linux-kernel - Re: [PATCHv3 9/9] zram: add dynamic device add/remove functionality

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Date:	Wed, 29 Apr 2015 16:02:18 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>, linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCHv3 9/9] zram: add dynamic device add/remove functionality

Hello Minchan,

On (04/29/15 15:48), Minchan Kim wrote:
[..]
> > 
> > 	CPU0			CPU1
> > 	umount
> > 	zram_remove()
> > 	zram_reset_device()	disksize_store()
> > 				mount
> > 	kfree zram
> > 
> > or
> > 
> > 	CPU0				CPU1
> > 	umount
> > 	zram_remove()
> > 	zram_reset_device()
> > 					cat /sys/block/zram0/_any_sysfs_node_
> > 	sysfs_remove_group()
> > 	kfree zram			_any_sysfs_node_read()
> > 
> > 
> > and so on. so removing sysfs group before zram_reset_device() makes sense.
> > 
> > at the same time we need to prevent `umount-zram_remove vs. mount' race and forbid
> > zram_remove() on active device. so we check ->bd_openers and perform device reset
> > under ->bd_mutex.
> > 
> 
> Could you explain in detail about unmount-zram_remove vs. mount race?
> I guess it should be done in upper layer(e,g. VFS). Anyway, I want to be
> more clear about that.
> 

sure. I was talking about this one:

	CPU0			CPU1
	umount
	zram_remove()
	lock ->bd_mutex
	zram_reset_device()
	unlock ->bd_mutex
				disksize_store
				mount
				echo 'test' > /mnt/test
	kfree zram
				zram write


w/o ->bd_mutex around zram_reset_device() it's evern simpler, I guess.
hm, I don't think VFS can help us here.

	-ss
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