Re: [PATCH 0/4] RFC: attempt to remove md deadlocks with metadata without

[NeilBrown <neilb@suse.com>]

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On Fri, Oct 06 2017, Xiao Ni wrote:

> On 10/05/2017 01:17 PM, NeilBrown wrote:
>> On Thu, Sep 14 2017, Xiao Ni wrote:
>>
>>>> What do
>>>>   cat /proc/8987/stack
>>>>   cat /proc/8983/stack
>>>>   cat /proc/8966/stack
>>>>   cat /proc/8381/stack
>>>>
>>>> show??
>> ...
>>
>>> /usr/sbin/mdadm --grow --continue /dev/md0. Is it the reason to add lockdep_assert_held(&mddev->reconfig_mutex)?
>>> [root@dell-pr1700-02 ~]# cat /proc/8983/stack
>>> [<ffffffffa0a3464c>] mddev_suspend+0x12c/0x160 [md_mod]
>>> [<ffffffffa0a379ec>] suspend_lo_store+0x7c/0xe0 [md_mod]
>>> [<ffffffffa0a3b7d0>] md_attr_store+0x80/0xc0 [md_mod]
>>> [<ffffffff812ec8da>] sysfs_kf_write+0x3a/0x50
>>> [<ffffffff812ec39f>] kernfs_fop_write+0xff/0x180
>>> [<ffffffff81260457>] __vfs_write+0x37/0x170
>>> [<ffffffff812619e2>] vfs_write+0xb2/0x1b0
>>> [<ffffffff81263015>] SyS_write+0x55/0xc0
>>> [<ffffffff810037c7>] do_syscall_64+0x67/0x150
>>> [<ffffffff81777527>] entry_SYSCALL64_slow_path+0x25/0x25
>>> [<ffffffffffffffff>] 0xffffffffffffffff
>>>
>>> [jbd2/md0-8]
>>> [root@dell-pr1700-02 ~]# cat /proc/8966/stack
>>> [<ffffffffa0a39b20>] md_write_start+0xf0/0x220 [md_mod]
>>> [<ffffffffa0972b49>] raid5_make_request+0x89/0x8b0 [raid456]
>>> [<ffffffffa0a34175>] md_make_request+0xf5/0x260 [md_mod]
>>> [<ffffffff81376427>] generic_make_request+0x117/0x2f0
>>> [<ffffffff81376675>] submit_bio+0x75/0x150
>>> [<ffffffff8129e0b0>] submit_bh_wbc+0x140/0x170
>>> [<ffffffff8129e683>] submit_bh+0x13/0x20
>>> [<ffffffffa0957e29>] jbd2_write_superblock+0x109/0x230 [jbd2]
>>> [<ffffffffa0957f8b>] jbd2_journal_update_sb_log_tail+0x3b/0x80 [jbd2]
>>> [<ffffffffa09517ff>] jbd2_journal_commit_transaction+0x16ef/0x19e0 [jbd2]
>>> [<ffffffffa0955d02>] kjournald2+0xd2/0x260 [jbd2]
>>> [<ffffffff810c73f9>] kthread+0x109/0x140
>>> [<ffffffff817776c5>] ret_from_fork+0x25/0x30
>>> [<ffffffffffffffff>] 0xffffffffffffffff
>> Thanks for this (and sorry it took so long to get to it).
>> It looks like
>>
>> Commit: cc27b0c78c79 ("md: fix deadlock between mddev_suspend() and md_write_start()")
>>
>> is badly broken.  I wonder how it ever passed testing.
>>
>> In write_start() is change the wait_event() call to
>>
>> 	wait_event(mddev->sb_wait,
>> 		   !test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags) && !mddev->suspended);
>>
>>
>> That should be
>>
>> 	wait_event(mddev->sb_wait,
>> 		   !test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags) || mddev->suspended);
> Hi Neil
>
> Do we want write bio can be handled when mddev->suspended is 1? After 
> changing to this,
> write bio can be handled when mddev->suspended is 1.

This is OK.
New write bios will not get past md_handle_request().
A write bios that did get past md_handle_request() is still allowed
through md_write_start().  The mddev_suspend() call won't complete until
that write bio has finished.

>
> When the stuck happens, mddev->suspended is 0 and MD_SB_CHANGE_PENDING 
> is set. So
> the patch can't fix this problem. I tried the patch, the problem still 
> exists.
>


I need to see all the stack traces.


> [ 7710.589274] mddev suspend : 0
> [ 7710.592228] mddev ro : 0
> [ 7710.594746] mddev insync : 0
> [ 7710.597620] mddev SB CHANGE PENDING is set
> [ 7710.601698] mddev SB CHANGE CLEAN is set
> [ 7710.605601] mddev->persistent : 1
> [ 7710.608905] mddev->external : 0
> [ 7710.612030] conf quiesce : 2
>
> raid5 is still spinning.
>
> Hmm, I have a question. Why can't call md_check_recovery when 
> MD_SB_CHANGE_PENDING
> is set in raid5d?

When MD_SB_CHANGE_PENDING is not set, there is no need to call
md_check_recovery().  I wouldn't hurt except that it would be a waste of
time.

NeilBrown


>
>                  if (mddev->sb_flags & ~(1 << MD_SB_CHANGE_PENDING)) {
>                          spin_unlock_irq(&conf->device_lock);
>                          md_check_recovery(mddev);
>                          spin_lock_irq(&conf->device_lock);
>                  }
>
> Best Regards
> Xiao
>
>
>>
>> i.e. it was (!A && !B), it should be (!A || B) !!!!!
>>
>> Could you please make that change and try again.
> Hi Neil
>
> I tried the patch and it can't work.
>>
>> Thanks,
>> NeilBrown

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