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Date: Wed, 31 May 2023 15:55:13 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Xiao Ni <xni@...hat.com>, Yu Kuai <yukuai1@...weicloud.com>
Cc: song@...nel.org, neilb@...e.de, akpm@...l.org,
linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
yi.zhang@...wei.com, yangerkun@...wei.com,
"yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH -next v3 6/7] md/raid1-10: don't handle pluged bio by
daemon thread
Hi,
在 2023/05/31 15:50, Xiao Ni 写道:
> On Mon, May 29, 2023 at 9:14 PM Yu Kuai <yukuai1@...weicloud.com> wrote:
>>
>> From: Yu Kuai <yukuai3@...wei.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@...wei.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?
>
> 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
>>
>
> .
>
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