The following commit has been merged into the core/rcu branch of tip: Commit-ID: 06a3ec9205d570526665c2071d1a5492c3091a54 Gitweb: https://git.kernel.org/tip/06a3ec9205d570526665c2071d1a5492c3091a54 Author: Paul E. McKenney <paulmck@xxxxxxxxxx> AuthorDate: Thu, 04 Mar 2021 14:41:47 -08:00 Committer: Paul E. McKenney <paulmck@xxxxxxxxxx> CommitterDate: Mon, 10 May 2021 16:04:24 -07:00 rcu-tasks: Add block comment laying out RCU Tasks design This commit adds a block comment that gives a high-level overview of how RCU tasks grace periods progress. It also adds a note about how exiting tasks are handled, plus it gives an overview of the memory ordering. Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> --- kernel/rcu/tasks.h | 40 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 40 insertions(+) diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index 350ebf5..94d2c2c 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -377,6 +377,46 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp) // Finally, this implementation does not support high call_rcu_tasks() // rates from multiple CPUs. If this is required, per-CPU callback lists // will be needed. +// +// The implementation uses rcu_tasks_wait_gp(), which relies on function +// pointers in the rcu_tasks structure. The rcu_spawn_tasks_kthread() +// function sets these function pointers up so that rcu_tasks_wait_gp() +// invokes these functions in this order: +// +// rcu_tasks_pregp_step(): +// Invokes synchronize_rcu() in order to wait for all in-flight +// t->on_rq and t->nvcsw transitions to complete. This works because +// all such transitions are carried out with interrupts disabled. +// rcu_tasks_pertask(), invoked on every non-idle task: +// For every runnable non-idle task other than the current one, use +// get_task_struct() to pin down that task, snapshot that task's +// number of voluntary context switches, and add that task to the +// holdout list. +// rcu_tasks_postscan(): +// Invoke synchronize_srcu() to ensure that all tasks that were +// in the process of exiting (and which thus might not know to +// synchronize with this RCU Tasks grace period) have completed +// exiting. +// check_all_holdout_tasks(), repeatedly until holdout list is empty: +// Scans the holdout list, attempting to identify a quiescent state +// for each task on the list. If there is a quiescent state, the +// corresponding task is removed from the holdout list. +// rcu_tasks_postgp(): +// Invokes synchronize_rcu() in order to ensure that all prior +// t->on_rq and t->nvcsw transitions are seen by all CPUs and tasks +// to have happened before the end of this RCU Tasks grace period. +// Again, this works because all such transitions are carried out +// with interrupts disabled. +// +// For each exiting task, the exit_tasks_rcu_start() and +// exit_tasks_rcu_finish() functions begin and end, respectively, the SRCU +// read-side critical sections waited for by rcu_tasks_postscan(). +// +// Pre-grace-period update-side code is ordered before the grace via the +// ->cbs_lock and the smp_mb__after_spinlock(). Pre-grace-period read-side +// code is ordered before the grace period via synchronize_rcu() call +// in rcu_tasks_pregp_step() and by the scheduler's locks and interrupt +// disabling. /* Pre-grace-period preparation. */ static void rcu_tasks_pregp_step(void)
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