Re: [PATCH 4/5] jbd2: Fix wrongly judgement for buffer head removing while doing checkpoint

[Zhihao Cheng <chengzhihao1@huawei.com>]

在 2023/6/2 0:31, Jan Kara 写道:
> On Thu 01-06-23 22:20:38, Zhang Yi wrote:
>> On 2023/6/1 21:44, Zhihao Cheng wrote:
>>> 在 2023/6/1 17:41, Jan Kara 写道:
>>>
>>> Hi, Jan
>>>> On Wed 31-05-23 19:50:59, Zhang Yi wrote:
>>>>> From: Zhihao Cheng <chengzhihao1@huawei.com>
>>>>>
>>>>> Following process,
>>>>>
>>>>> jbd2_journal_commit_transaction
>>>>> // there are several dirty buffer heads in transaction->t_checkpoint_list
>>>>>             P1                   wb_workfn
>>>>> jbd2_log_do_checkpoint
>>>>>    if (buffer_locked(bh)) // false
>>>>>                               __block_write_full_page
>>>>>                                trylock_buffer(bh)
>>>>>                                test_clear_buffer_dirty(bh)
>>>>>    if (!buffer_dirty(bh))
>>>>>     __jbd2_journal_remove_checkpoint(jh)
>>>>>      if (buffer_write_io_error(bh)) // false
>>>>>                                >> bh IO error occurs <<
>>>>>    jbd2_cleanup_journal_tail
>>>>>     __jbd2_update_log_tail
>>>>>      jbd2_write_superblock
>>>>>      // The bh won't be replayed in next mount.
>>>>> , which could corrupt the ext4 image, fetch a reproducer in [Link].
>>>>>
>>>>> Since writeback process clears buffer dirty after locking buffer head,
>>>>> we can fix it by checking buffer dirty firstly and then checking buffer
>>>>> locked, the buffer head can be removed if it is neither dirty nor locked.
>>>>>
>>>>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217490
>>>>> Fixes: 470decc613ab ("[PATCH] jbd2: initial copy of files from jbd")
>>>>> Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
>>>>> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
>>>>
>>>> OK, the analysis is correct but I'm afraid the fix won't be that easy.  The
>>>> reordering of tests you did below doesn't really help because CPU or the
>>>> compiler are free to order the loads (and stores) in whatever way they
>>>> wish. You'd have to use memory barriers when reading and modifying bh flags
>>>> (although the modification side is implicitely handled by the bitlock
>>>> code) to make this work reliably. But that is IMHO too subtle for this
>>>> code.
>>>>
>>>
>>
>> I write two litmus-test files in tools/memory-model to check the memory module
>> of these two cases as jbd2_log_do_checkpoint() and __cp_buffer_busy() does.
> 
> <snip litmus tests>
> 
>> So it looks like the out-of-order situation cannot happen, am I miss something?
> 
> After thinking about it a bit, indeed CPU cannot reorder the two loads
> because they are from the same location in memory. Thanks for correcting me
> on this. I'm not sure whether a C compiler still could not reorder the
> tests - technically I suspect the C standard does not forbid this although
> it would have to be really evil compiler to do this.
> 
> But still I think with the helper function I've suggested things are much
> more obviously correct.

Thanks for suggestions, we will modify it in next iteration.