Re: [PATCH 2/2] Btrfs: fix deadlock on sb->s_umount when doing umount

[Miao Xie <miaox@cn.fujitsu.com>]

On tue, 6 Dec 2011 16:36:11 -0500, Chris Mason wrote:
> On Tue, Dec 06, 2011 at 06:23:23AM -0500, Christoph Hellwig wrote:
>> On Tue, Dec 06, 2011 at 07:06:40PM +0800, Miao Xie wrote:
>>>> I can't see why you need the writeout when the trylocks fails.  Umount
>>>> needs to take care of writing out all pending file data anyway, so doing
>>>> it from the cleaner thread in addition doesn't sound like it would help.
>>>
>>> umount invokes sync_fs() and write out all the dirty file data. For the
>>> other file systems, its OK because the file system does not introduce dirty pages
>>> by itself. But btrfs is different. Its automatic defragment will make lots of dirty
>>> pages after sync_fs() and reserve lots of meta-data space for those pages.
>>> And then the cleaner thread may find there is no enough space to reserve, it must
>>> sync the dirty file data and release the reserved space which is for the dirty
>>> file data.
>>
>> I think the safest way to fix is is to write out all dirty data again
>> once the cleaner thread has been safely stopped.
>>
> 
> Said another way we want to stop the autodefrag code before the unmount
> is ready to continue.  We also want to stop balancing, scrub etc.

But there is no good interface to do it before umount gets s_umount lock.
I think trylock(in writeback_inodes_sb_nr_if_idle()) + dirty data flush
can help us to fix the bug perfectly.

Thanks
Miao

> 
> -chris
> 
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