Re: [PATCH RFC] Btrfs: fix deadlock between sys_sync and freeze

[Marco Stornelli <marco.stornelli@gmail.com>]

Il 14/08/2012 15:53, Liu Bo ha scritto:
> On 08/14/2012 08:59 PM, Marco Stornelli wrote:
>> Il 14/08/2012 07:01, liub.liubo@gmail.com ha scritto:
>>> From: Liu Bo <bo.li.liu@oracle.com>
>>>
>>> I found this while testing xfstests 068, the story is
>>>
>>>       t1                                            t2
>>>     sys_sync                                    thaw_super
>>>       iterate_supers
>>>         down_read(sb->s_umount)                   down_write(sb->s_umount) --->wait for t1
>>>         sync_fs (with wait mode)
>>>           start_transaction
>>>             sb_start_intwrite --------------------> wait for t2 to set s_writers.frozen to SB_UNFROZEN
>>>
>>> In this patch, I add an helper sb_start_intwrite_trylock() and use it before we
>>> start_transaction in sync_fs() with wait mode so that we won't hit the deadlock.
>>>
>>
>> IMHO, we should avoid to call the sync operation on a frozen fs. The freeze operation, indeed, already include a sync operation.
>> According to man page, no other operation should modify the fs after the freeze.
>> So for me the modification is inside sync_filesystem (and sync_one_sb).
>
> Do you mean that we should add the trylock check in sync_filesystem?
>
> But it seems to be useless because we already run into down_read(sb->s_umount) before starting sync_one_sb().
>
> thanks,
> liubo
>

I meant that we should check if there are in a "complete" freeze state 
(according to the "states" of a freeze transaction) and simply skip the 
sync operation.

Marco