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

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Date:   Tue, 1 Oct 2019 17:06:56 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Qian Cai <cai@....pw>, 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

> Subject: sched: Avoid spurious lock dependencies
> 
> While seemingly harmless, __sched_fork() does hrtimer_init(), which,
> when DEBUG_OBJETS, can end up doing allocations.
> 

NIT: s/DEBUG_OBJETS/DEBUG_OBJECTS

> 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;
> 

Given that there is a comment just after this which says
"init_task() gets called multiple times on a task",
should we add a check if rq->idle is present and bail out?

if (rq->idle) {
    raw_spin_unlock(&rq->lock);
    raw_spin_unlock_irqrestore(&idle->pi_lock, flags);
    return;
}

Also can we also move the above 3 statements before the lock?


-- 
Thanks and Regards
Srikar Dronamraju