Re: [PATCH] blk-cgroup: Remove unnecessary rcu_read_lock/unlock()

From: Jens Axboe <axboe@kernel.dk>
To: Marek Szyprowski <m.szyprowski@samsung.com>,
	bh1scw@gmail.com, tj@kernel.org
Cc: cgroups@vger.kernel.org, linux-block@vger.kernel.org,
	linux-kernel@vger.kernel.org, songmuchun@bytedance.com
Subject: Re: [PATCH] blk-cgroup: Remove unnecessary rcu_read_lock/unlock()
Date: Wed, 18 May 2022 16:31:53 -0600	[thread overview]
Message-ID: <de6b0f81-67e1-b167-a618-193c2303f149@kernel.dk> (raw)
In-Reply-To: <1dad86bb-ae31-5bf8-5810-9e81c68be8ff@kernel.dk>

On 5/18/22 4:29 PM, Jens Axboe wrote:
> On 5/18/22 1:28 PM, Marek Szyprowski wrote:
>> On 16.05.2022 19:39, bh1scw@gmail.com wrote:
>>> From: Fanjun Kong <bh1scw@gmail.com>
>>>
>>> spin_lock_irq/spin_unlock_irq contains preempt_disable/enable().
>>> Which can serve as RCU read-side critical region, so remove
>>> rcu_read_lock/unlock().
>>>
>>> Signed-off-by: Fanjun Kong <bh1scw@gmail.com>
>>
>> This patch landed in today's linux next-20220518 as commit 77c570a1ea85 
>> ("blk-cgroup: Remove unnecessary rcu_read_lock/unlock()").
>>
>> Unfortunately it triggers the following warning on ARM64 based Raspberry 
>> Pi 4B board:>
>> ------------[ cut here ]------------
>> WARNING: CPU: 0 PID: 1 at block/blk-cgroup.c:301 blkg_create+0x398/0x4e0
> 
> Should this use rcu_read_lock_any_held() rather than
> rcu_read_lock_held()?

I think the better alternative is just to delete the WARN_ON(), we have
a:

lockdep_assert_held(&q->queue_lock);

right after it. Since the queue_lock is IRQ disabling, having two checks
serves no purpose. I'll kill the line.

-- 
Jens Axboe