rcu-tasks: Provide rcu_trace_implies_rcu_gp()

As an accident of implementation, an RCU Tasks Trace grace period also
acts as an RCU grace period.  However, this could change at any time.
This commit therefore creates an rcu_trace_implies_rcu_gp() that currently
returns true to codify this accident.  Code relying on this accident
must call this function to verify that this accident is still happening.

Reported-by: Hou Tao <houtao@huaweicloud.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Link: https://lore.kernel.org/r/20221014113946.965131-2-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 08605ce..8822f06 100644 (file)
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -241,6 +241,18 @@ static inline void exit_tasks_rcu_finish(void) { }
 #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
 
 /**
+ * rcu_trace_implies_rcu_gp - does an RCU Tasks Trace grace period imply an RCU grace period?
+ *
+ * As an accident of implementation, an RCU Tasks Trace grace period also
+ * acts as an RCU grace period.  However, this could change at any time.
+ * Code relying on this accident must call this function to verify that
+ * this accident is still happening.
+ *
+ * You have been warned!
+ */
+static inline bool rcu_trace_implies_rcu_gp(void) { return true; }
+
+/**
  * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU
  *
  * This macro resembles cond_resched(), except that it is defined to

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index f5bf6fb..9435e5a 100644 (file)
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1535,6 +1535,8 @@ static void rcu_tasks_trace_postscan(struct list_head *hop)
 {
        // Wait for late-stage exiting tasks to finish exiting.
        // These might have passed the call to exit_tasks_rcu_finish().
+
+       // If you remove the following line, update rcu_trace_implies_rcu_gp()!!!
        synchronize_rcu();
        // Any tasks that exit after this point will set
        // TRC_NEED_QS_CHECKED in ->trc_reader_special.b.need_qs.