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authorPaul E. McKenney2021-03-04 14:41:47 -0800
committerPaul E. McKenney2021-05-10 16:04:24 -0700
commit06a3ec9205d570526665c2071d1a5492c3091a54 (patch)
tree94cc4b178885a928dbebf243d5fb6e421dc29bf9 /kernel/rcu
parent6efb943b8616ec53a5e444193dccf1af9ad627b5 (diff)
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@kernel.org>
Diffstat (limited to 'kernel/rcu')
-rw-r--r--kernel/rcu/tasks.h40
1 files changed, 40 insertions, 0 deletions
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 350ebf5051f9..94d2c2c7f0ab 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)