entry/rcu: Check TIF_RESCHED _after_ delayed RCU wake-up

RCU sometimes needs to perform a delayed wake up for specific kthreads
handling offloaded callbacks (RCU_NOCB).  These wakeups are performed
by timers and upon entry to idle (also to guest and to 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@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230315194349.10798-3-joel@joelfernandes.org

diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 1314894d2efada8a72c9b817c4aad95dd6c4026b..be61332c66b54a02b37d7a1847001b23a82d910e 100644 (file)
--- 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);