Re: [PATCH V2] md: don't unregister sync_thread with reconfig_mutex held

[Guoqing Jiang <guoqing.jiang@cloud.ionos.com>]

On 2/24/21 10:09, Song Liu wrote:
> On Mon, Feb 15, 2021 at 3:08 AM Paul Menzel <pmenzel@molgen.mpg.de> wrote:
>>
>> [+cc Donald]
>>
>> Am 13.02.21 um 01:49 schrieb Guoqing Jiang:
>>> Unregister sync_thread doesn't need to hold reconfig_mutex since it
>>> doesn't reconfigure array.
>>>
>>> And it could cause deadlock problem for raid5 as follows:
>>>
>>> 1. process A tried to reap sync thread with reconfig_mutex held after echo
>>>      idle to sync_action.
>>> 2. raid5 sync thread was blocked if there were too many active stripes.
>>> 3. SB_CHANGE_PENDING was set (because of write IO comes from upper layer)
>>>      which causes the number of active stripes can't be decreased.
>>> 4. SB_CHANGE_PENDING can't be cleared since md_check_recovery was not able
>>>      to hold reconfig_mutex.
>>>
>>> More details in the link:
>>> https://lore.kernel.org/linux-raid/5ed54ffc-ce82-bf66-4eff-390cb23bc1ac@molgen.mpg.de/T/#t
>>>
>>> And add one parameter to md_reap_sync_thread since it could be called by
>>> dm-raid which doesn't hold reconfig_mutex.
>>>
>>> Reported-and-tested-by: Donald Buczek <buczek@molgen.mpg.de>
>>> Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
> 
> I don't really like this fix. But I haven't got a better (and not too
> complicated)
> alternative.
> 
>>> ---
>>> V2:
>>> 1. add one parameter to md_reap_sync_thread per Jack's suggestion.
>>>
>>>    drivers/md/dm-raid.c |  2 +-
>>>    drivers/md/md.c      | 14 +++++++++-----
>>>    drivers/md/md.h      |  2 +-
>>>    3 files changed, 11 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/drivers/md/dm-raid.c b/drivers/md/dm-raid.c
>>> index cab12b2..0c4cbba 100644
>>> --- a/drivers/md/dm-raid.c
>>> +++ b/drivers/md/dm-raid.c
>>> @@ -3668,7 +3668,7 @@ static int raid_message(struct dm_target *ti, unsigned int argc, char **argv,
>>>        if (!strcasecmp(argv[0], "idle") || !strcasecmp(argv[0], "frozen")) {
>>>                if (mddev->sync_thread) {
>>>                        set_bit(MD_RECOVERY_INTR, &mddev->recovery);
>>> -                     md_reap_sync_thread(mddev);
>>> +                     md_reap_sync_thread(mddev, false);
> 
> I think we can add mddev_lock() and mddev_unlock() here and then we don't
> need the extra parameter?
> 

I thought it too, but I would prefer get the input from DM people first.

@ Mike or Alasdair


Thanks,
Guoqing