linux-kernel - Re: [PATCH v2 1/2] block: Avoid sleeping function called from invalid context bug

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Message-ID: <YboeqlSsoFRbjEv2@linutronix.de>
Date:   Wed, 15 Dec 2021 17:58:18 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Wander Lairson Costa <wander@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-rt-users@...r.kernel.org
Subject: Re: [PATCH v2 1/2] block: Avoid sleeping function called from
 invalid context bug

On 2021-12-13 09:37:36 [-0300], Wander Lairson Costa wrote:
> ---
>  block/blk-cgroup.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index 663aabfeba18..0a532bb3003c 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -1911,7 +1911,7 @@ void blk_cgroup_bio_start(struct bio *bio)
>  	struct blkg_iostat_set *bis;
>  	unsigned long flags;
>  
> -	cpu = get_cpu();
> +	cpu = get_cpu_light();
>  	bis = per_cpu_ptr(bio->bi_blkg->iostat_cpu, cpu);
>  	flags = u64_stats_update_begin_irqsave(&bis->sync);
>  
> @@ -1928,7 +1928,7 @@ void blk_cgroup_bio_start(struct bio *bio)
>  	u64_stats_update_end_irqrestore(&bis->sync, flags);
>  	if (cgroup_subsys_on_dfl(io_cgrp_subsys))
>  		cgroup_rstat_updated(bio->bi_blkg->blkcg->css.cgroup, cpu);
> -	put_cpu();
> +	put_cpu_light();
>  }

Are you sure patch and backtrace match? There is also
u64_stats_update_begin_irqsave() which disables preemption on RT. So by
doing what you are suggesting, you only avoid disabling preemption in
cgroup_rstat_updated() which acquires a raw_spinlock_t.

Sebastian