linux-kernel - Re: [PATCHv4 00/10] add on-demand device creation

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Date:	Thu, 7 May 2015 21:09:44 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 00/10] add on-demand device creation

On (05/07/15 16:41), Minchan Kim wrote:
> 
> Unfortunately, I can reproduce in a few second with this patch.
> 

so I googled a bit, and it seems that zram is not the only one who suffered
from add_disk() behaviour.

http://www.spinics.net/lists/dm-devel/msg23465.html

and there is
http://git.kernel.dk/?p=linux-block.git;a=commitdiff;h=6cd18e71;hp=393a33970540ac6a2c894b0d6ef3f5d485860884

that looks interesting:

|
| Because of the peculiar way that md devices are created (automatically
| when the device node is opened), a new device can be created and
| registered immediately after the
| blk_unregister_region(disk_devt(disk), disk->minors);
| call in del_gendisk().
|
| Therefore it is important that all visible artifacts of the previous
| device are removed before this call.  In particular, the 'bdi'.
|

basically, it destroys bdi during queue cleanup.


> moved the
>     device_unregister(bdi->dev);
> call from bdi_unregister() to bdi_destroy() it has been quite easy to
> lose a race and have a new (e.g.) "md127" be created after the
> blk_unregister_region() call and before bdi_destroy() is ultimately
> called by the final 'put_disk', which must come after del_gendisk().
>
> The new device finds that the bdi name is already registered in sysfs
> and complains
>
>> [ 9627.630029] WARNING: CPU: 18 PID: 3330 at fs/sysfs/dir.c:31 sysfs_warn_dup+0x5a/0x70()
>> [ 9627.630032] sysfs: cannot create duplicate filename '/devices/virtual/bdi/9:127'
>
> We can fix this by moving the bdi_destroy() call out of
> blk_release_queue() (which can happen very late when a refcount
> reaches zero) and into blk_cleanup_queue() - which happens exactly when the md
> device driver calls it.


that does look like something that can happen in our case.


and here:
http://www.spinics.net/lists/dm-devel/msg23415.html


is there any chance to ask you to test with these patches (no rush, take your time)?
as I'm still unable to reproduce it locally.


	-ss
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