[PATCH RT 2/8] sched: Switch wait_task_inactive to HRTIMER_MODE_REL_HARD

[zanussi@kernel.org]

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

v5.4.161-rt67-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------


[ Upstream commit 39609ed79d420e0b966e16a1d695733c2d3b9a7f ]

With PREEMPT_RT enabled all hrtimers callbacks will be invoked in
softirq mode unless they are explicitly marked as HRTIMER_MODE_HARD.
During boot kthread_bind() is used for the creation of per-CPU threads
and then hangs in wait_task_inactive() if the ksoftirqd is not
yet up and running.
The hang disappeared since commit
   26c7295be0c5e ("kthread: Do not preempt current task if it is going to call schedule()")

but enabling function trace on boot reliably leads to the freeze on boot
behaviour again.
The timer in wait_task_inactive() can not be directly used by an user
interface to abuse it and create a mass wake of several tasks at the
same time which would to long sections with disabled interrupts.
Therefore it is safe to make the timer HRTIMER_MODE_REL_HARD.

Switch the timer to HRTIMER_MODE_REL_HARD.

Cc: stable-rt@vger.kernel.org
Link: https://lkml.kernel.org/r/20210826170408.vm7rlj7odslshwch@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
---
 kernel/sched/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9b32fbded588..022c7b78642d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2158,7 +2158,7 @@ unsigned long wait_task_inactive(struct task_struct *p, long match_state)
 			ktime_t to = NSEC_PER_SEC / HZ;
 
 			set_current_state(TASK_UNINTERRUPTIBLE);
-			schedule_hrtimeout(&to, HRTIMER_MODE_REL);
+			schedule_hrtimeout(&to, HRTIMER_MODE_REL_HARD);
 			continue;
 		}
 
-- 
2.17.1