linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: remove circular locking between sb_internal and fs

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Date:   Fri, 22 Oct 2021 23:32:16 +0800
From:   Chao Yu <chao@...nel.org>
To:     Daeho Jeong <daeho43@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, kernel-team@...roid.com,
        Daeho Jeong <daehojeong@...gle.com>
Subject: Re: [f2fs-dev] [PATCH] f2fs: remove circular locking between
 sb_internal and fs_reclaim

On 2021/10/22 0:44, Daeho Jeong wrote:
> There is a deadlock between sb_internal lock (sb_start_intwrite()) and
> dquot related lock.
> It's because we call f2fs_truncate(), which eventually calls
> dquot_initialize(), while holding sb_internal lock.
> So, I called dquot_initialize() in advance to make the 2nd calling of
> it in f2fs_truncate() ineffective.
> This is similar with the thing in f2fs_evict_inode() in inode.c

Well, if dquot_initialize() fails in f2fs_drop_inode(), will we still run
into deadlock?

Thanks,

> 
> Thanks,
> 
> On Thu, Oct 21, 2021 at 5:11 AM Chao Yu <chao@...nel.org> wrote:
>>
>> On 2021/10/15 3:05, Daeho Jeong wrote:
>>> From: Daeho Jeong <daehojeong@...gle.com>
>>>
>>> We detected the below circular locking dependency between sb_internal
>>> and fs_reclaim. So, removed it by calling dquot_initialize() before
>>> sb_start_intwrite().
>>>
>>>    ======================================================
>>>    WARNING: possible circular locking dependency detected
>>>    ------------------------------------------------------
>>>    kswapd0/133 is trying to acquire lock:
>>> ffffff80d5fb9680 (sb_internal#2){.+.+}-{0:0}, at: evict+0xd4/0x2f8
>>>
>>> but task is already holding lock:
>>> ffffffda597c93a8 (fs_reclaim){+.+.}-{0:0}, at:
>>> __fs_reclaim_acquire+0x4/0x50
>>>
>>> which lock already depends on the new lock.
>>> ...
>>> other info that might help us debug this:
>>>
>>>    Chain exists of:
>>>
>>> sb_internal#2 --> &s->s_dquot.dqio_sem --> fs_reclaim
>>>
>>>     Possible unsafe locking scenario:
>>>
>>>           CPU0                    CPU1
>>>           ----                    ----
>>>      lock(fs_reclaim);
>>>                                   lock(&s->s_dquot.dqio_sem);
>>>                                   lock(fs_reclaim);
>>>      lock(sb_internal#2);
>>
>> Sorry, I still didn't get the root cause of this deadlock issue, could
>> you please explain more about this?
>>
>> And why calling dquot_initialize() in drop_inode() could break the
>> circular locking dependency?
>>
>> Thanks,
>>
>>>
>>> Signed-off-by: Daeho Jeong <daehojeong@...gle.com>
>>> ---
>>>    fs/f2fs/super.c | 2 ++
>>>    1 file changed, 2 insertions(+)
>>>
>>> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
>>> index 86eeb019cc52..a133932333c5 100644
>>> --- a/fs/f2fs/super.c
>>> +++ b/fs/f2fs/super.c
>>> @@ -1370,6 +1370,8 @@ static int f2fs_drop_inode(struct inode *inode)
>>>                        /* should remain fi->extent_tree for writepage */
>>>                        f2fs_destroy_extent_node(inode);
>>>
>>> +                     dquot_initialize(inode);
>>> +
>>>                        sb_start_intwrite(inode->i_sb);
>>>                        f2fs_i_size_write(inode, 0);
>>>
>>>