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

Sasha Levin
2024-03-24T23:35:03Z

From: "Paul E. McKenney" <paulmck@kernel.org>

[ 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@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>
Stable-dep-of: 876673364161 ("bpf: Defer the free of inner map when necessary")
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 10108826191ab30388e8ae9d54505a628f78a7ec)
Signed-off-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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