The following commit has been merged into the core/rcu branch of tip: Commit-ID: a434dd10cd843c7348e7c54c77eb0fac27beceb4 Gitweb: https://git.kernel.org/tip/a434dd10cd843c7348e7c54c77eb0fac27beceb4 Author: Paul E. McKenney <paulmck@xxxxxxxxxx> AuthorDate: Thu, 25 Feb 2021 10:26:00 -08:00 Committer: Paul E. McKenney <paulmck@xxxxxxxxxx> CommitterDate: Mon, 08 Mar 2021 14:22:02 -08:00 rcu-tasks: Add block comment laying out RCU Tasks Trace design This commit adds a block comment that gives a high-level overview of how RCU tasks trace grace periods progress. It also adds a note about how exiting tasks are handled, plus it gives an overview of the memory ordering. Reported-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Reported-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> [ paulmck: Fix commit log per Mathieu Desnoyers feedback. ] Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> --- kernel/rcu/tasks.h | 36 ++++++++++++++++++++++++++++++++++++ 1 file changed, 36 insertions(+) diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index 17c8ebe..350ebf5 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -726,6 +726,42 @@ EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread); // flavors, rcu_preempt and rcu_sched. The fact that RCU Tasks Trace // readers can operate from idle, offline, and exception entry/exit in no // way allows rcu_preempt and rcu_sched readers to also do so. +// +// The implementation uses rcu_tasks_wait_gp(), which relies on function +// pointers in the rcu_tasks structure. The rcu_spawn_tasks_trace_kthread() +// function sets these function pointers up so that rcu_tasks_wait_gp() +// invokes these functions in this order: +// +// rcu_tasks_trace_pregp_step(): +// Initialize the count of readers and block CPU-hotplug operations. +// rcu_tasks_trace_pertask(), invoked on every non-idle task: +// Initialize per-task state and attempt to identify an immediate +// quiescent state for that task, or, failing that, attempt to +// set that task's .need_qs flag so that task's next outermost +// rcu_read_unlock_trace() will report the quiescent state (in which +// case the count of readers is incremented). If both attempts fail, +// the task is added to a "holdout" list. +// rcu_tasks_trace_postscan(): +// Initialize state and attempt to identify an immediate quiescent +// state as above (but only for idle tasks), unblock CPU-hotplug +// operations, and wait for an RCU grace period to avoid races with +// tasks that are in the process of exiting. +// check_all_holdout_tasks_trace(), 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_trace_postgp(): +// Wait for the count of readers do drop to zero, reporting any stalls. +// Also execute full memory barriers to maintain ordering with code +// executing after the grace period. +// +// The exit_tasks_rcu_finish_trace() synchronizes with exiting tasks. +// +// Pre-grace-period update-side code is ordered before the grace +// period via the ->cbs_lock and barriers in rcu_tasks_kthread(). +// Pre-grace-period read-side code is ordered before the grace period by +// atomic_dec_and_test() of the count of readers (for IPIed readers) and by +// scheduler context-switch ordering (for locked-down non-running readers). // The lockdep state must be outside of #ifdef to be useful. #ifdef CONFIG_DEBUG_LOCK_ALLOC
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