Re: [PATCH -next] md: fix duplicate filename for rdev

[Song Liu]

On Fri, Apr 28, 2023 at 12:13 AM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> commit 5792a2856a63 ("[PATCH] md: avoid a deadlock when removing a device
> from an md array via sysfs") delay the deleting of rdev, however, this
> introduce a window that rdev can be added again while the deleting is
> not done yet, and sysfs will complain about duplicate filename.
>
> Follow up patches try to fix this problem by flush workqueue, however,
> flush_rdev_wq() is just dead code, the progress in
> md_kick_rdev_from_array():
>
> 1) list_del_rcu(&rdev->same_set);
> 2) synchronize_rcu();
> 3) queue_work(md_rdev_misc_wq, &rdev->del_work);
>
> So in flush_rdev_wq(), if rdev is found in the list, work_pending() can
> never pass, in the meantime, if work is queued, then rdev can never be
> found in the list.
>
> flush_rdev_wq() can be replaced by flush_workqueue() directly, however,
> this approach is not good:
> - the workqueue is global, this synchronization for all raid disks is
>   not necessary.
> - flush_workqueue can't be called under 'reconfig_mutex', there is still
>   a small window between flush_workqueue() and mddev_lock() that other
>   context can queue new work, hence the problem is not solved completely.
>
> sysfs already have apis to support delete itself through writer, and
> these apis, specifically sysfs_break/unbreak_active_protection(), is used
> so support deleting rdev synchronously. Therefore, the above commit can be
> reverted, and sysfs duplicate filename can be avoided.
>
> A new mdadm regression test [1] is proposed as well.
>
> Link: https://lore.kernel.org/linux-raid/20230428062845.1975462-1-yukuai1@xxxxxxxxxxxxxxx/
> Fixes: 5792a2856a63 ("[PATCH] md: avoid a deadlock when removing a device from an md array via sysfs")
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
>  drivers/md/md.c | 85 +++++++++++++++++++++++++------------------------
>  drivers/md/md.h |  8 +++++
>  2 files changed, 51 insertions(+), 42 deletions(-)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 0a4a10d4c0e0..e1bc223d58b3 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -83,12 +83,12 @@ static struct module *md_cluster_mod;
>  static DECLARE_WAIT_QUEUE_HEAD(resync_wait);
>  static struct workqueue_struct *md_wq;
>  static struct workqueue_struct *md_misc_wq;
> -static struct workqueue_struct *md_rdev_misc_wq;
>
>  static int remove_and_add_spares(struct mddev *mddev,
>                                  struct md_rdev *this);
>  static void mddev_detach(struct mddev *mddev);
>  static void md_wakeup_thread_directly(struct md_thread __rcu *thread);
> +static void export_rdev(struct md_rdev *rdev);

There is some conflict with md_wakeup_thread_directly().
I guess you run format-patch on top of the "protect md_thread" set
which is not merged yet (and we are waiting for v8 of it?).

Please redo the patch on top of the latest md-next. I just updated it.

Thanks,
Song