linux-kernel - Re: Race condition in Kernel

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Date:   Wed, 24 Mar 2021 17:58:30 -0700
From:   Junxiao Bi <junxiao.bi@...cle.com>
To:     Ming Lei <ming.lei@...hat.com>,
        Gulam Mohamed <gulam.mohamed@...cle.com>
Cc:     "hch@...radead.org" <hch@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        Martin Petersen <martin.petersen@...cle.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>
Subject: Re: Race condition in Kernel

On 3/24/21 5:37 PM, Ming Lei wrote:

> On Wed, Mar 24, 2021 at 12:37:03PM +0000, Gulam Mohamed wrote:
>> Hi All,
>>
>> We are facing a stale link (of the device) issue during the iscsi-logout process if we use parted command just before the iscsi logout. Here are the details:
>> 	 	
>> As part of iscsi logout, the partitions and the disk will be removed. The parted command, used to list the partitions, will open the disk in RW mode which results in systemd-udevd re-reading the partitions. This will trigger the rescan partitions which will also delete and re-add the partitions. So, both iscsi logout processing and the parted (through systemd-udevd) will be involved in add/delete of partitions. In our case, the following sequence of operations happened (the iscsi device is /dev/sdb with partition sdb1):
>> 	
>> 	1. sdb1 was removed by PARTED
>> 	2. kworker, as part of iscsi logout, couldn't remove sdb1 as it was already removed by PARTED
>> 	3. sdb1 was added by parted
>> 	4. sdb was NOW removed as part of iscsi logout (the last part of the device removal after remoing the partitions)
>>
>> Since the symlink /sys/class/block/sdb1 points to /sys/class/devices/platform/hostx/sessionx/targetx:x:x:x/x:x:x:x/block/sdb/sdb1 and since sdb is already removed, the symlink /sys/class/block/sdb1 will be orphan and stale. So, this stale link is a result of the race condition in kernel between the systemd-udevd and iscsi-logout processing as described above. We are able to reproduce this even with latest upstream kernel.
>> 	
>> We have come across a patch from Ming Lei which was created for "avoid to drop & re-add partitions if partitions aren't changed":
>> https://lore.kernel.org/linux-block/20210216084430.GA23694@lst.de/T/
> BTW,  there is a newer version of this patchset:
>
> https://lore.kernel.org/linux-block/20210224081825.GA1339@lst.de/#r
>
>> 	
>> This patch could resolve our problem of stale link but it just seems to be a work-around and not the actual fix for the race. We were looking for help to fix this race in kernel. Do you have any idea how to fix this race condition?
>>
> IMO, that isn't a work-around, kernel shouldn't drop partitions if
> partition table isn't changed. But Christoph thought the current approach
> is taken since beginning of kernel, and he suggested to fix systemd-udev.

This is a real kernel bug. Whatever BLK_RRPART do, it should not cause 
this sysfs stale link issue. After this issue happen, there is no way to 
remove that stale link except reboot. The situation is even worse when 
login back a new disk, since it will reuse the disk number of the old 
one, it will fail when it creates the symbol link because the stale link 
is still there.

Thanks,

Junxiao.

>
>
>
> Thanks,
> Ming
>