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

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Message-ID: <20170614152007.GD74571@jaegeuk-macbookpro.roam.corp.google.com>
Date:   Wed, 14 Jun 2017 08:20:07 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <chao@...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 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
by adding dc->ref-- appropriately?

Thanks,

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