Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230523012727.3042247-1-yukuai1@huaweicloud.com>
In-Reply-To: <20230523012727.3042247-1-yukuai1@huaweicloud.com>
From:   Song Liu <song@kernel.org>
Date:   Tue, 23 May 2023 11:05:18 -0700
Message-ID: <CAPhsuW5AERnf5D4804ksgZEB2ZP7gVAG8f5Fghwnn0FKseBG+g@mail.gmail.com>
Subject: Re: [PATCH v3] md: fix duplicate filename for rdev
To:     Yu Kuai <yukuai1@huaweicloud.com>
Cc:     pmenzel@molgen.mpg.de, akpm@linux-foundation.org, neilb@suse.de,
        linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org,
        yukuai3@huawei.com, yi.zhang@huawei.com, yangerkun@huawei.com
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Mon, May 22, 2023 at 6:30=E2=80=AFPM Yu Kuai <yukuai1@huaweicloud.com> w=
rote:
>
> From: Yu Kuai <yukuai3@huawei.com>
>
> Commit 5792a2856a63 ("[PATCH] md: avoid a deadlock when removing a device
> from an md array via sysfs") delays the deletion of rdev, however, this
> introduces a window that rdev can be added again while the deletion is
> not done yet, and sysfs will complain about duplicate filename.
>
> Follow up patches try to fix this problem by flushing 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
>   contexts can queue new work, hence the problem is not solved completely=
.
>
> sysfs already has apis to support delete itself through writer, and
> these apis, specifically sysfs_break/unbreak_active_protection(), is used
> to support deleting rdev synchronously. Therefore, the above commit can b=
e
> reverted, and sysfs duplicate filename can be avoided.
>
> A new mdadm regression test is proposed as well([1]).
>
> [1] https://lore.kernel.org/linux-raid/20230428062845.1975462-1-yukuai1@h=
uaweicloud.com/
> Fixes: 5792a2856a63 ("[PATCH] md: avoid a deadlock when removing a device=
 from an md array via sysfs")
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>

Thanks for the fix! I made the following changes and applied it
to md-next:

1. remove md_rdev->del_work, which is not used any more;
2. change list_empty_safe to list_empty protected by the mutex, as
   list_empty_safe doesn't seem safe here.

Please let me know if either change doesn't make sense.

Thanks,
Song