Commit a434dd10 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

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: default avatarPeter Zijlstra <peterz@infradead.org>
Reported-by: default avatarMathieu Desnoyers <mathieu.desnoyers@efficios.com>
[ paulmck: Fix commit log per Mathieu Desnoyers feedback. ]
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 85b86994

kernel/rcu/tasks.h

+36 −0
Original line number Diff line number Diff line
@@ -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