From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Sat, 22 Jun 2019 00:09:22 +0200
Subject: [PATCH] softirq: Avoid a cancel dead-lock in tasklet handling due to
 preemptible-softirq
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
Git-commit: da77ceac3d20f27310a07a7c346a4ee6b40d6c28
Patch-mainline: Queued in subsystem maintainer repository
References: SLE Realtime Extension

A pending / active tasklet which is preempted by a task on the same CPU
will spin indefinitely because the tasklet makes no progress.
To avoid this deadlock we can disable BH which will acquire the
softirq-lock which will force the completion of the softirq and so the
tasklet.
The BH off/on in tasklet_kill() will force tasklets which are not yet
running but scheduled (because ksoftirqd was preempted before it could
start the tasklet).
The BH off/on in  tasklet_unlock_wait() will force tasklets which got
preempted while running.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Daniel Wagner <dwagner@suse.de>
---
 include/linux/interrupt.h |    5 ++++-
 kernel/softirq.c          |    3 ++-
 2 files changed, 6 insertions(+), 2 deletions(-)

--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -626,7 +626,10 @@ static inline void tasklet_unlock(struct
 
 static inline void tasklet_unlock_wait(struct tasklet_struct *t)
 {
-	while (test_bit(TASKLET_STATE_RUN, &(t)->state)) { barrier(); }
+	while (test_bit(TASKLET_STATE_RUN, &(t)->state)) {
+		local_bh_disable();
+		local_bh_enable();
+	}
 }
 #else
 #define tasklet_trylock(t) 1
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -711,7 +711,8 @@ void tasklet_kill(struct tasklet_struct
 
 	while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
 		do {
-			yield();
+			local_bh_disable();
+			local_bh_enable();
 		} while (test_bit(TASKLET_STATE_SCHED, &t->state));
 	}
 	tasklet_unlock_wait(t);