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From c0e34a5ef4e854220a216e934ab90f04d4432bdd Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri, 23 Oct 2020 12:12:12 +0200
Subject: [PATCH 015/323] sched, lockdep: Annotate ->pi_lock recursion
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.10/older/patches-5.10.204-rt100.tar.xz
There's a valid ->pi_lock recursion issue where the actual PI code
tries to wake up the stop task. Make lockdep aware so it doesn't
complain about this.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
kernel/sched/core.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2517a003295b..abbf01f77a76 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2670,6 +2670,7 @@ int select_task_rq(struct task_struct *p, int cpu, int sd_flags, int wake_flags)
void sched_set_stop_task(int cpu, struct task_struct *stop)
{
+ static struct lock_class_key stop_pi_lock;
struct sched_param param = { .sched_priority = MAX_RT_PRIO - 1 };
struct task_struct *old_stop = cpu_rq(cpu)->stop;
@@ -2685,6 +2686,20 @@ void sched_set_stop_task(int cpu, struct task_struct *stop)
sched_setscheduler_nocheck(stop, SCHED_FIFO, ¶m);
stop->sched_class = &stop_sched_class;
+
+ /*
+ * The PI code calls rt_mutex_setprio() with ->pi_lock held to
+ * adjust the effective priority of a task. As a result,
+ * rt_mutex_setprio() can trigger (RT) balancing operations,
+ * which can then trigger wakeups of the stop thread to push
+ * around the current task.
+ *
+ * The stop task itself will never be part of the PI-chain, it
+ * never blocks, therefore that ->pi_lock recursion is safe.
+ * Tell lockdep about this by placing the stop->pi_lock in its
+ * own class.
+ */
+ lockdep_set_class(&stop->pi_lock, &stop_pi_lock);
}
cpu_rq(cpu)->stop = stop;
--
2.43.0
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