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From 733206b1d60fe02ac81756b268804c19445362d2 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Tue, 9 Mar 2021 09:42:10 +0100
Subject: [PATCH 047/191] 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 ee3ce4f852b7..d6876e1aa6c1 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -654,7 +654,7 @@ 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);
@@ -662,16 +662,8 @@ 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 1d910294faf7..47321dde8f32 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -616,6 +616,32 @@ 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())
@@ -629,7 +655,7 @@ 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();
--
2.19.1
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