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

[Liu Bo <bo.li.liu@oracle.com>]

(CCed Jan, the author of freeze code)

On 08/14/2012 10:12 PM, Marco Stornelli wrote:
> 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.
> 

I'm ok with it.

What do you think about it, Jan?  Any comments?

thanks,
liubo

> Marco