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

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

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

> 
> Marco
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