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Date: Sun, 18 Feb 2024 15:47:12 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Song Liu <song@...nel.org>, Li Nan <linan666@...weicloud.com>
Cc: axboe@...nel.dk, linux-raid@...r.kernel.org, linux-block@...r.kernel.org,
linux-kernel@...r.kernel.org, yi.zhang@...wei.com, houtao1@...wei.com,
yangerkun@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH] block: fix deadlock between bd_link_disk_holder and
partition scan
Hi,
在 2024/02/17 3:03, Song Liu 写道:
> On Thu, Feb 8, 2024 at 4:49 PM Song Liu <song@...nel.org> wrote:
>>
>> On Thu, Feb 8, 2024 at 12:44 AM Li Nan <linan666@...weicloud.com> wrote:
>>>
>>>
>>>
>>> 在 2024/2/8 14:50, Song Liu 写道:
>>>> On Wed, Feb 7, 2024 at 1:32 AM <linan666@...weicloud.com> wrote:
>>>>>
>>>>> From: Li Nan <linan122@...wei.com>
>>>>>
>>>>> 'open_mutex' of gendisk is used to protect open/close block devices. But
>>>>> in bd_link_disk_holder(), it is used to protect the creation of symlink
>>>>> between holding disk and slave bdev, which introduces some issues.
>>>>>
>>>>> When bd_link_disk_holder() is called, the driver is usually in the process
>>>>> of initialization/modification and may suspend submitting io. At this
>>>>> time, any io hold 'open_mutex', such as scanning partitions, can cause
>>>>> deadlocks. For example, in raid:
>>>>>
>>>>> T1 T2
>>>>> bdev_open_by_dev
>>>>> lock open_mutex [1]
>>>>> ...
>>>>> efi_partition
>>>>> ...
>>>>> md_submit_bio
>>>>> md_ioctl mddev_syspend
>>>>> -> suspend all io
>>>>> md_add_new_disk
>>>>> bind_rdev_to_array
>>>>> bd_link_disk_holder
>>>>> try lock open_mutex [2]
>>>>> md_handle_request
>>>>> -> wait mddev_resume
>>>>>
>>>>> T1 scan partition, T2 add a new device to raid. T1 waits for T2 to resume
>>>>> mddev, but T2 waits for open_mutex held by T1. Deadlock occurs.
>>>>>
>>>>> Fix it by introducing a local mutex 'holder_mutex' to replace 'open_mutex'.
>>>>
>>>> Is this to fix [1]? Do we need some Fixes and/or Closes tags?
>>>>
>>>
>>> No. Just use another way to fix [2], and both [2] and this patch can fix
>>> the issue. I am not sure about the root cause of [1] yet.
>>>
>>> [2] https://patchwork.kernel.org/project/linux-raid/list/?series=812045
>>>
>>>> Could you please add steps to reproduce this issue?
>>>
>>> We need to modify the kernel, add sleep in md_submit_bio() and md_ioctl()
>>> as below, and then:
>>> 1. mdadm -CR /dev/md0 -l1 -n2 /dev/sd[bc] #create a raid
>>> 2. echo 1 > /sys/module/md_mod/parameters/error_inject #enable sleep
>>> 3. 'mdadm --add /dev/md0 /dev/sda' #add a disk to raid
>>> 4. submit ioctl BLKRRPART to raid within 10s.
>>
>> The analysis makes sense. I also hit the issue a couple times without adding
>> extra delays. But I am not sure whether this is the best fix (I didn't find real
>> issues with it either).
>
> To be extra safe and future proof, we can do something like the
> following to only
> suspend the array for ADD_NEW_DISK on not-running arrays.
>
> This appear to solve the problem reported in
>
> https://bugzilla.kernel.org/show_bug.cgi?id=218459
>
> Thanks,
> Song
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 9e41a9aaba8b..395911d5f4d6 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -7570,10 +7570,11 @@ static inline bool md_ioctl_valid(unsigned int cmd)
> }
> }
>
> -static bool md_ioctl_need_suspend(unsigned int cmd)
> +static bool md_ioctl_need_suspend(struct mddev *mddev, unsigned int cmd)
> {
> switch (cmd) {
> case ADD_NEW_DISK:
> + return mddev->pers != NULL;
Did you check already that this problem is not related that 'active_io'
is leaked for flush IO?
I don't understand the problem reported yet. If 'mddev->pers' is not set
yet, md_submit_bio() will return directly, and 'active_io' should not be
grabbed in the first place.
md_run() is the only place to convert 'mddev->pers' from NULL to a real
personality, and it's protected by 'reconfig_mutex', however,
md_ioctl_need_suspend() is called without 'reconfig_mutex', hence there
is a race condition:
md_ioctl_need_suspend array_state_store
// mddev->pers is NULL, return false
mddev_lock
do_md_run
mddev->pers = xxx
mddev_unlock
// mddev_suspend is not called
mddev_lock
md_add_new_disk
if (mddev->pers)
md_import_device
bind_rdev_to_array
add_bound_rdev
mddev->pers->hot_add_disk
-> hot add disk without suspending
Thanks,
Kuai
> case HOT_ADD_DISK:
> case HOT_REMOVE_DISK:
> case SET_BITMAP_FILE:
> @@ -7625,6 +7626,7 @@ static int md_ioctl(struct block_device *bdev,
> blk_mode_t mode,
> void __user *argp = (void __user *)arg;
> struct mddev *mddev = NULL;
> bool did_set_md_closing = false;
> + bool need_suspend;
>
> if (!md_ioctl_valid(cmd))
> return -ENOTTY;
> @@ -7716,8 +7718,11 @@ static int md_ioctl(struct block_device *bdev,
> blk_mode_t mode,
> if (!md_is_rdwr(mddev))
> flush_work(&mddev->sync_work);
>
> - err = md_ioctl_need_suspend(cmd) ? mddev_suspend_and_lock(mddev) :
> - mddev_lock(mddev);
> + need_suspend = md_ioctl_need_suspend(mddev, cmd);
> + if (need_suspend)
> + err = mddev_suspend_and_lock(mddev);
> + else
> + err = mddev_lock(mddev);
> if (err) {
> pr_debug("md: ioctl lock interrupted, reason %d, cmd %d\n",
> err, cmd);
> @@ -7846,8 +7851,10 @@ static int md_ioctl(struct block_device *bdev,
> blk_mode_t mode,
> err != -EINVAL)
> mddev->hold_active = 0;
>
> - md_ioctl_need_suspend(cmd) ? mddev_unlock_and_resume(mddev) :
> - mddev_unlock(mddev);
> + if (need_suspend)
> + mddev_unlock_and_resume(mddev);
> + else
> + mddev_unlock(mddev);
>
> out:
> if(did_set_md_closing)
> .
>
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