From a8868d2a50f8ba67c0fc4816234b3c99626d7156 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Tue, 9 Mar 2021 09:42:10 +0100
Subject: [PATCH 138/327] tasklets: Prevent tasklet_unlock_spin_wait() deadlock
 on RT

tasklet_unlock_spin_wait() spin waits for the TASKLET_STATE_SCHED bit in
the tasklet state to be cleared. This works on !RT nicely because the
corresponding execution can only happen on a different CPU.

On RT softirq processing is preemptible, therefore a task preempting the
softirq processing thread can spin forever.

Prevent this by invoking local_bh_disable()/enable() inside the loop. In
case that the softirq processing thread was preempted by the current task,
current will block on the local lock which yields the CPU to the preempted
softirq processing thread. If the tasklet is processed on a different CPU
then the local_bh_disable()/enable() pair is just a waste of processor
cycles.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/interrupt.h | 12 ++----------
 kernel/softirq.c          | 28 +++++++++++++++++++++++++++-
 2 files changed, 29 insertions(+), 11 deletions(-)

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index c4fafbfa28a6..6eae70ec3a17 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -658,7 +658,7 @@ enum
 	TASKLET_STATE_RUN	/* Tasklet is running (SMP only) */
 };
 
-#ifdef CONFIG_SMP
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT)
 static inline int tasklet_trylock(struct tasklet_struct *t)
 {
 	return !test_and_set_bit(TASKLET_STATE_RUN, &(t)->state);
@@ -666,16 +666,8 @@ static inline int tasklet_trylock(struct tasklet_struct *t)
 
 void tasklet_unlock(struct tasklet_struct *t);
 void tasklet_unlock_wait(struct tasklet_struct *t);
+void tasklet_unlock_spin_wait(struct tasklet_struct *t);
 
-/*
- * Do not use in new code. Waiting for tasklets from atomic contexts is
- * error prone and should be avoided.
- */
-static inline void tasklet_unlock_spin_wait(struct tasklet_struct *t)
-{
-	while (test_bit(TASKLET_STATE_RUN, &t->state))
-		cpu_relax();
-}
 #else
 static inline int tasklet_trylock(struct tasklet_struct *t) { return 1; }
 static inline void tasklet_unlock(struct tasklet_struct *t) { }
diff --git a/kernel/softirq.c b/kernel/softirq.c
index ecc3ac4091c8..fcb201ceed71 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -614,6 +614,32 @@ void tasklet_init(struct tasklet_struct *t,
 }
 EXPORT_SYMBOL(tasklet_init);
 
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT)
+/*
+ * Do not use in new code. Waiting for tasklets from atomic contexts is
+ * error prone and should be avoided.
+ */
+void tasklet_unlock_spin_wait(struct tasklet_struct *t)
+{
+	while (test_bit(TASKLET_STATE_RUN, &(t)->state)) {
+		if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
+			/*
+			 * Prevent a live lock when current preempted soft
+			 * interrupt processing or prevents ksoftirqd from
+			 * running. If the tasklet runs on a different CPU
+			 * then this has no effect other than doing the BH
+			 * disable/enable dance for nothing.
+			 */
+			local_bh_disable();
+			local_bh_enable();
+		} else {
+			cpu_relax();
+		}
+	}
+}
+EXPORT_SYMBOL(tasklet_unlock_spin_wait);
+#endif
+
 void tasklet_kill(struct tasklet_struct *t)
 {
 	if (in_interrupt())
@@ -627,7 +653,7 @@ void tasklet_kill(struct tasklet_struct *t)
 }
 EXPORT_SYMBOL(tasklet_kill);
 
-#ifdef CONFIG_SMP
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT)
 void tasklet_unlock(struct tasklet_struct *t)
 {
 	smp_mb__before_atomic();
-- 
2.49.0

