[bpf-next,v2,1/4] rcu-tasks: Provide rcu_trace_implies_rcu_gp()

Commit Message

Hou Tao Oct. 14, 2022, 11:39 a.m. UTC

From: "Paul E. McKenney" <[e-mail]>

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 <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>
Cc: Alexei Starovoitov <[e-mail]>
Cc: Martin KaFai Lau <[e-mail]>
---
 include/linux/rcupdate.h | 12 ++++++++++++
 kernel/rcu/tasks.h       |  2 ++
 2 files changed, 14 insertions(+)

Patch

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 08605ce7379d..8822f06e4b40 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -240,6 +240,18 @@  static inline void exit_tasks_rcu_start(void) { }
 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
  *
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index f5bf6fb430da..9435e5a7b53e 100644
--- 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.