linux-ext4 - Re: [PATCH v2 6/7] quota: simplify drop_dquot

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Message-ID: <939155cb-33bd-debd-02b8-d50c540ccc97@huawei.com>
Date:   Thu, 29 Jun 2023 22:16:08 +0800
From:   Baokun Li <libaokun1@...wei.com>
To:     Jan Kara <jack@...e.cz>
CC:     <linux-fsdevel@...r.kernel.org>, <linux-ext4@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <yi.zhang@...wei.com>,
        <yangerkun@...wei.com>, <chengzhihao1@...wei.com>,
        <yukuai3@...wei.com>, Baokun Li <libaokun1@...wei.com>
Subject: Re: [PATCH v2 6/7] quota: simplify drop_dquot_ref()

On 2023/6/29 22:09, Jan Kara wrote:
> On Thu 29-06-23 20:13:05, Baokun Li wrote:
>> On 2023/6/29 19:08, Jan Kara wrote:
>>> On Wed 28-06-23 21:21:54, Baokun Li wrote:
>>>> Now when dqput() drops the last reference count, it will call
>>>> synchronize_srcu(&dquot_srcu) in quota_release_workfn() to ensure that
>>>> no other user will use the dquot after the last reference count is dropped,
>>>> so we don't need to call synchronize_srcu(&dquot_srcu) in drop_dquot_ref()
>>>> and remove the corresponding logic directly to simplify the code.
>>> Nice simplification!  It is also important that dqput() now cannot sleep
>>> which was another reason for the logic with tofree_head in
>>> remove_inode_dquot_ref().
>> I don't understand this sentence very well, so I would appreciate it
>>
>> if you could explain it in detail. 🤔
> OK, let me phrase it in a "changelog" way :):
>
> remove_inode_dquot_ref() currently does not release the last dquot
> reference but instead adds the dquot to tofree_head list. This is because
> dqput() can sleep while dropping of the last dquot reference (writing back
> the dquot and calling ->release_dquot()) and that must not happen under
> dq_list_lock. Now that dqput() queues the final dquot cleanup into a
> workqueue, remove_inode_dquot_ref() can call dqput() unconditionally
> and we can significantly simplify it.
>
> 								Honza
I suddenly understand what you mean, you mean that now dqput() doesn't have
any possible sleep operation. So it can be called in spin_lock at will.

I was confused because I understood that dqput() cannot be called in the 
context
of possible sleep.

Thank you very much for the detailed explanation!
Now there is only the problem in patch 5.

Thanks!
-- 
With Best Regards,
Baokun Li
.