This is a note to let you know that I've just added the patch titled rcu/nocb: Perform deferred wake up before last idle's need_resched() check to the 5.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: rcu-nocb-perform-deferred-wake-up-before-last-idle-s-need_resched-check.patch and it can be found in the queue-5.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 43789ef3f7d61aa7bed0cb2764e588fc990c30ef Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker <frederic@xxxxxxxxxx> Date: Mon, 1 Feb 2021 00:05:45 +0100 Subject: rcu/nocb: Perform deferred wake up before last idle's need_resched() check From: Frederic Weisbecker <frederic@xxxxxxxxxx> commit 43789ef3f7d61aa7bed0cb2764e588fc990c30ef upstream. Entering RCU idle mode may cause a deferred wake up of an RCU NOCB_GP kthread (rcuog) to be serviced. Usually a local wake up happening while running the idle task is handled in one of the need_resched() checks carefully placed within the idle loop that can break to the scheduler. Unfortunately the call to rcu_idle_enter() is already beyond the last generic need_resched() check and we may halt the CPU with a resched request unhandled, leaving the task hanging. Fix this with splitting the rcuog wakeup handling from rcu_idle_enter() and place it before the last generic need_resched() check in the idle loop. It is then assumed that no call to call_rcu() will be performed after that in the idle loop until the CPU is put in low power mode. Fixes: 96d3fd0d315a (rcu: Break call_rcu() deadlock involving scheduler and perf) Reported-by: Paul E. McKenney <paulmck@xxxxxxxxxx> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx Link: https://lkml.kernel.org/r/20210131230548.32970-3-frederic@xxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- include/linux/rcupdate.h | 2 ++ kernel/rcu/tree.c | 3 --- kernel/rcu/tree_plugin.h | 5 +++++ kernel/sched/idle.c | 1 + 4 files changed, 8 insertions(+), 3 deletions(-) --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -96,8 +96,10 @@ static inline void rcu_user_exit(void) { #ifdef CONFIG_RCU_NOCB_CPU void rcu_init_nohz(void); +void rcu_nocb_flush_deferred_wakeup(void); #else /* #ifdef CONFIG_RCU_NOCB_CPU */ static inline void rcu_init_nohz(void) { } +static inline void rcu_nocb_flush_deferred_wakeup(void) { } #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */ /** --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -599,10 +599,7 @@ static void rcu_eqs_enter(bool user) */ void rcu_idle_enter(void) { - struct rcu_data *rdp = this_cpu_ptr(&rcu_data); - lockdep_assert_irqs_disabled(); - do_nocb_deferred_wakeup(rdp); rcu_eqs_enter(false); } --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -2190,6 +2190,11 @@ static void do_nocb_deferred_wakeup(stru do_nocb_deferred_wakeup_common(rdp); } +void rcu_nocb_flush_deferred_wakeup(void) +{ + do_nocb_deferred_wakeup(this_cpu_ptr(&rcu_data)); +} + void __init rcu_init_nohz(void) { int cpu; --- a/kernel/sched/idle.c +++ b/kernel/sched/idle.c @@ -249,6 +249,7 @@ static void do_idle(void) } arch_cpu_idle_enter(); + rcu_nocb_flush_deferred_wakeup(); /* * In poll mode we reenable interrupts and spin. Also if we Patches currently in stable-queue which might be from frederic@xxxxxxxxxx are queue-5.4/rcu-nocb-perform-deferred-wake-up-before-last-idle-s-need_resched-check.patch queue-5.4/rcu-nocb-trigger-self-ipi-on-late-deferred-wake-up-before-user-resume.patch queue-5.4/rcu-pull-deferred-rcuog-wake-up-to-rcu_eqs_enter-callers.patch