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

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Date:   Thu, 15 Jun 2023 17:05:22 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Xiao Ni <xni@...hat.com>, 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

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,
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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