[PATCH 5.15 003/317] rcu-tasks: Provide rcu_trace_implies_rcu_gp()

[Sasha Levin]

From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>

[ Upstream commit e6c86c513f440bec5f1046539c7e3c6c653842da ]

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@xxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Martin KaFai Lau <martin.lau@xxxxxxxxx>
Link: https://lore.kernel.org/r/20221014113946.965131-2-houtao@xxxxxxxxxxxxxxx
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Stable-dep-of: 876673364161 ("bpf: Defer the free of inner map when necessary")
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
(cherry picked from commit 10108826191ab30388e8ae9d54505a628f78a7ec)
Signed-off-by: Robert Kolchmeyer <rkolchmeyer@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 include/linux/rcupdate.h | 12 ++++++++++++
 kernel/rcu/tasks.h       |  2 ++
 2 files changed, 14 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 13bddb841ceb1..e3b12de36e924 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -205,6 +205,18 @@ static inline void exit_tasks_rcu_stop(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 28f628c702452..b24ef77325eed 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1098,6 +1098,8 @@ 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_reader_checked.
 }
-- 
2.43.0