[tip: core/rcu] rcu: Do full report for .need_qs for strict GPs

["tip-bot2 for Paul E. McKenney" <tip-bot2@linutronix.de>]

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

Commit-ID:     44bad5b3cca2d452d17ef82841b20b42a2cf11a0
Gitweb:        https://git.kernel.org/tip/44bad5b3cca2d452d17ef82841b20b42a2cf11a0
Author:        Paul E. McKenney <paulmck@kernel.org>
AuthorDate:    Thu, 06 Aug 2020 15:12:50 -07:00
Committer:     Paul E. McKenney <paulmck@kernel.org>
CommitterDate: Mon, 24 Aug 2020 18:40:25 -07:00

rcu: Do full report for .need_qs for strict GPs

The rcu_preempt_deferred_qs_irqrestore() function is invoked at
the end of an RCU read-side critical section (for example, directly
from rcu_read_unlock()) and, if .need_qs is set, invokes rcu_qs() to
report the new quiescent state.  This works, except that rcu_qs() only
updates per-CPU state, leaving reporting of the actual quiescent state
to a later call to rcu_report_qs_rdp(), for example from within a later
RCU_SOFTIRQ instance.  Although this approach is exactly what you want if
you are more concerned about efficiency than about short grace periods,
in CONFIG_RCU_STRICT_GRACE_PERIOD=y kernels, short grace periods are
the name of the game.

This commit therefore makes rcu_preempt_deferred_qs_irqrestore() directly
invoke rcu_report_qs_rdp() in CONFIG_RCU_STRICT_GRACE_PERIOD=y, thus
shortening grace periods.

Historical note:  To the best of my knowledge, causing rcu_read_unlock()
to directly report a quiescent state first appeared in Jim Houston's
and Joe Korty's JRCU.  This is the second instance of a Linux-kernel RCU
feature being inspired by JRCU, the first being RCU callback offloading
(as in the RCU_NOCB_CPU Kconfig option).

Reported-by Jann Horn <jannh@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree_plugin.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 668bbd2..dfdb902 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -459,8 +459,12 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
 		return;
 	}
 	t->rcu_read_unlock_special.s = 0;
-	if (special.b.need_qs)
-		rcu_qs();
+	if (special.b.need_qs) {
+		if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
+			rcu_report_qs_rdp(rdp->cpu, rdp);
+		else
+			rcu_qs();
+	}
 
 	/*
 	 * Respond to a request by an expedited grace period for a