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 <[email protected]> Signed-off-by: Paul E. McKenney <[email protected]> Cc: Alexei Starovoitov <[email protected]> Cc: Martin KaFai Lau <[email protected]> Link: https://lore.kernel.org/r/[email protected] Signed-off-by: Alexei Starovoitov <[email protected]>
majek · Oct 18, 2022 · e6c86c5 · e6c86c5
1 parent 62c69e8
commit e6c86c5
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diff --git 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
@@ -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.