[PATCH tip/core/rcu 07/12] rcu: Attempt QS when CPU discovers GP for strict GPs

From: paulmck
Date: Wed Aug 12 2020 - 18:57:57 EST

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

A given CPU normally notes a new grace period during one RCU_SOFTIRQ,
but avoids reporting the corresponding quiescent state until some later
RCU_SOFTIRQ. This leisurly approach improves efficiency by increasing
the number of update requests served by each grace period, but is not
what is needed for kernels built with CONFIG_RCU_STRICT_GRACE_PERIOD=y.

This commit therefore adds a new rcu_strict_gp_check_qs() function
which, in CONFIG_RCU_STRICT_GRACE_PERIOD=y kernels, simply enters and
immediately exist an RCU read-side critical section. If the CPU is
in a quiescent state, the rcu_read_unlock() will attempt to report an
immediate quiescent state. This rcu_strict_gp_check_qs() function is
invoked from note_gp_changes(), so that a CPU just noticing a new grace
period might immediately report a quiescent state for that grace period.

Reported-by Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
kernel/rcu/tree.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 08cc91c..4353a1a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1557,6 +1557,19 @@ static void __maybe_unused rcu_advance_cbs_nowake(struct rcu_node *rnp,

+ * In CONFIG_RCU_STRICT_GRACE_PERIOD=y kernels, attempt to generate a
+ * quiescent state. This is intended to be invoked when the CPU notices
+ * a new grace period.
+ */
+static void rcu_strict_gp_check_qs(void)
+ rcu_read_lock();
+ rcu_read_unlock();
+ }
* Update CPU-local rcu_data state to record the beginnings and ends of
* grace periods. The caller must hold the ->lock of the leaf rcu_node
* structure corresponding to the current CPU, and must have irqs disabled.
@@ -1626,6 +1639,7 @@ static void note_gp_changes(struct rcu_data *rdp)
needwake = __note_gp_changes(rnp, rdp);
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+ rcu_strict_gp_check_qs();
if (needwake)