linux-kernel - [PATCH 3/3] rcu: Fix preemption mode check on synchronize_rcu[

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Date:   Mon, 14 Mar 2022 14:37:38 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Marco Elver <elver@...gle.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Boqun Feng <boqun.feng@...il.com>,
        Uladzislau Rezki <uladzislau.rezki@...y.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: [PATCH 3/3] rcu: Fix preemption mode check on synchronize_rcu[_expedited]()

An early check on synchronize_rcu[_expedited]() tries to determine if
the current CPU is in UP mode on an SMP no-preempt kernel, in which case
there is no need to start a grace period since the current assumed
quiescent state is all we need.

However the preemption mode doesn't take into account the boot selected
preemption mode under CONFIG_PREEMPT_DYNAMIC=y, missing a possible
early return if the running flavour is "none" or "voluntary".

Use the shiny new preempt mode accessors to fix this.

Reported-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Uladzislau Rezki <uladzislau.rezki@...y.com>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
Cc: Valentin Schneider <valentin.schneider@....com>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d2db709f83d9..8fe2f9843139 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3769,7 +3769,7 @@ static int rcu_blocking_is_gp(void)
 {
 	int ret;
 
-	if (IS_ENABLED(CONFIG_PREEMPTION))
+	if (preempt_mode_full() || preempt_mode_rt())
 		return rcu_scheduler_active == RCU_SCHEDULER_INACTIVE;
 	might_sleep();  /* Check for RCU read-side critical section. */
 	preempt_disable();
-- 
2.25.1