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Message-ID: <05aa3b09-7bb9-a65a-6231-4707b4b078a0@redhat.com>
Date: Tue, 13 Jun 2023 22:50:35 +0800
From: Xiao Ni <xni@...hat.com>
To: Yu Kuai <yukuai1@...weicloud.com>, guoqing.jiang@...ux.dev,
agk@...hat.com, snitzer@...nel.org, dm-devel@...hat.com,
song@...nel.org
Cc: yi.zhang@...wei.com, yangerkun@...wei.com,
linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
yukuai3@...wei.com
Subject: Re: [dm-devel] [PATCH -next v2 4/6] md: refactor
idle/frozen_sync_thread() to fix deadlock
在 2023/5/29 下午9:20, Yu Kuai 写道:
> From: Yu Kuai <yukuai3@...wei.com>
>
> Our test found a following deadlock in raid10:
>
> 1) Issue a normal write, and such write failed:
>
> raid10_end_write_request
> set_bit(R10BIO_WriteError, &r10_bio->state)
> one_write_done
> reschedule_retry
>
> // later from md thread
> raid10d
> handle_write_completed
> list_add(&r10_bio->retry_list, &conf->bio_end_io_list)
>
> // later from md thread
> raid10d
> if (!test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags))
> list_move(conf->bio_end_io_list.prev, &tmp)
> r10_bio = list_first_entry(&tmp, struct r10bio, retry_list)
> raid_end_bio_io(r10_bio)
>
> Dependency chain 1: normal io is waiting for updating superblock
Hi Kuai
It looks like the above situation is more complex. It only needs a
normal write and md_write_start needs to
wait until the metadata is written to member disks, right? If so, it
doesn't need to introduce raid10 write failure
here. I guess, it should be your test case. It's nice, if you can put
your test steps in the patch. But for the analysis
of the deadlock here, it's better to be simple.
>
> 2) Trigger a recovery:
>
> raid10_sync_request
> raise_barrier
>
> Dependency chain 2: sync thread is waiting for normal io
>
> 3) echo idle/frozen to sync_action:
>
> action_store
> mddev_lock
> md_unregister_thread
> kthread_stop
>
> Dependency chain 3: drop 'reconfig_mutex' is waiting for sync thread
>
> 4) md thread can't update superblock:
>
> raid10d
> md_check_recovery
> if (mddev_trylock(mddev))
> md_update_sb
>
> Dependency chain 4: update superblock is waiting for 'reconfig_mutex'
>
> Hence cyclic dependency exist, in order to fix the problem, we must
> break one of them. Dependency 1 and 2 can't be broken because they are
> foundation design. Dependency 4 may be possible if it can be guaranteed
> that no io can be inflight, however, this requires a new mechanism which
> seems complex. Dependency 3 is a good choice, because idle/frozen only
> requires sync thread to finish, which can be done asynchronously that is
> already implemented, and 'reconfig_mutex' is not needed anymore.
>
> This patch switch 'idle' and 'frozen' to wait sync thread to be done
> asynchronously, and this patch also add a sequence counter to record how
> many times sync thread is done, so that 'idle' won't keep waiting on new
> started sync thread.
In the patch, sync_seq is added in md_reap_sync_thread. In
idle_sync_thread, if sync_seq isn't equal
mddev->sync_seq, it should mean there is someone that stops the sync
thread already, right? Why do
you say 'new started sync thread' here?
Regards
Xiao
>
> Noted that raid456 has similiar deadlock([1]), and it's verified[2] this
> deadlock can be fixed by this patch as well.
>
> [1] https://lore.kernel.org/linux-raid/5ed54ffc-ce82-bf66-4eff-390cb23bc1ac@molgen.mpg.de/T/#t
> [2] https://lore.kernel.org/linux-raid/e9067438-d713-f5f3-0d3d-9e6b0e9efa0e@huaweicloud.com/
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
> drivers/md/md.c | 23 +++++++++++++++++++----
> drivers/md/md.h | 2 ++
> 2 files changed, 21 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 63a993b52cd7..7912de0e4d12 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -652,6 +652,7 @@ void mddev_init(struct mddev *mddev)
> timer_setup(&mddev->safemode_timer, md_safemode_timeout, 0);
> atomic_set(&mddev->active, 1);
> atomic_set(&mddev->openers, 0);
> + atomic_set(&mddev->sync_seq, 0);
> spin_lock_init(&mddev->lock);
> atomic_set(&mddev->flush_pending, 0);
> init_waitqueue_head(&mddev->sb_wait);
> @@ -4776,19 +4777,27 @@ static void stop_sync_thread(struct mddev *mddev)
> if (work_pending(&mddev->del_work))
> flush_workqueue(md_misc_wq);
>
> - if (mddev->sync_thread) {
> - set_bit(MD_RECOVERY_INTR, &mddev->recovery);
> - md_reap_sync_thread(mddev);
> - }
> + set_bit(MD_RECOVERY_INTR, &mddev->recovery);
> + /*
> + * Thread might be blocked waiting for metadata update which will now
> + * never happen
> + */
> + md_wakeup_thread_directly(mddev->sync_thread);
>
> mddev_unlock(mddev);
> }
>
> static void idle_sync_thread(struct mddev *mddev)
> {
> + int sync_seq = atomic_read(&mddev->sync_seq);
> +
> mutex_lock(&mddev->sync_mutex);
> clear_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
> stop_sync_thread(mddev);
> +
> + wait_event(resync_wait, sync_seq != atomic_read(&mddev->sync_seq) ||
> + !test_bit(MD_RECOVERY_RUNNING, &mddev->recovery));
> +
> mutex_unlock(&mddev->sync_mutex);
> }
>
> @@ -4797,6 +4806,10 @@ static void frozen_sync_thread(struct mddev *mddev)
> mutex_init(&mddev->delete_mutex);
> set_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
> stop_sync_thread(mddev);
> +
> + wait_event(resync_wait, mddev->sync_thread == NULL &&
> + !test_bit(MD_RECOVERY_RUNNING, &mddev->recovery));
> +
> mutex_unlock(&mddev->sync_mutex);
> }
>
> @@ -9472,6 +9485,8 @@ void md_reap_sync_thread(struct mddev *mddev)
>
> /* resync has finished, collect result */
> md_unregister_thread(&mddev->sync_thread);
> + atomic_inc(&mddev->sync_seq);
> +
> if (!test_bit(MD_RECOVERY_INTR, &mddev->recovery) &&
> !test_bit(MD_RECOVERY_REQUESTED, &mddev->recovery) &&
> mddev->degraded != mddev->raid_disks) {
> diff --git a/drivers/md/md.h b/drivers/md/md.h
> index 2fa903de5bd0..7cab9c7c45b8 100644
> --- a/drivers/md/md.h
> +++ b/drivers/md/md.h
> @@ -539,6 +539,8 @@ struct mddev {
>
> /* Used to synchronize idle and frozen for action_store() */
> struct mutex sync_mutex;
> + /* The sequence number for sync thread */
> + atomic_t sync_seq;
>
> bool has_superblocks:1;
> bool fail_last_dev:1;
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