| Date | Fri, 23 Oct 2020 12:12:12 +0200 | From | Peter Zijlstra <> | Subject | [PATCH v4 14/19] sched, lockdep: Annotate ->pi_lock recursion |
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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> --- kernel/sched/core.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+)
--- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2602,6 +2602,7 @@ int select_task_rq(struct task_struct *p 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; @@ -2617,6 +2618,20 @@ void sched_set_stop_task(int cpu, struct 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;
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