched read-side critical sections have completed. These read-side critical sections are delimited by rcu_read_lock_sched and rcu_read_unlock_sched, and may be nested. Note that preempt_disable, local_irq_disable, and so on may be used in place of rcu_read_lock_sched.

This means that all preempt_disable code sequences, including NMI and non-threaded hardware-interrupt handlers, in progress on entry will have completed before this primitive returns. However, this does not guarantee that softirq handlers will have completed, since in some kernels, these handlers can run in process context, and can block.

Note that this guarantee implies further memory-ordering guarantees. On systems with more than one CPU, when synchronize_sched returns, each CPU is guaranteed to have executed a full memory barrier since the end of its last RCU-sched read-side critical section whose beginning preceded the call to synchronize_sched. In addition, each CPU having an RCU read-side critical section that extends beyond the return from synchronize_sched is guaranteed to have executed a full memory barrier after the beginning of synchronize_sched and before the beginning of that RCU 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_sched, 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_sched -- even if CPU A and CPU B are the same CPU (but again only if the system has more than one CPU).

This primitive provides the guarantees made by the (now removed) synchronize_kernel API. In contrast, synchronize_rcu only guarantees that rcu_read_lock sections will have completed. In “classic RCU”, these two guarantees happen to be one and the same, but can differ in realtime RCU implementations.


January 2017 Kernel Hackers Manual 4.8.