linux-kernel - Re: [PATCH] io-wq: Remove unnecessary rcu_read_lock/unlock() in raw spinlock critical section

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Message-ID: <c4bcf2fa-b72a-5e3a-efe9-544457a9816a@gmail.com>
Date:   Wed, 27 Oct 2021 15:06:00 +0800
From:   Zqiang <qiang.zhang1211@...il.com>
To:     Jens Axboe <axboe@...nel.dk>,
        Muchun Song <songmuchun@...edance.com>
Cc:     asml.silence@...il.com, io-uring@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] io-wq: Remove unnecessary rcu_read_lock/unlock() in raw
 spinlock critical section


On 2021/10/26 下午10:47, Jens Axboe wrote:
> On 10/26/21 4:32 AM, Muchun Song wrote:
>> On Tue, Oct 26, 2021 at 11:23 AM Zqiang <qiang.zhang1211@...il.com> wrote:
>>> Due to raw_spin_lock/unlock() contains preempt_disable/enable() action,
>>> already regarded as RCU critical region, so remove unnecessary
>>> rcu_read_lock/unlock().
>>>
>>> Signed-off-by: Zqiang <qiang.zhang1211@...il.com>
>>> ---
>>>   fs/io-wq.c | 2 --
>>>   1 file changed, 2 deletions(-)
>>>
>>> diff --git a/fs/io-wq.c b/fs/io-wq.c
>>> index cd88602e2e81..401be005d089 100644
>>> --- a/fs/io-wq.c
>>> +++ b/fs/io-wq.c
>>> @@ -855,9 +855,7 @@ static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work)
>>>          io_wqe_insert_work(wqe, work);
>>>          clear_bit(IO_ACCT_STALLED_BIT, &acct->flags);
>>>
>>> -       rcu_read_lock();
>> Add a comment like:
>> /* spin_lock can serve as an RCU read-side critical section. */
> Note that it's a raw spinlock. Honestly I'd probably prefer if we just leave
> it as-is. There are plans to improve the io-wq locking, and a rcu lock/unlock
> is pretty cheap.
>
> That said, if resend with a comment fully detailing why it's OK currently,
> then I'd be fine with that as well.
>
Thanks Jens Axboe, Muchun

  I  will  add a comment fully detailing and resend.