linux-kernel - Re: [PATCH -next 1/6] Revert "md: unlock mddev before reap sync_thread in action

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Message-ID: <3aa073e9-5145-aae2-2201-5ba48c09c693@huaweicloud.com>
Date:   Thu, 23 Mar 2023 14:32:50 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Guoqing Jiang <guoqing.jiang@...ux.dev>,
        Yu Kuai <yukuai1@...weicloud.com>, logang@...tatee.com,
        pmenzel@...gen.mpg.de, agk@...hat.com, snitzer@...nel.org,
        song@...nel.org
Cc:     linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
        yi.zhang@...wei.com, yangerkun@...wei.com,
        Marc Smith <msmith626@...il.com>,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH -next 1/6] Revert "md: unlock mddev before reap
 sync_thread in action_store"

Hi,

在 2023/03/23 11:50, Guoqing Jiang 写道:

> Combined your debug patch with above steps. Seems you are
> 
> 1. add delay to action_store, so it can't get lock in time.
> 2. echo "want_replacement"**triggers md_check_recovery which can grab lock
>      to start sync thread.
> 3. action_store finally hold lock to clear RECOVERY_RUNNING in reap sync 
> thread.
> 4. Then the new added BUG_ON is invoked since RECOVERY_RUNNING is cleared
>      in step 3.

Yes, this is exactly what I did.

> sync_thread can be interrupted once MD_RECOVERY_INTR is set which means 
> the RUNNING
> can be cleared, so I am not sure the added BUG_ON is reasonable. And 
> change BUG_ON

I think BUG_ON() is reasonable because only md_reap_sync_thread can
clear it, md_do_sync will exit quictly if MD_RECOVERY_INTR is set, but
md_do_sync should not see that MD_RECOVERY_RUNNING is cleared, otherwise
there is no gurantee that only one sync_thread can be in progress.

> like this makes more sense to me.
> 
> +BUG_ON(!test_bit(MD_RECOVERY_RUNNING, &mddev->recovery) &&
> +!test_bit(MD_RECOVERY_INTR, &mddev->recovery));

I think this can be reporduced likewise, md_check_recovery clear
MD_RECOVERY_INTR, and new sync_thread triggered by echo
"want_replacement" won't set this bit.

> 
> I think there might be racy window like you described but it should be 
> really small, I prefer
> to just add a few lines like this instead of revert and introduce new 
> lock to resolve the same
> issue (if it is).

The new lock that I add in this patchset is just try to synchronize idle
and forzen from action_store（patch 3), I can drop it if you think this
is not necessary.

The main changes is patch 4, new lines is not much and I really don't
like to add new flags unless we have to, current code is already hard
to understand...

By the way, I'm concerned that drop the mutex to unregister sync_thread
might not be safe, since the mutex protects lots of stuff, and there
might exist other implicit dependencies.

> 
> TBH, I am reluctant to see the changes in the series, it can only be 
> considered
> acceptable with conditions:
> 
> 1. the previous raid456 bug can be fixed in this way too, hopefully Marc 
> or others
>      can verify it.
> 2. pass all the tests in mdadm

I already test this patchset with mdadm, If there are reporducer for
raid456 bug, I can try to verify it myself.

Thanks,
Kuai
> 
> Thanks,
> Guoqing
> .
>