linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: fix ref of discard command

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Date:   Wed, 14 Jun 2017 23:40:21 +0800
From:   Chao Yu <chao@...nel.org>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     Chao Yu <yuchao0@...wei.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH] f2fs: fix ref of discard command

On 2017/6/14 23:20, Jaegeuk Kim wrote:
> On 06/14, Chao Yu wrote:
>> On 2017/6/14 22:26, Jaegeuk Kim wrote:
>>> On 06/12, Chao Yu wrote:
>>>> Hi Jaegeuk,
>>>>
>>>> On 2017/6/12 11:04, Jaegeuk Kim wrote:
>>>>> This patch resolves kernel panic for xfstests/081, caused by recent f2fs_bug_on
>>>>>
>>>>>   f2fs: add f2fs_bug_on in __remove_discard_cmd
>>>>>
>>>>> Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
>>>>> ---
>>>>>  fs/f2fs/segment.c | 2 ++
>>>>>  1 file changed, 2 insertions(+)
>>>>>
>>>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>>>> index 86a0c1095939..a6d77388a806 100644
>>>>> --- a/fs/f2fs/segment.c
>>>>> +++ b/fs/f2fs/segment.c
>>>>> @@ -1025,6 +1025,8 @@ static void __wait_discard_cmd(struct f2fs_sb_info *sbi, bool wait_cond)
>>>>>  	list_for_each_entry_safe(dc, tmp, wait_list, list) {
>>>>>  		if (!wait_cond || (dc->state == D_DONE && !dc->ref)) {
>>>>>  			wait_for_completion_io(&dc->wait);
>>>>> +			if (dc->state == D_DONE && dc->ref)
>>>>> +				dc->ref--;
>>>>
>>>> Should set dc->ref to 0 to avoid panic once we add other referrers?
>>>
>>> Sorry, could you please explain this in more detail?
>>
>> Oh, I just assume later we may add another referrer for some reason
>> which will make dc->ref = 2, so dc->ref-- is not enough to avoid the
>> bug_on in __remove_discard_cmd. I think reseting dc->ref is more safe
>> here, how do you think?
> 
> Well, for now, it makes more sense to do like this when considering ref flow,
> IIUC. What will make dc->ref = 2 later? Even in that case, why not making zero

It's just assumption, till now, I do not have it in my mind ;)

> by adding dc->ref-- appropriately?

You mean as below?

dc->ref--;
if (dc->ref > 0)
	dc->ref--;

Thanks,

> 
> Thanks,
> 
>>
>> Thanks,
>>
>>>
>>> Thanks,
>>>
>>>>
>>>> Thanks,
>>>>
>>>>>  			__remove_discard_cmd(sbi, dc);
>>>>>  		} else {
>>>>>  			dc->ref++;
>>>>>
>>>
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