linux-kernel - [PATCH] entry/rcu: Check TIF_RESCHED _after

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Message-Id: <20221223211507.84249-1-frederic@kernel.org>
Date:   Fri, 23 Dec 2022 22:15:07 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E . McKenney" <paulmck@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>, rcu@...r.kernel.org
Subject: [PATCH] entry/rcu: Check TIF_RESCHED _after_ delayed RCU wake-up

RCU sometimes needs to perform a delayed wake up for specific kthreads
handling callbacks offloading (RCU_NOCB). This is handled through timers
and upon entry to idle (also guest and user on nohz_full).

However the delayed wake-up on kernel exit is actually performed after
the thread flags are fetched towards the fast path check for work to
do on exit to user. As a result, and if there is no other pending work
to do upon that kernel exit, the current task will resume to userspace
with TIF_RESCHED set and the pending wake up ignored.

Fix this with fetching the thread flags _after_ the delayed RCU-nocb
kthread wake-up.

Fixes: 47b8ff194c1f ("entry: Explicitly flush pending rcuog wakeup before last rescheduling point")
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 kernel/entry/common.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 846add8394c4..a134e26b58c6 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -192,13 +192,14 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,

 static void exit_to_user_mode_prepare(struct pt_regs *regs)
 {
-	unsigned long ti_work = read_thread_flags();
+	unsigned long ti_work;

 	lockdep_assert_irqs_disabled();

 	/* Flush pending rcuog wakeup before the last need_resched() check */
 	tick_nohz_user_enter_prepare();

+	ti_work = read_thread_flags();
 	if (unlikely(ti_work & EXIT_TO_USER_MODE_WORK))
 		ti_work = exit_to_user_mode_loop(regs, ti_work);

-- 
2.25.1