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

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Message-ID: <157363958888.29376.9190587096871610849.tip-bot2@tip-bot2>
Date:   Wed, 13 Nov 2019 10:06:28 -0000
From:   "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Qian Cai <cai@....pw>, Thomas Gleixner <tglx@...utronix.de>,
        akpm@...ux-foundation.org, bigeasy@...utronix.de, cl@...ux.com,
        keescook@...omium.org, penberg@...nel.org, rientjes@...gle.com,
        thgarnie@...gle.com, tytso@....edu, will@...nel.org,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        linux-kernel@...r.kernel.org
Subject: [tip: sched/urgent] sched/core: Avoid spurious lock dependencies

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@...radead.org>
AuthorDate:    Tue, 01 Oct 2019 11:18:37 +02:00
Committer:     Ingo Molnar <mingo@...nel.org>
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@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Qian Cai <cai@....pw>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: akpm@...ux-foundation.org
Cc: bigeasy@...utronix.de
Cc: cl@...ux.com
Cc: keescook@...omium.org
Cc: penberg@...nel.org
Cc: rientjes@...gle.com
Cc: thgarnie@...gle.com
Cc: tytso@....edu
Cc: will@...nel.org
Fixes: b7d5dc21072c ("random: add a spinlock_t to struct batched_entropy")
Link: https://lkml.kernel.org/r/20191001091837.GK4536@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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;