RE: [GIT PULL] Fix for btrfs/070 checksum error

[Zhao Lei <zhaolei@cn.fujitsu.com>]

Hi, Chris

> -----Original Message-----
> From: linux-btrfs-owner@vger.kernel.org
> [mailto:linux-btrfs-owner@vger.kernel.org] On Behalf Of Qu Wenruo
> Sent: Tuesday, July 28, 2015 3:11 PM
> To: Chris Mason; btrfs
> Subject: Re: [GIT PULL] Fix for btrfs/070 checksum error
> 
> Chris Mason wrote on 2015/07/23 21:57 -0400:
> > On Fri, Jul 24, 2015 at 08:29:05AM +0800, Qu Wenruo wrote:
> >
> > [ deadlock with the 070 patches ]
> >
> >> Thanks Chris
> >>
> >> We will investigate it with highest priority.
> >>
> >> Thanks,
> >> Qu
> >>
> >
> > Thanks!  I'm doing a few more runs to make sure the lockup is new with
> > these patches.
> >
> > -chris
> >
> Hi Chris,
> 
> I'm very sorry that we are unable to fix the lockup in a short time, so it may not
> fit in the v4.2 merge window.
> 
> Please ignore this patchset for now.
> 

Sorry for taking quite a long time for investigate because it is
randomly happened.

We got reason of process blocking:
1: In some case, this patch caused __btrfs_cow_block()->btrfs_reloc_cow_block()
  failed from btrfs_balance operation.(need more investigation)

2: __btrfs_cow_block()'s error handle code hadn't unlock/free
  new_allocated tree block before return error.

3: do_relocation(), which is caller of __btrfs_cow_block(), have error handle
  code, but also can't work in this case, because new_allocated eb is not
  returned.

4: subsequent code in do_relocation() try to lock above eb again,
  and caused dead lock.

In short:
do_relocation()
-> __btrfs_cow_block() failed without unlock eb *1
...
-> btrfs_search_slot() try to lock above eb again
...
*1: this fail is caused by scrub

Because eb locking code is not normal lock, we can't get information
from lockldep in this case.

Things to do:
1: Fix this patch to avoid making __btrfs_cow_block() fails.
2: Fix __btrfs_cow_block() to do enough cleanup in error handle code.
3: Some enhance for eb locking, to report some information to helps
  similar error.

Thanks
Zhaolei

> Thanks,
> Qu
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