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

[Yu Kuai]

Hi,

在 2025/06/30 16:25, Xiao Ni 写道:
> On Mon, Jun 30, 2025 at 10:25 AM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> 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?

This can happen for raw disk, however, if there are filesystem, discard
and write can never race. And for raw disk, if user really issue write
and discard concurrently, the result really is uncertain, and it's fine
the bit result in dirty or unwritten.

Thanks,
Kuai

> Regards
> Xiao
> > Thanks,
> > Kuai
>
>
> .