[tip: core/rcu] rcu: Reinstate synchronize_rcu_mult()

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     b3d73156b075014ce5b2609f4f47723d6c0c23d6
Gitweb:        https://git.kernel.org/tip/b3d73156b075014ce5b2609f4f47723d6c0c23d6
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Fri, 06 Mar 2020 13:58:27 -08:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 27 Apr 2020 11:03:51 -07:00

rcu: Reinstate synchronize_rcu_mult()

With the advent and likely usage of synchronize_rcu_rude(), there is
again a need to wait on multiple types of RCU grace periods, for
example, call_rcu_tasks() and call_rcu_tasks_rude().  This commit
therefore reinstates synchronize_rcu_mult() in order to allow these
grace periods to be straightforwardly waited on concurrently.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 include/linux/rcupdate_wait.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/include/linux/rcupdate_wait.h b/include/linux/rcupdate_wait.h
index c0578ba..699b938 100644
--- a/include/linux/rcupdate_wait.h
+++ b/include/linux/rcupdate_wait.h
@@ -31,4 +31,23 @@ do {									\
 
 #define wait_rcu_gp(...) _wait_rcu_gp(false, __VA_ARGS__)
 
+/**
+ * synchronize_rcu_mult - Wait concurrently for multiple grace periods
+ * @...: List of call_rcu() functions for different grace periods to wait on
+ *
+ * This macro waits concurrently for multiple types of RCU grace periods.
+ * For example, synchronize_rcu_mult(call_rcu, call_rcu_tasks) would wait
+ * on concurrent RCU and RCU-tasks grace periods.  Waiting on a given SRCU
+ * domain requires you to write a wrapper function for that SRCU domain's
+ * call_srcu() function, with this wrapper supplying the pointer to the
+ * corresponding srcu_struct.
+ *
+ * The first argument tells Tiny RCU's _wait_rcu_gp() not to
+ * bother waiting for RCU.  The reason for this is because anywhere
+ * synchronize_rcu_mult() can be called is automatically already a full
+ * grace period.
+ */
+#define synchronize_rcu_mult(...) \
+	_wait_rcu_gp(IS_ENABLED(CONFIG_TINY_RCU), __VA_ARGS__)
+
 #endif /* _LINUX_SCHED_RCUPDATE_WAIT_H */