linux-kernel - Re: [PATCH] sched: Avoid spurious lock dependencies

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Message-ID: <8160833a-d01b-a942-5087-65831c9f96e9@arm.com>
Date:   Tue, 1 Oct 2019 11:01:38 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peter Zijlstra <peterz@...radead.org>, Qian Cai <cai@....pw>
Cc:     akpm@...ux-foundation.org, bigeasy@...utronix.de,
        tglx@...utronix.de, thgarnie@...gle.com, tytso@....edu,
        cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        mingo@...hat.com, will@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, keescook@...omium.org
Subject: Re: [PATCH] sched: Avoid spurious lock dependencies

On 01/10/2019 10:18, Peter Zijlstra wrote:
> 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@...radead.org>

Funky dependency, but the change looks fine to me.
Reviewed-by: Valentin Schneider <valentin.schneider@....com>

> ---
>  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;
> 
>