linux-kernel - Re: md_raid: mdX_raid6 looping after sync

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Date:   Wed, 15 Mar 2023 17:53:35 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Guoqing Jiang <guoqing.jiang@...ux.dev>,
        Marc Smith <msmith626@...il.com>
Cc:     Donald Buczek <buczek@...gen.mpg.de>, Song Liu <song@...nel.org>,
        linux-raid@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        it+raid@...gen.mpg.de, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: md_raid: mdX_raid6 looping after sync_action "check" to "idle"
 transition

Hi,

在 2023/03/15 17:30, Guoqing Jiang 写道:
> 
>> Just borrow this thread to discuss, I think this commit might have
>> problem in some corner cases:
>>
>> t1:                t2:
>> action_store
>>  mddev_lock
>>   if (mddev->sync_thread)
>>    mddev_unlock
>>    md_unregister_thread
>>                 md_check_recovery
>>                  set_bit(MD_RECOVERY_RUNNING, &mddev->recovery)
>>                  queue_work(md_misc_wq, &mddev->del_work)
>>    mddev_lock_nointr
>>    md_reap_sync_thread
>>    // clear running
>>  mddev_lock
>>
>> t3:
>> md_start_sync
>> // running is not set
> 
> What does 'running' mean? MD_RECOVERY_RUNNING?
> 
>> Our test report a problem that can be cause by this in theory, by we
>> can't be sure for now...
> 
> I guess you tried to describe racy between
> 
> action_store -> md_register_thread
> 
> and
> 
> md_start_sync -> md_register_thread
> 
> Didn't you already fix them in the series?
> 
> [PATCH -next 0/5] md: fix uaf for sync_thread
> 
> Sorry, I didn't follow the problem and also your series, I might try your
> test with latest mainline kernel if the test is available somewhere.
> 
>> We thought about how to fix this, instead of calling
>> md_register_thread() here to wait for sync_thread to be done
>> synchronisely,
> 
> IMO, md_register_thread just create and wake a thread, not sure why it
> waits for sync_thread.
> 
>> we do this asynchronously like what md_set_readonly() and do_md_stop() 
>> does.
> 
> Still, I don't have clear picture about the problem, so I can't judge it.
> 

Sorry that I didn't explain the problem clear. Let me explain the
problem we meet first:

1) raid10d is waiting for sync_thread to stop:
   raid10d
    md_unregister_thread
     kthread_stop

2) sync_thread is waiting for io to finish:
   md_do_sync
    wait_event(... atomic_read(&mddev->recovery_active) == 0)

3) io is waiting for raid10d to finish(online crash found 2 io in 
conf->retry_list)

Additional information from online crash:
mddev->recovery = 29, // DONE, RUNING, INTR is set

PID: 138293  TASK: ffff0000de89a900  CPU: 7   COMMAND: "md0_resync"
  #0 [ffffa00107c178a0] __switch_to at ffffa0010001d75c
  #1 [ffffa00107c178d0] __schedule at ffffa001017c7f14
  #2 [ffffa00107c179f0] schedule at ffffa001017c880c
  #3 [ffffa00107c17a20] md_do_sync at ffffa0010129cdb4
  #4 [ffffa00107c17d50] md_thread at ffffa00101290d9c
  #5 [ffffa00107c17e50] kthread at ffffa00100187a74

PID: 138294  TASK: ffff0000eba13d80  CPU: 5   COMMAND: "md0_resync"
  #0 [ffffa00107e47a60] __switch_to at ffffa0010001d75c
  #1 [ffffa00107e47a90] __schedule at ffffa001017c7f14
  #2 [ffffa00107e47bb0] schedule at ffffa001017c880c
  #3 [ffffa00107e47be0] schedule_timeout at ffffa001017d1298
  #4 [ffffa00107e47d50] md_thread at ffffa00101290ee8
  #5 [ffffa00107e47e50] kthread at ffffa00100187a74
// there are two sync_thread for md0

I believe the root cause is that two sync_thread exist for the same
mddev, and this is how I think this is possible:

t1:			t2:
action_store
  mddev_lock
   if (mddev->sync_thread)
    mddev_unlock
    md_unregister_thread
    // first sync_thread is done
			md_check_recovery
                  	 set_bit(MD_RECOVERY_RUNNING, &mddev->recovery)
                  	 queue_work(md_misc_wq, &mddev->del_work)
    mddev_lock_nointr
    md_reap_sync_thread
    // MD_RECOVERY_RUNNING is cleared
  mddev_unlock

t3:
md_start_sync
// second sync_thread is registed

t3:
md_check_recovery
  queue_work(md_misc_wq, &mddev->del_work)
  // MD_RECOVERY_RUNNING  is not set, a new sync_thread can be started

This is just guess, I can't reporduce the problem yet. Please let me
know if you have any questions

Thanks,
Kuai