Comment the current understanding of barriers and locking role around
task snapshot.
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
kernel/rcu/tasks.h | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 6a9ee35a282e..05413b37dd6e 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1738,9 +1738,21 @@ static void rcu_tasks_trace_pregp_step(struct list_head *hop)
for_each_online_cpu(cpu) {
rcu_read_lock();
/*
- * RQ must be locked because no ordering exists/can be relied upon
- * between rq->curr write and subsequent read sides. This ensures that
- * further context switching tasks will see update side pre-GP accesses.
+ * RQ lock + smp_mb__after_spinlock() before reading rq->curr serve
+ * two purposes:
+ *
+ * 1) Ordering against previous tasks accesses (though already enforced
+ * by upcoming IPIs and post-gp synchronize_rcu()).
+ *
+ * 2) Make sure not to miss latest context switch, because no ordering
+ * exists/can be relied upon between rq->curr write and subsequent read
+ * sides.
+ *
+ * 3) Make sure subsequent context switching tasks will see update side
+ * pre-GP accesses.
+ *
+ * smp_mb() after reading rq->curr doesn't play a significant role and might
+ * be considered for removal in the future.
*/
t = cpu_curr_snapshot(cpu);
if (rcu_tasks_trace_pertask_prep(t, true))
--
2.44.0
