linux-kernel - Re: [PATCH 16/23] md/md-llbitmap: implement bit state machine

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Message-ID: <CALTww2-RT64+twHo3=Djpuj81jArmePQShGynDrRtYab3c1i2w@mail.gmail.com>
Date: Mon, 30 Jun 2025 16:25:07 +0800
From: Xiao Ni <xni@...hat.com>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: hch@....de, colyli@...nel.org, song@...nel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org, yi.zhang@...wei.com, 
	yangerkun@...wei.com, johnny.chenyi@...wei.com, 
	"yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH 16/23] md/md-llbitmap: implement bit state machine

On Mon, Jun 30, 2025 at 10:25 AM Yu Kuai <yukuai1@...weicloud.com> wrote:
>
> Hi,
>
> 在 2025/06/30 10:14, Xiao Ni 写道:
> > For reload action, it runs continue here.
>
> No one can concurent with reload.
>
> >
> > And doesn't it need a lock when reading the state?
>
> Notice that from IO path, all concurrent context are doing the same
> thing, it doesn't matter if old state or new state are read. If old
> state is read, it will write new state in memory again; if new state is
> read, it just do nothing.

Hi Kuai

This is the last place that I don't understand well. Is it the reason
that it only changes one byte at a time and the system can guarantee
the atomic when updating one byte?

If so, it only needs to concern the old and new data you mentioned
above. For example:
raid1 is created without --assume-clean, so all bits are BitUnwritten.
And a write bio comes, the bit changes to dirty. Then a discard is
submitted in another cpu context and it reads the old status
unwritten. From the status change table, the discard doesn't do
anything. In fact, discard should update dirty to unwritten. Can such
a case happen?

Regards
Xiao
>
> Thanks,
> Kuai
>