linux-kernel - [PATCH AUTOSEL 4.19 10/25] sched/core: Avoid spurious lock dependencies

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Date:   Fri, 22 Nov 2019 14:48:43 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Peter Zijlstra <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>, Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 4.19 10/25] sched/core: Avoid spurious lock dependencies

From: Peter Zijlstra <peterz@...radead.org>

[ Upstream commit ff51ff84d82aea5a889b85f2b9fb3aa2b8691668 ]

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>
Signed-off-by: Sasha Levin <sashal@...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 78ecdfae25b69..2befd2c4ce9e6 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5413,10 +5413,11 @@ 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;
-- 
2.20.1