Message-ID: <CALTww28G_CmSxzJZVDqHmPgdmT1e2X8+QcToiUOGV1msAisTcQ@mail.gmail.com>
Date: Wed, 24 Jan 2024 20:19:08 +0800
From: Xiao Ni <xni@...hat.com>
To: Yu Kuai <yukuai3@...wei.com>
Cc: agk@...hat.com, snitzer@...nel.org, mpatocka@...hat.com, 
	dm-devel@...ts.linux.dev, song@...nel.org, jbrassow@....redhat.com, 
	neilb@...e.de, heinzm@...hat.com, shli@...com, akpm@...l.org, 
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org, 
	yukuai1@...weicloud.com, yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH v2 00/11] dm-raid: fix v6.7 regressions

On Wed, Jan 24, 2024 at 5:18 PM Yu Kuai <yukuai3@...wei.com> wrote:
>
> First regression related to stop sync thread:
>
> The lifetime of sync_thread is designed as following:
>
> 1) Decide want to start sync_thread, set MD_RECOVERY_NEEDED, and wake up
> daemon thread;
> 2) Daemon thread detect that MD_RECOVERY_NEEDED is set, then set
> MD_RECOVERY_RUNNING and register sync_thread;
> 3) Execute md_do_sync() for the actual work, if it's done or
> interrupted, it will set MD_RECOVERY_DONE and wake up daemone thread;
> 4) Daemon thread detect that MD_RECOVERY_DONE is set, then clear
> MD_RECOVERY_RUNNING and unregister sync_thread;
>
> In v6.7, we fix md/raid to follow this design by commit f52f5c71f3d4
> ("md: fix stopping sync thread"), however, dm-raid is not considered at
> that time, and following test will hang:

Hi Kuai

Thanks very much for the patch set. I reported the dm raid deadlock
when stopping dm raid and we had the patch set "[PATCH v5 md-fixes
0/3] md: fix stopping sync thread" which has patch f52f5c71f3d4. So we
indeed considered dm-raid that time. Because we want to resolve the
deadlock problem. I re-look patch f52f5c71f3d4. It has two major
changes. One is to use a common function stop_sync_thread for stopping
sync thread. This can fix the deadlock problem. The second change
changes the way to reap sync thread. mdraid and dmraid reap sync
thread in __md_stop_writes. So the patch looks overweight.

Before f52f5c71f3d4  do_md_stop release reconfig_mutex before waiting
sync_thread to finish. So there should not be the deadlock problem
which has been fixed in 130443d60b1b ("md: refactor
idle/frozen_sync_thread() to fix deadlock"). So we only need to change
__md_stop_writes to stop sync thread like do_md_stop and reap sync
thread directly.

Maybe this can avoid deadlock? I'll try this way and give the test result.
>
> shell/integrity-caching.sh
> shell/lvconvert-raid-reshape.sh
>
> This patch set fix the broken test by patch 1-4;
>  - patch 1 fix that step 4) is broken by suspended array;
>  - patch 2 fix that step 4) is broken by read-only array;
>  - patch 3 fix that step 3) is broken that md_do_sync() doesn't set
>  MD_RECOVERY_DONE; Noted that this patch will introdece new problem that
>  data will be corrupted, which will be fixed in later patches.
>  - patch 4 fix that setp 1) is broken that sync_thread is register and
>  MD_RECOVERY_RUNNING is set directly;
>
> With patch 1-4, the above test won't hang anymore, however, the test
> will still fail and complain that ext4 is corrupted;

For patch3, as I mentioned today, the root cause is
dm-raid->rs_start_reshape sets MD_RECOVERY_WAIT. So md_do_sync returns
when MD_RECOVERY_WAIT is set. It's the reason why dm-raid can't stop
sync thread when start a new reshape. . The way in patch3 looks like a
workaround. We need to figure out if dm raid really needs to set
MD_RECOVERY_WAIT. Because now we stop sync thread in an asynchronous
way. So the deadlock problem which was fixed in 644e2537f (dm raid:
fix stripe adding reshape deadlock) may disappear. Maybe we can revert
the patch.

Best Regards
Xiao

>
> Second regression related to frozen sync thread:
>
> Noted that for raid456, if reshape is interrupted, then call
> "pers->start_reshape" will corrupt data. This is because dm-raid rely on
> md_do_sync() doesn't set MD_RECOVERY_DONE so that new sync_thread won't
> be registered, and patch 3 just break this.
>
>  - Patch 5-6 fix this problem by interrupting reshape and frozen
>  sync_thread in dm_suspend(), then unfrozen and continue reshape in
> dm_resume(). It's verified that dm-raid tests won't complain that
> ext4 is corrupted anymore.
>  - Patch 7 fix the problem that raid_message() call
>  md_reap_sync_thread() directly, without holding 'reconfig_mutex'.
>
> Last regression related to dm-raid456 IO concurrent with reshape:
>
> For raid456, if reshape is still in progress, then IO across reshape
> position will wait for reshape to make progress. However, for dm-raid,
> in following cases reshape will never make progress hence IO will hang:
>
> 1) the array is read-only;
> 2) MD_RECOVERY_WAIT is set;
> 3) MD_RECOVERY_FROZEN is set;
>
> After commit c467e97f079f ("md/raid6: use valid sector values to determine
> if an I/O should wait on the reshape") fix the problem that IO across
> reshape position doesn't wait for reshape, the dm-raid test
> shell/lvconvert-raid-reshape.sh start to hang at raid5_make_request().
>
> For md/raid, the problem doesn't exist because:
>
> 1) If array is read-only, it can switch to read-write by ioctl/sysfs;
> 2) md/raid never set MD_RECOVERY_WAIT;
> 3) If MD_RECOVERY_FROZEN is set, mddev_suspend() doesn't hold
>    'reconfig_mutex' anymore, it can be cleared and reshape can continue by
>    sysfs api 'sync_action'.
>
> However, I'm not sure yet how to avoid the problem in dm-raid yet.
>
>  - patch 9-11 fix this problem by detecting the above 3 cases in
>  dm_suspend(), and fail those IO directly.
>
> If user really meet the IO error, then it means they're reading the wrong
> data before c467e97f079f. And it's safe to read/write the array after
> reshape make progress successfully.
>
> Tests:
>
> I already run the following two tests many times and verified that they
> won't fail anymore:
>
> shell/integrity-caching.sh
> shell/lvconvert-raid-reshape.sh
>
> For other tests, I'm still running. However, I'm sending this patchset
> in case people think the fixes is not appropriate. Running the full
> tests will cost lots of time in my VM, and I'll update full test results
> soon.
>
> Yu Kuai (11):
>   md: don't ignore suspended array in md_check_recovery()
>   md: don't ignore read-only array in md_check_recovery()
>   md: make sure md_do_sync() will set MD_RECOVERY_DONE
>   md: don't register sync_thread for reshape directly
>   md: export helpers to stop sync_thread
>   dm-raid: really frozen sync_thread during suspend
>   md/dm-raid: don't call md_reap_sync_thread() directly
>   dm-raid: remove mddev_suspend/resume()
>   dm-raid: add a new helper prepare_suspend() in md_personality
>   md: export helper md_is_rdwr()
>   md/raid456: fix a deadlock for dm-raid456 while io concurrent with
>     reshape
>
>  drivers/md/dm-raid.c |  76 +++++++++++++++++++++----------
>  drivers/md/md.c      | 104 ++++++++++++++++++++++++++++---------------
>  drivers/md/md.h      |  16 +++++++
>  drivers/md/raid10.c  |  16 +------
>  drivers/md/raid5.c   |  61 +++++++++++++------------
>  5 files changed, 171 insertions(+), 102 deletions(-)
>
> --
> 2.39.2
>

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