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

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

On Wed, 7 Dec 2011 13:11:58 +0200, Ilya Dryomov wrote:
> On Wed, Dec 07, 2011 at 10:31:35AM +0800, Miao Xie wrote:
>> 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.
> 
> But it won't fix the umount while balancing family of deadlocks (they
> are really of the same nature, vfs grabs s_umount mutex and we need it
> to proceed).  (Balance cancelling code is part of restriper patches,
> it's just a hook in close_ctree() that waits until we are done
> relocating a chunk - very similar to cleaner wait)

I will change the logic, we will add a while loop to check if something is
running(xxx_running is not zero), if yes, invoke btrfs_sync_fs() to do
dirty page flush.

> 
> One example would be that balancing code while dirtying pages calls
> balance_dirty_pages_ratelimited() for each dirtied page, as it should.
> And if balance_dirty_pages() then decides to initiate writeback we are
> stuck schedule()ing forever, because writeback can't proceed w/o
> read-taking s_umount mutex which is fully held by vfs - it just skips
> the relocation inode.

AFAIK, balance_dirty_pages() just wake up the flush thread, and the flush
thread also doesn't grab s_umount. So we needn't worry about it.I think.

Thanks
Miao