Re: [f2fs-dev] [PATCH] f2fs: remove circular locking between sb_internal and fs_reclaim

[Chao Yu]

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@xxxxxxxxxx> wrote:
> > On 2021/10/15 3:05, Daeho Jeong wrote:
> > > From: Daeho Jeong <daehojeong@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > > ---
> > >    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);