linux-kernel - Re: [PATCH v2 1/2] md: factor out a new helper to put mddev

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Date:   Tue, 26 Sep 2023 20:54:01 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Mariusz Tkaczyk <mariusz.tkaczyk@...ux.intel.com>,
        Yu Kuai <yukuai1@...weicloud.com>
Cc:     xni@...hat.com, song@...nel.org, linux-raid@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        yangerkun@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v2 1/2] md: factor out a new helper to put mddev

Hi,

在 2023/09/26 20:45, Mariusz Tkaczyk 写道:
> On Tue, 26 Sep 2023 10:58:26 +0800
> Yu Kuai <yukuai1@...weicloud.com> wrote:
> 
>> From: Yu Kuai <yukuai3@...wei.com>
>>
>> There are no functional changes, the new helper will still hold
>> 'all_mddevs_lock' after putting mddev, and it will be used to simplify
>> md_seq_ops.
>>
>> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
>> ---
>>   drivers/md/md.c | 18 +++++++++++++++---
>>   1 file changed, 15 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/md/md.c b/drivers/md/md.c
>> index 10cb4dfbf4ae..a5ef6f7da8ec 100644
>> --- a/drivers/md/md.c
>> +++ b/drivers/md/md.c
>> @@ -616,10 +616,15 @@ static inline struct mddev *mddev_get(struct mddev
>> *mddev)
>>   static void mddev_delayed_delete(struct work_struct *ws);
>>   
>> -void mddev_put(struct mddev *mddev)
>> +static void __mddev_put(struct mddev *mddev, bool locked)
>>   {
>> -	if (!atomic_dec_and_lock(&mddev->active, &all_mddevs_lock))
>> +	if (locked) {
>> +		spin_lock(&all_mddevs_lock);
>> +		if (!atomic_dec_and_test(&mddev->active))
>> +			return;
> 
> It is "locked" and we are taking lock? It seems weird to me. Perhaps "do_lock"
> would be better? Do you meant "lockdep_assert_held(&all_mddevs_lock);"

Yes, do_lock is a better name, true means this function will return with
lock held.
> 
> Something is wrong here, we have two paths and in both cases we are
> taking lock.

No, in the first path, lock is held unconditionaly, that's what we
expected in md_seq_show(); in the next path, lock will only be held if
active is decreased to 0.

Thanks,
Kuai

> 
>> +	} else if (!atomic_dec_and_lock(&mddev->active, &all_mddevs_lock))
>>   		return;
>> +
>>   	if (!mddev->raid_disks && list_empty(&mddev->disks) &&
>>   	    mddev->ctime == 0 && !mddev->hold_active) {
>>   		/* Array is not configured at all, and not held active,
>> @@ -633,7 +638,14 @@ void mddev_put(struct mddev *mddev)
>>   		 */
>>   		queue_work(md_misc_wq, &mddev->del_work);
>>   	}
>> -	spin_unlock(&all_mddevs_lock);
>> +
>> +	if (!locked)
>> +		spin_unlock(&all_mddevs_lock);
> As above, I'm not sure if it is correct.
> 
> Thanks,
> Mariusz
> 
> .
>