linux-kernel - Re: [PATCH -next] md: synchronize flush io with array reconfiguration

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Message-ID: <baef0c45-9136-ed83-fab5-e2ea7108f1a3@huaweicloud.com>
Date:   Sat, 25 Nov 2023 14:48:08 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Song Liu <song@...nel.org>, Yu Kuai <yukuai1@...weicloud.com>
Cc:     xni@...hat.com, maan@...temlinux.org, neilb@...e.de,
        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] md: synchronize flush io with array reconfiguration

Hi,

在 2023/11/25 1:36, Song Liu 写道:
> On Wed, Nov 8, 2023 at 2:07 AM Yu Kuai <yukuai1@...weicloud.com> wrote:
>>
>> From: Yu Kuai <yukuai3@...wei.com>
>>
>> Currently rcu is used to protect iterating rdev from submit_flushes():
>>
>> submit_flushes                  remove_and_add_spares
>>                                   synchronize_rcu
>>                                   pers->hot_remove_disk()
>>   rcu_read_lock()
>>   rdev_for_each_rcu
>>    if (rdev->raid_disk >= 0)
>>                                   rdev->radi_disk = -1;
>>     atomic_inc(&rdev->nr_pending)
>>     rcu_read_unlock()
>>     bi = bio_alloc_bioset()
>>     bi->bi_end_io = md_end_flush
>>     bi->private = rdev
>>     submit_bio
>>     // issue io for removed rdev
>>
>> Fix this problem by grabbing 'acive_io' before iterating rdev, make sure
>> that remove_and_add_spares() won't concurrent with submit_flushes().
>>
>> Fixes: a2826aa92e2e ("md: support barrier requests on all personalities.")
>> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> 
> LGTM.
> 
>> ---
>>   drivers/md/md.c | 21 +++++++++++++++------
>>   1 file changed, 15 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/md/md.c b/drivers/md/md.c
>> index 4ee4593c874a..eb3e455bcbae 100644
>> --- a/drivers/md/md.c
>> +++ b/drivers/md/md.c
>> @@ -529,6 +529,9 @@ static void md_end_flush(struct bio *bio)
>>          rdev_dec_pending(rdev, mddev);
>>
>>          if (atomic_dec_and_test(&mddev->flush_pending)) {
>> +               /* The pair is percpu_ref_tryget() from md_flush_request() */
>> +               percpu_ref_put(&mddev->active_io);
>> +
>>                  /* The pre-request flush has finished */
>>                  queue_work(md_wq, &mddev->flush_work);
>>          }
>> @@ -548,12 +551,8 @@ static void submit_flushes(struct work_struct *ws)
>>          rdev_for_each_rcu(rdev, mddev)
>>                  if (rdev->raid_disk >= 0 &&
>>                      !test_bit(Faulty, &rdev->flags)) {
>> -                       /* Take two references, one is dropped
>> -                        * when request finishes, one after
>> -                        * we reclaim rcu_read_lock
>> -                        */
>>                          struct bio *bi;
>> -                       atomic_inc(&rdev->nr_pending);
>> +
>>                          atomic_inc(&rdev->nr_pending);
>>                          rcu_read_unlock();
>>                          bi = bio_alloc_bioset(rdev->bdev, 0,
>> @@ -564,7 +563,6 @@ static void submit_flushes(struct work_struct *ws)
>>                          atomic_inc(&mddev->flush_pending);
>>                          submit_bio(bi);
>>                          rcu_read_lock();
>> -                       rdev_dec_pending(rdev, mddev);
>>                  }
>>          rcu_read_unlock();
>>          if (atomic_dec_and_test(&mddev->flush_pending))
>> @@ -617,6 +615,17 @@ bool md_flush_request(struct mddev *mddev, struct bio *bio)
>>          /* new request after previous flush is completed */
>>          if (ktime_after(req_start, mddev->prev_flush_start)) {
>>                  WARN_ON(mddev->flush_bio);
>> +               /*
>> +                * Grab a reference to make sure mddev_suspend() will wait for
>> +                * this flush to be done.
>> +                *
>> +                * md_flush_reqeust() is called under md_handle_request() and
>> +                * 'active_io' is already grabbed, hence percpu_ref_tryget()
>> +                * won't fail, percpu_ref_tryget_live() can't be used because
>> +                * percpu_ref_kill() can be called by mddev_suspend()
>> +                * concurrently.
>> +                */
>> +               percpu_ref_tryget(&mddev->active_io);
> 
> Probably add an warn_on here to catch any issues in the future.

Will do this in v2.

Thanks,
Kuai

> 
> Thanks,
> Song
> 
>>                  mddev->flush_bio = bio;
>>                  bio = NULL;
>>          }
>> --
>> 2.39.2
>>
> .
>