[tip: core/rcu] rcu: IPI all CPUs at GP start for strict GPs

[tip-bot2 for Paul E. McKenney]

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

Commit-ID:     933ada2c3310aa88807e65c8d498b74a2159a9a2
Gitweb:        https://git.kernel.org/tip/933ada2c3310aa88807e65c8d498b74a2159a9a2
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Thu, 06 Aug 2020 19:21:48 -07:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 24 Aug 2020 18:40:26 -07:00

rcu: IPI all CPUs at GP start for strict GPs

Currently, each CPU discovers the beginning of a given grace period
on its own time, which is again good for efficiency but bad for fast
grace periods.  This commit therefore uses on_each_cpu() to IPI each
CPU after grace-period initialization in order to inform each CPU of
the new grace period in a timely manner, but only in kernels build with
CONFIG_RCU_STRICT_GRACE_PERIOD=y.

Reported-by Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/tree.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 36a860c..88f4fa6 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1696,6 +1696,15 @@ static void rcu_gp_torture_wait(void)
 }
 
 /*
+ * Handler for on_each_cpu() to invoke the target CPU's RCU core
+ * processing.
+ */
+static void rcu_strict_gp_boundary(void *unused)
+{
+	invoke_rcu_core();
+}
+
+/*
  * Initialize a new grace period.  Return false if no grace period required.
  */
 static bool rcu_gp_init(void)
@@ -1823,6 +1832,10 @@ static bool rcu_gp_init(void)
 		WRITE_ONCE(rcu_state.gp_activity, jiffies);
 	}
 
+	// If strict, make all CPUs aware of new grace period.
+	if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
+		on_each_cpu(rcu_strict_gp_boundary, NULL, 0);
+
 	return true;
 }