Re: [PATCH] Remove blkg node after destroying blkg

Yu Kuai
2023-04-25T11:10:16Z

Hi,

在 2023/04/25 17:41, Tao Su 写道:
> On Tue, Apr 25, 2023 at 04:09:34PM +0800, Yu Kuai wrote:
>> Hi,
>>
>> 在 2023/04/25 15:59, Tao Su 写道:
>>> Kernel hang when poweroff or reboot, due to infinite restart in function
>>> blkg_destroy_all. It will goto restart label when a batch of blkgs are
>>> destroyed, but not remove blkg node in blkg_list. So the blkg_list is
>>> same in every 'restart' and result in kernel hang.
>>>
>>> By adding list_del to remove blkg node after destroying, can solve this
>>> kernel hang issue and satisfy the previous will to 'restart'.
>>>
>>> Reported-by: Xiangfei Ma <xiangfeix.ma@intel.com>
>>> Tested-by: Xiangfei Ma <xiangfeix.ma@intel.com>
>>> Tested-by: Farrah Chen <farrah.chen@intel.com>
>>> Signed-off-by: Tao Su <tao1.su@linux.intel.com>
>>> ---
>>> block/blk-cgroup.c | 1 +
>>> 1 file changed, 1 insertion(+)
>>>
>>> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
>>> index bd50b55bdb61..960eb538a704 100644
>>> --- a/block/blk-cgroup.c
>>> +++ b/block/blk-cgroup.c
>>> @@ -530,6 +530,7 @@ static void blkg_destroy_all(struct gendisk *disk)
>>> spin_lock(&blkcg->lock);
>>> blkg_destroy(blkg);
>>> + list_del(&blkg->q_node);
>>
>> blkg should stay on the queue list until blkg_free_workfn(), otherwise
>> parent blkg can be freed before child, which will cause some known
>> issue.
>
> Yes, directly removing blkg node is not appropriate, which I noticed some
> comments in blkg_destroy(), thanks for pointing out this issue.
>
>>
>> I think this hung happens when total blkg is greater than
>> BLKG_DESTROY_BATCH_SIZE, right?
>
> Yes, you are right.
>
>>
>> Can you try if following patch fix your problem?
>
> This patch can also fix my problem, and indeed is a more secure way.

Thanks for the test, for a better solution, I think 'blkcg_mutex' can
be used to protect 'blkg->q_node' list instead of 'queue_lock', so that
the 'restart' can be removed because softlockup can be avoided.

Thanks,
Kuai

>
> Thanks,
> Tao
>
>>
>> index 1c1ebeb51003..0ecb4cce8af2 100644
>> --- a/block/blk-cgroup.c
>> +++ b/block/blk-cgroup.c
>> @@ -527,6 +527,9 @@ static void blkg_destroy_all(struct gendisk *disk)
>> list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {
>> struct blkcg *blkcg = blkg->blkcg;
>>
>> + if (hlist_unhashed(&blkg->blkcg_node))
>> + continue;
>> +
>> spin_lock(&blkcg->lock);
>> blkg_destroy(blkg);
>> spin_unlock(&blkcg->lock);
>>
>>> spin_unlock(&blkcg->lock);
>>> /*
>>>
>
> .
>