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

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

On 2/11/21 08:28, Song Liu wrote:
> On Tue, Feb 9, 2021 at 6:22 PM Guoqing Jiang
> <guoqing.jiang@cloud.ionos.com> wrote:
>>
>> 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:
>> issu://lore.kernel.org/linux-raid/5ed54ffc-ce82-bf66-4eff-390cb23bc1ac@molgen.mpg.de/T/#t
>>
>> Reported-and-tested-by: Donald Buczek <buczek@molgen.mpg.de>
>> Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
> 
> Thanks for debugging the issue. However, I am not sure whether this is
> the proper
> fix. For example, would this break dm-raid.c:raid_message()? IIUC,
> raid_message()
> calls md_reap_sync_thread() without holding reconfigure_mutex, no?

Oops, I didn't notice dm-raid calls it though md did call it with 
reconfig_mutex held. But on the other side, it proves we don't need to 
call md_reap_sync_thread with the mutex held.

Thanks,
Guoqing