linux-kernel - Re: [dm-devel] [PATCH -next v2 4/6] md: refactor idle/frozen_sync_thread() to fix deadlock

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Message-ID: <CALTww2-C6vjiSZqd=gko84mbf3tN3UjX7izPFE2vr3VgNdaQ7g@mail.gmail.com>
Date:   Thu, 15 Jun 2023 17:14:10 +0800
From:   Xiao Ni <xni@...hat.com>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     yi.zhang@...wei.com, yangerkun@...wei.com, snitzer@...nel.org,
        linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
        song@...nel.org, dm-devel@...hat.com, guoqing.jiang@...ux.dev,
        agk@...hat.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [dm-devel] [PATCH -next v2 4/6] md: refactor idle/frozen_sync_thread()
 to fix deadlock

On Thu, Jun 15, 2023 at 5:05 PM Yu Kuai <yukuai1@...weicloud.com> wrote:
>
> Hi,
>
> 在 2023/06/15 16:17, Xiao Ni 写道:
> >> Thanks for the example. I can understand the usage of it. It's the
> >> side effect that removes the mutex protection for idle_sync_thread.
> >>
> >> There is a problem. New sync thread is started in md_check_recovery.
> >> After your patch, md_reap_sync_thread is called in md_check_recovery
> >> too. So it looks like they can't happen at the same time?
>
> Of course they can't. md_check_recovery() can only do one thing at a
> time.
>
> >
> > After thinking a while, there is still a race possibility.
> >
> > md_reap_sync_thread is called in pers deamon (e.g. raid10d ->
> > md_check_recovery) and md_check_recovery returns. Before
> > idle_sync_thread is woken, the new sync thread can be started in
> > md_check_recovery again.
> >
> > But it's really strange, when one people echo idle to sync_action.
> > It's better to add some messages to notify the users that they need to
> > echo idle to sync_action again to have a try. Is there a way that
> > md_reap_sync_thread can wait idle_sync_thread?
>
> I don't think this is a problem, echo idle only make sure to interupt
> current sync_thread, there is no gurantee that sync_thread is not
> running after "echo idle" is done with or without this patchset, before
> this patchset, new sync thread can still start after the mutex is
> released.
>
> User shoud "echo forzen" instead of "echo idle" if they really what to
> avoid new sync_thread to start.

Thanks for all the explanations and patience.

Regards
Xiao
>
> Thanks,
> Kuai
> >
> > Regards
> > Xiao
> >>
> >> Regards
> >> Xiao
> >>
> >>>
> >>> Thanks,
> >>> Kuai
> >>>
> >>> --
> >>> dm-devel mailing list
> >>> dm-devel@...hat.com
> >>> https://listman.redhat.com/mailman/listinfo/dm-devel
> >
> > .
> >
>

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