Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <e8a09f5d-afce-608f-220b-6b32b3ae37b9@kernel.dk>
Date:   Thu, 8 Dec 2022 16:00:10 -0700
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:102.0) Gecko/20100101
 Thunderbird/102.5.1
Subject: Re: [PATCH-block 3/3] blk-cgroup: Flush stats at blkgs destruction
 path
Content-Language: en-US
To:     Waiman Long <longman@redhat.com>, Tejun Heo <tj@kernel.org>,
        Josef Bacik <josef@toxicpanda.com>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>
References: <20221208220141.2625775-1-longman@redhat.com>
 <20221208220141.2625775-4-longman@redhat.com>
From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <20221208220141.2625775-4-longman@redhat.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 12/8/22 3:01?PM, Waiman Long wrote:
> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
> index 793ecff29038..910e633869b0 100644
> --- a/kernel/cgroup/rstat.c
> +++ b/kernel/cgroup/rstat.c
> @@ -281,6 +281,26 @@ void cgroup_rstat_flush_release(void)
>  	spin_unlock_irq(&cgroup_rstat_lock);
>  }
>  
> +/**
> + * cgroup_rstat_css_cpu_flush - flush stats for the given css and cpu
> + * @css: target css to be flush
> + * @cpu: the cpu that holds the stats to be flush
> + *
> + * A lightweight rstat flush operation for a given css and cpu.
> + * Only the cpu_lock is being held for mutual exclusion, the cgroup_rstat_lock
> + * isn't used.
> + */
> +void cgroup_rstat_css_cpu_flush(struct cgroup_subsys_state *css, int cpu)
> +{
> +	raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu);
> +
> +	raw_spin_lock_irq(cpu_lock);
> +	rcu_read_lock();
> +	css->ss->css_rstat_flush(css, cpu);
> +	rcu_read_unlock();
> +	raw_spin_unlock_irq(cpu_lock);
> +}
> +
>  int cgroup_rstat_init(struct cgroup *cgrp)
>  {
>  	int cpu;

As I mentioned last time, raw_spin_lock_irq() will be equivalent to an
RCU protected section anyway, so you don't need to do both. Just add a
comment on why rcu_read_lock()/rcu_read_unlock() isn't needed inside the
raw irq safe lock.

-- 
Jens Axboe