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

[Guoqing Jiang <guoqing.jiang@linux.dev>]

On 12/10/21 10:16 PM, Donald Buczek wrote:
> Dear Guoqing,
>
> On 13.02.21 01:49, Guoqing Jiang 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:
>> 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>
>> ---
>> 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);
>>           }
>>       } else if (decipher_sync_action(mddev, mddev->recovery) != 
>> st_idle)
>>           return -EBUSY;
>> diff --git a/drivers/md/md.c b/drivers/md/md.c
>> index ca40942..0c12b7f 100644
>> --- a/drivers/md/md.c
>> +++ b/drivers/md/md.c
>> @@ -4857,7 +4857,7 @@ action_store(struct mddev *mddev, const char 
>> *page, size_t len)
>>                   flush_workqueue(md_misc_wq);
>>               if (mddev->sync_thread) {
>>                   set_bit(MD_RECOVERY_INTR, &mddev->recovery);
>> -                md_reap_sync_thread(mddev);
>> +                md_reap_sync_thread(mddev, true);
>>               }
>>               mddev_unlock(mddev);
>>           }
>> @@ -6234,7 +6234,7 @@ static void __md_stop_writes(struct mddev *mddev)
>>           flush_workqueue(md_misc_wq);
>>       if (mddev->sync_thread) {
>>           set_bit(MD_RECOVERY_INTR, &mddev->recovery);
>> -        md_reap_sync_thread(mddev);
>> +        md_reap_sync_thread(mddev, true);
>>       }
>>         del_timer_sync(&mddev->safemode_timer);
>> @@ -9256,7 +9256,7 @@ void md_check_recovery(struct mddev *mddev)
>>                * ->spare_active and clear saved_raid_disk
>>                */
>>               set_bit(MD_RECOVERY_INTR, &mddev->recovery);
>> -            md_reap_sync_thread(mddev);
>> +            md_reap_sync_thread(mddev, true);
>>               clear_bit(MD_RECOVERY_RECOVER, &mddev->recovery);
>>               clear_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
>>               clear_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags);
>> @@ -9291,7 +9291,7 @@ void md_check_recovery(struct mddev *mddev)
>>               goto unlock;
>>           }
>>           if (mddev->sync_thread) {
>> -            md_reap_sync_thread(mddev);
>> +            md_reap_sync_thread(mddev, true);
>>               goto unlock;
>>           }
>>           /* Set RUNNING before clearing NEEDED to avoid
>> @@ -9364,14 +9364,18 @@ void md_check_recovery(struct mddev *mddev)
>>   }
>>   EXPORT_SYMBOL(md_check_recovery);
>>   -void md_reap_sync_thread(struct mddev *mddev)
>> +void md_reap_sync_thread(struct mddev *mddev, bool reconfig_mutex_held)
>>   {
>>       struct md_rdev *rdev;
>>       sector_t old_dev_sectors = mddev->dev_sectors;
>>       bool is_reshaped = false;
>>         /* resync has finished, collect result */
>> +    if (reconfig_mutex_held)
>> +        mddev_unlock(mddev);
>
>
> If one thread got here, e.g. via action_store( /* "idle" */ ), now 
> that the mutex is unlocked, is there anything which would prevent 
> another thread getting  here as well, e.g. via the same path?
>
> If not, they both might call
>
>> md_unregister_thread(&mddev->sync_thread);
>
> Which is not reentrant:
>
> void md_unregister_thread(struct md_thread **threadp)
> {
>     struct md_thread *thread = *threadp;
>     if (!thread)
>         return;
>     pr_debug("interrupting MD-thread pid %d\n", 
> task_pid_nr(thread->tsk));
>     /* Locking ensures that mddev_unlock does not wake_up a
>      * non-existent thread
>      */
>     spin_lock(&pers_lock);
>     *threadp = NULL;
>     spin_unlock(&pers_lock);
>
>     kthread_stop(thread->tsk);
>     kfree(thread);
> }
>
> This might be a preexisting problem, because the call site in 
> dm-raid.c, which you updated to `md_reap_sync_thread(mddev, false);`, 
> didn't hold the mutex anyway.
>
> Am I missing something? Probably, I do.
>
> Otherwise: Move the deref of threadp in md_unregister_thread() into 
> the spinlock scope?

Good point, I think you are right.

And actually pers_lock does extra service to protect accesses to 
mddev->thread (I think it
also suitable for mddev->sync_thread ) when the mutex can't be held. 
Care to send a patch
for it?

Thanks,
Guoqing