linux-kernel - Re: [PATCH RFC md-6.16 v3 15/19] md/md-llbitmap: implement APIs to dirty bits and clear bits

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Message-ID: <0de7efeb-6d4a-2fa5-ed14-e2c0bec0257b@huaweicloud.com>
Date: Mon, 12 May 2025 16:23:55 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Christoph Hellwig <hch@....de>, Yu Kuai <yukuai1@...weicloud.com>
Cc: xni@...hat.com, colyli@...nel.org, agk@...hat.com, snitzer@...nel.org,
 mpatocka@...hat.com, song@...nel.org, linux-kernel@...r.kernel.org,
 dm-devel@...ts.linux.dev, linux-raid@...r.kernel.org, yi.zhang@...wei.com,
 yangerkun@...wei.com, johnny.chenyi@...wei.com,
 "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH RFC md-6.16 v3 15/19] md/md-llbitmap: implement APIs to
 dirty bits and clear bits

Hi,

在 2025/05/12 13:17, Christoph Hellwig 写道:
> On Mon, May 12, 2025 at 09:19:23AM +0800, Yu Kuai wrote:
>> +static void llbitmap_unplug(struct mddev *mddev, bool sync)
>> +{
>> +	DECLARE_COMPLETION_ONSTACK(done);
>> +	struct llbitmap *llbitmap = mddev->bitmap;
>> +	struct llbitmap_unplug_work unplug_work = {
>> +		.llbitmap = llbitmap,
>> +		.done = &done,
>> +	};
>> +
>> +	if (!llbitmap_dirty(llbitmap))
>> +		return;
>> +
>> +	INIT_WORK_ONSTACK(&unplug_work.work, llbitmap_unplug_fn);
>> +	queue_work(md_llbitmap_unplug_wq, &unplug_work.work);
>> +	wait_for_completion(&done);
>> +	destroy_work_on_stack(&unplug_work.work);
> 
> Why is this deferring the work to a workqueue, but then synchronously
> waits on it?

This is the same as old bitmap, by the fact that issue new IO and wait
for such IO to be done from submit_bio() context will deadlock.

1) bitmap bio must be done before this bio can be issued;
2) bitmap bio will be added to current->bio_list, and wait for this bio
to be issued;

Do you have a better sulution to this problem?

Thanks,
Kuai

> 
> .
>