[tip: sched/urgent] sched/core: Avoid spurious lock dependencies

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     ff51ff84d82aea5a889b85f2b9fb3aa2b8691668
Gitweb:        https://git.kernel.org/tip/ff51ff84d82aea5a889b85f2b9fb3aa2b8691668
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Tue, 01 Oct 2019 11:18:37 +02:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Wed, 13 Nov 2019 08:01:30 +01:00

sched/core: Avoid spurious lock dependencies

While seemingly harmless, __sched_fork() does hrtimer_init(), which,
when DEBUG_OBJETS, can end up doing allocations.

This then results in the following lock order:

  rq->lock
    zone->lock.rlock
      batched_entropy_u64.lock

Which in turn causes deadlocks when we do wakeups while holding that
batched_entropy lock -- as the random code does.

Solve this by moving __sched_fork() out from under rq->lock. This is
safe because nothing there relies on rq->lock, as also evident from the
other __sched_fork() callsite.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Qian Cai <cai@xxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: akpm@xxxxxxxxxxxxxxxxxxxx
Cc: bigeasy@xxxxxxxxxxxxx
Cc: cl@xxxxxxxxx
Cc: keescook@xxxxxxxxxxxx
Cc: penberg@xxxxxxxxxx
Cc: rientjes@xxxxxxxxxx
Cc: thgarnie@xxxxxxxxxx
Cc: tytso@xxxxxxx
Cc: will@xxxxxxxxxx
Fixes: b7d5dc21072c ("random: add a spinlock_t to struct batched_entropy")
Link: https://lkml.kernel.org/r/20191001091837.GK4536@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/sched/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0f2eb36..33cd250 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6019,10 +6019,11 @@ void init_idle(struct task_struct *idle, int cpu)
 	struct rq *rq = cpu_rq(cpu);
 	unsigned long flags;
 
+	__sched_fork(0, idle);
+
 	raw_spin_lock_irqsave(&idle->pi_lock, flags);
 	raw_spin_lock(&rq->lock);
 
-	__sched_fork(0, idle);
 	idle->state = TASK_RUNNING;
 	idle->se.exec_start = sched_clock();
 	idle->flags |= PF_IDLE;