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 4436857..36a860c 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1575,6 +1575,19 @@ 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) +{ + if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) { + 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. @@ -1644,6 +1657,7 @@ 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) rcu_gp_kthread_wake(); } -- 2.9.5