linux-kernel - Re: [MD] Crash with 4.12+ kernel and high disk load -- bisected to 4ad23a976413: MD: use per-cpu counter for writes

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Date:   Tue, 08 Aug 2017 17:01:28 +1000
From:   NeilBrown <neilb@...e.com>
To:     Dominik Brodowski <linux@...inikbrodowski.net>,
        Shaohua Li <shli@...com>
Cc:     David R <david@...olicited.net>, linux-kernel@...r.kernel.org,
        linux-raid@...r.kernel.org
Subject: Re: [MD] Crash with 4.12+ kernel and high disk load -- bisected to 4ad23a976413: MD: use per-cpu counter for writes_pending

On Mon, Aug 07 2017, Dominik Brodowski wrote:

> Neil, Shaohua,
>
> following up on David R's bug message: I have observed something similar
> on v4.12.[345] and v4.13-rc4, but not on v4.11. This is a RAID1 (on bare
> metal partitions, /dev/sdaX and /dev/sdbY linked together). In case it
> matters: Further upwards are cryptsetup, a DM volume group, then logical
> volumes, and then filesystems (ext4, but also happened with xfs).
>
> In a tedious bisect (the bug wasn't as quickly reproducible as I would like,
> but happened when I repeatedly created large lvs and filled them with some
> content, while compiling kernels in parallel), I was able to track this
> down to:
>
>
> commit 4ad23a976413aa57fe5ba7a25953dc35ccca5b71
> Author: NeilBrown <neilb@...e.com>
> Date:   Wed Mar 15 14:05:14 2017 +1100
>
>     MD: use per-cpu counter for writes_pending
>     
>     The 'writes_pending' counter is used to determine when the
>     array is stable so that it can be marked in the superblock
>     as "Clean".  Consequently it needs to be updated frequently
>     but only checked for zero occasionally.  Recent changes to
>     raid5 cause the count to be updated even more often - once
>     per 4K rather than once per bio.  This provided
>     justification for making the updates more efficient.
>
>     ...

Thanks for the report... and for bisecting and for re-sending...

I believe I have found the problem, and have sent a patch separately.

If mddev->safemode == 1 and mddev->in_sync != 0, md_check_recovery()
causes the thread that calls it to spin.
Prior to the patch you found, that couldn't happen.  Now it can,
so it needs to be handled more carefully.

While I was examining the code, I found another bug - so that is a win!

Thanks,
NeilBrown

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