rcu-tasks: Update comments

// 3.	Avoids expensive read-side instructions, having overhead similar

//	to that of Preemptible RCU.

//

// There are of course downsides.  The grace-period code can send IPIs to

// CPUs, even when those CPUs are in the idle loop or in nohz_full userspace.

// It is necessary to scan the full tasklist, much as for Tasks RCU.  There

// is a single callback queue guarded by a single lock, again, much as for

// Tasks RCU.  If needed, these downsides can be at least partially remedied.

// There are of course downsides.  For example, the grace-period code

// can send IPIs to CPUs, even when those CPUs are in the idle loop or

// in nohz_full userspace.  If needed, these downsides can be at least

// partially remedied.

//

// Perhaps most important, this variant of RCU does not affect the vanilla

// flavors, rcu_preempt and rcu_sched.  The fact that RCU Tasks Trace

// 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.  Note that IPIs are used

//	to invoke trc_read_check_handler() in the context of running tasks

//	in order to avoid ordering overhead on common-case shared-variable

//	accessses.

//	Disables CPU hotplug, adds all currently executing tasks to the

//	holdout list, then checks the state of all tasks that blocked

//	or were preempted within their current RCU Tasks Trace read-side

//	critical section, adding them to the holdout list if appropriate.

//	Finally, this function re-enables CPU hotplug.

// The ->pertask_func() pointer is NULL, so there is no per-task processing.

// 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.

//	Invokes synchronize_rcu() to wait for late-stage exiting tasks

//	to finish 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.

//	corresponding task is removed from the holdout list.  Once this

//	list is empty, the grace period has completed.

// 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.

//	Provides the needed full memory barrier and does debug checks.

//

// 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).

// 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 operations

// on .b.need_qs flag of each task involved in this process, or 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

}

EXPORT_SYMBOL_GPL(rcu_trc_cmpxchg_need_qs);

/* If we are the last reader, wake up the grace-period kthread. */

/*

 * If we are the last reader, signal the grace-period kthread.

 * Also remove from the per-CPU list of blocked tasks.

 */

void rcu_read_unlock_trace_special(struct task_struct *t)

{

	unsigned long flags;

	if (unlikely(nesting < 0))

		goto reset_ipi;

	// Get here if the task is in a read-side critical section.  Set

	// its state so that it will awaken the grace-period kthread upon

	// exit from that critical section.

	// Get here if the task is in a read-side critical section.

	// Set its state so that it will update state for the grace-period

	// kthread upon exit from that critical section.

	rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED);

reset_ipi:

		return 0;  // In QS, so done.

	}

	if (nesting < 0)

		return -EINVAL; //  QS transitioning, try again later.

		return -EINVAL; // Reader transitioning, try again later.

	// The task is in a read-side critical section, so set up its

	// state so that it will update state upon exit from that critical

	for_each_possible_cpu(cpu)

		WARN_ON_ONCE(per_cpu(trc_ipi_to_cpu, cpu));

	// Disable CPU hotplug across the CPU scan.

	// This also waits for all readers in CPU-hotplug code paths.

	// Disable CPU hotplug across the CPU scan for the benefit of

	// any IPIs that might be needed.  This also waits for all readers

	// in CPU-hotplug code paths.

	cpus_read_lock();

	// These smp_call_function_single() calls are serialized to

	// These rcu_tasks_trace_pertask_prep() calls are serialized to

	// allow safe access to the hop list.

	for_each_online_cpu(cpu) {

		rcu_read_lock();

{

	struct task_struct *g, *t;

	// Disable CPU hotplug across the holdout list scan.

	// Disable CPU hotplug across the holdout list scan for IPIs.

	cpus_read_lock();

	list_for_each_entry_safe(t, g, hop, trc_holdout_list) {