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Subject: Re: [PATCH -next v3 6/7] md/raid1-10: don't handle pluged bio by
 daemon thread
To:     Xiao Ni <xni@redhat.com>, Yu Kuai <yukuai1@huaweicloud.com>
Cc:     song@kernel.org, neilb@suse.de, akpm@osdl.org,
        linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org,
        yi.zhang@huawei.com, yangerkun@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20230529131106.2123367-1-yukuai1@huaweicloud.com>
 <20230529131106.2123367-7-yukuai1@huaweicloud.com>
 <CALTww29ixKpcVknNe36D+x=2c1Aw-=z32SP-dJ_Hj8WxL2n4bg@mail.gmail.com>
 <830352e1-ecfa-f5dc-ce7c-349553bd3003@huaweicloud.com>
 <CALTww28_w3h1_viTp5L9SCytY7WmwmABqkXOmyvH_UD8T5odMg@mail.gmail.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <f5cc14ed-e9d9-42ed-22e1-186973b991b1@huaweicloud.com>
Date:   Wed, 31 May 2023 16:06:35 +0800
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Hi,

在 2023/05/31 16:00, Xiao Ni 写道:
> On Wed, May 31, 2023 at 3:55 PM Yu Kuai <yukuai1@huaweicloud.com> wrote:
>>
>> Hi,
>>
>> 在 2023/05/31 15:50, Xiao Ni 写道:
>>> On Mon, May 29, 2023 at 9:14 PM Yu Kuai <yukuai1@huaweicloud.com> wrote:
>>>>
>>>> From: Yu Kuai <yukuai3@huawei.com>
>>>>
>>>> current->bio_list will be set under submit_bio() context, in this case
>>>> bitmap io will be added to the list and wait for current io submission to
>>>> finish, while current io submission must wait for bitmap io to be done.
>>>> commit 874807a83139 ("md/raid1{,0}: fix deadlock in bitmap_unplug.") fix
>>>> the deadlock by handling plugged bio by daemon thread.
>>>
>>> Thanks for the historic introduction. I did a test and printed the
>>> logs in raid10_unplug. The tools I used are dd and mkfs. from_schedule
>>> is always true during I/O and it's 0 when io finishes. So I have a
>>> question here, how can I trigger the condition that from_schedule is 0
>>> and current->list is not NULL? In other words, is there really a
>>> deadlock here? Before your patch it looks like all bios are merged
>>> into conf->pending_bio_list and are handled by raid10d. It can't
>>> submit bio directly in the originating process which mentioned in
>>> 57c67df48866
>>>
>> As I mentioned below, after commit a214b949d8e3, this deadlock doesn't
>> exist anymore, and without this patch, patch 7 will introduce this
>> scenario again.
>>
>> Thanks,
>> Kuai
>>>>
>>>> On the one hand, the deadlock won't exist after commit a214b949d8e3
>>>> ("blk-mq: only flush requests from the plug in blk_mq_submit_bio"). On
>>>> the other hand, current solution makes it impossible to flush plugged bio
>>>> in raid1/10_make_request(), because this will cause that all the writes
>>>> will goto daemon thread.
>>>>
>>>> In order to limit the number of plugged bio, commit 874807a83139
>>>> ("md/raid1{,0}: fix deadlock in bitmap_unplug.") is reverted, and the
>>>> deadlock is fixed by handling bitmap io asynchronously.
>>>>
>>>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
>>>> ---
>>>>    drivers/md/raid1-10.c | 14 ++++++++++++++
>>>>    drivers/md/raid1.c    |  4 ++--
>>>>    drivers/md/raid10.c   |  8 +++-----
>>>>    3 files changed, 19 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/drivers/md/raid1-10.c b/drivers/md/raid1-10.c
>>>> index 73cc3cb9154d..17e55c1fd5a1 100644
>>>> --- a/drivers/md/raid1-10.c
>>>> +++ b/drivers/md/raid1-10.c
>>>> @@ -151,3 +151,17 @@ static inline bool raid1_add_bio_to_plug(struct mddev *mddev, struct bio *bio,
>>>>
>>>>           return true;
>>>>    }
>>>> +
>>>> +/*
>>>> + * current->bio_list will be set under submit_bio() context, in this case bitmap
>>>> + * io will be added to the list and wait for current io submission to finish,
>>>> + * while current io submission must wait for bitmap io to be done. In order to
>>>> + * avoid such deadlock, submit bitmap io asynchronously.
>>>> + */
>>>> +static inline void raid1_prepare_flush_writes(struct bitmap *bitmap)
>>>> +{
>>>> +       if (current->bio_list)
>>>> +               md_bitmap_unplug_async(bitmap);
>>>> +       else
>>>> +               md_bitmap_unplug(bitmap);
>>>> +}
>>>> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
>>>> index 0778e398584c..006620fed595 100644
>>>> --- a/drivers/md/raid1.c
>>>> +++ b/drivers/md/raid1.c
>>>> @@ -794,7 +794,7 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
>>>>    static void flush_bio_list(struct r1conf *conf, struct bio *bio)
>>>>    {
>>>>           /* flush any pending bitmap writes to disk before proceeding w/ I/O */
>>>> -       md_bitmap_unplug(conf->mddev->bitmap);
>>>> +       raid1_prepare_flush_writes(conf->mddev->bitmap);
>>>
>>> If we unplug bitmap asynchronously, can we make sure the bitmap are
>>> flushed before the corresponding data?
> 
> Could you explain this question?

Sorry that I missed this... See the new helper in patch 5,
md_bitmap_unplug_async() will still wait for bitmap io to finish.

md_bitmap_unplug_async
  DECLARE_COMPLETION_ONSTACK(done)
  ...
  wait_for_completion(&done)

Thanks,
Kuai
> 
> Regards
> Xiao
> 
> 
>>>
>>> Regards
>>> Xiao
>>>
>>>>           wake_up(&conf->wait_barrier);
>>>>
>>>>           while (bio) { /* submit pending writes */
>>>> @@ -1166,7 +1166,7 @@ static void raid1_unplug(struct blk_plug_cb *cb, bool from_schedule)
>>>>           struct r1conf *conf = mddev->private;
>>>>           struct bio *bio;
>>>>
>>>> -       if (from_schedule || current->bio_list) {
>>>> +       if (from_schedule) {
>>>>                   spin_lock_irq(&conf->device_lock);
>>>>                   bio_list_merge(&conf->pending_bio_list, &plug->pending);
>>>>                   spin_unlock_irq(&conf->device_lock);
>>>> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
>>>> index 6640507ecb0d..fb22cfe94d32 100644
>>>> --- a/drivers/md/raid10.c
>>>> +++ b/drivers/md/raid10.c
>>>> @@ -902,9 +902,7 @@ static void flush_pending_writes(struct r10conf *conf)
>>>>                   __set_current_state(TASK_RUNNING);
>>>>
>>>>                   blk_start_plug(&plug);
>>>> -               /* flush any pending bitmap writes to disk
>>>> -                * before proceeding w/ I/O */
>>>> -               md_bitmap_unplug(conf->mddev->bitmap);
>>>> +               raid1_prepare_flush_writes(conf->mddev->bitmap);
>>>>                   wake_up(&conf->wait_barrier);
>>>>
>>>>                   while (bio) { /* submit pending writes */
>>>> @@ -1108,7 +1106,7 @@ static void raid10_unplug(struct blk_plug_cb *cb, bool from_schedule)
>>>>           struct r10conf *conf = mddev->private;
>>>>           struct bio *bio;
>>>>
>>>> -       if (from_schedule || current->bio_list) {
>>>> +       if (from_schedule) {
>>>>                   spin_lock_irq(&conf->device_lock);
>>>>                   bio_list_merge(&conf->pending_bio_list, &plug->pending);
>>>>                   spin_unlock_irq(&conf->device_lock);
>>>> @@ -1120,7 +1118,7 @@ static void raid10_unplug(struct blk_plug_cb *cb, bool from_schedule)
>>>>
>>>>           /* we aren't scheduling, so we can do the write-out directly. */
>>>>           bio = bio_list_get(&plug->pending);
>>>> -       md_bitmap_unplug(mddev->bitmap);
>>>> +       raid1_prepare_flush_writes(mddev->bitmap);
>>>>           wake_up(&conf->wait_barrier);
>>>>
>>>>           while (bio) { /* submit pending writes */
>>>> --
>>>> 2.39.2
>>>>
>>>
>>> .
>>>
>>
> 
> .
>