linux-kernel - Re: [PATCH] blk-cgroup: Remove unnecessary rcu_read

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Date:   Wed, 18 May 2022 16:31:53 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Marek Szyprowski <m.szyprowski@...sung.com>, bh1scw@...il.com,
        tj@...nel.org
Cc:     cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, songmuchun@...edance.com
Subject: Re: [PATCH] blk-cgroup: Remove unnecessary rcu_read_lock/unlock()

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@...il.com wrote:
>>> From: Fanjun Kong <bh1scw@...il.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@...il.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