From c0e34a5ef4e854220a216e934ab90f04d4432bdd Mon Sep 17 00:00:00 2001 From: Peter Zijlstra 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) Signed-off-by: Sebastian Andrzej Siewior --- 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