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From: Peter Zijlstra <peterz@infradead.org>
To: Qian Cai <cai@lca.pw>
Cc: akpm@linux-foundation.org, bigeasy@linutronix.de,
	tglx@linutronix.de, thgarnie@google.com, tytso@mit.edu,
	cl@linux.com, penberg@kernel.org, rientjes@google.com,
	mingo@redhat.com, will@kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, keescook@chromium.org
Subject: [PATCH] sched: Avoid spurious lock dependencies
Date: Tue, 1 Oct 2019 11:18:37 +0200
Message-ID: <20191001091837.GK4536@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <1569500974.5576.234.camel@lca.pw>

On Thu, Sep 26, 2019 at 08:29:34AM -0400, Qian Cai wrote:

> Oh, you were talking about took #3 while holding #2. Anyway, your patch is
> working fine so far. Care to post/merge it officially or do you want me to post
> it?

Does the below adequately describe the situation?

---
Subject: sched: 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.

Fixes: b7d5dc21072c ("random: add a spinlock_t to struct batched_entropy")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.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 7880f4f64d0e..1832fc0fbec5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6039,10 +6039,11 @@ 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;




  reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-13 16:27 [PATCH] mm/slub: fix a deadlock in shuffle_freelist() Qian Cai
2019-09-16  9:03 ` Sebastian Andrzej Siewior
2019-09-16 14:01   ` Qian Cai
2019-09-16 19:51     ` Sebastian Andrzej Siewior
2019-09-16 21:31       ` Qian Cai
2019-09-17  7:16         ` Sebastian Andrzej Siewior
2019-09-18 19:59           ` Qian Cai
2019-09-25  9:31 ` Peter Zijlstra
2019-09-25 15:18   ` Qian Cai
2019-09-25 16:45     ` Peter Zijlstra
2019-09-26 12:29       ` Qian Cai
2019-10-01  9:18         ` Peter Zijlstra [this message]
2019-10-01 10:01           ` [PATCH] sched: Avoid spurious lock dependencies Valentin Schneider
2019-10-01 11:22           ` Qian Cai
2019-10-01 11:36           ` Srikar Dronamraju
2019-10-01 13:44             ` Peter Zijlstra
2019-10-29 11:10           ` Qian Cai
2019-10-29 12:44             ` Peter Zijlstra
2019-11-12  0:54               ` Qian Cai

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