Re: [f2fs-dev] [PATCH] f2fs: avoid race condition for shinker count

[Chao Yu]

On 2020/11/10 12:19, Jaegeuk Kim wrote:
> On 11/10, Chao Yu wrote:
> > On 2020/11/10 1:00, Jaegeuk Kim wrote:
> > > Light reported sometimes shinker gets nat_cnt < dirty_nat_cnt resulting in
> >
> > I didn't get the problem clearly, did you mean __count_nat_entries() will
> > give the wrong shrink count due to race condition? should there be a lock
> > while reading these two variables?
> >
> > > wrong do_shinker work. Basically the two counts should not happen like that.
> > >
> > > So, I suspect this race condtion where:
> > > - f2fs_try_to_free_nats            __flush_nat_entry_set
> > >    nat_cnt=2, dirty_nat_cnt=2
> > >                                      __clear_nat_cache_dirty
> > >                                       spin_lock(nat_list_lock)
> > >                                       list_move()
> > >                                       spin_unlock(nat_list_lock)
> > >    spin_lock(nat_list_lock)
> > >    list_del()
> > >    spin_unlock(nat_list_lock)
> > >    nat_cnt=1, dirty_nat_cnt=2
> > >                                      nat_cnt=1, dirty_nat_cnt=1
> >
> > nm_i->nat_cnt and nm_i->dirty_nat_cnt were protected by
> > nm_i->nat_tree_lock, I didn't see why expanding nat_list_lock range
> > will help... since there are still places nat_list_lock() didn't
> > cover these two reference counts.
>
> Yeah, I missed nat_tree_lock, and indeed it should cover this. So, the problem
> is Light reported subtle case of nat_cnt < dirty_nat_cnt in shrink_count.
> We may need to use nat_tree_lock in shrink_count?

change like this?

__count_nat_entries()

	down_read(&nm_i->nat_tree_lock);
	count = NM_I(sbi)->nat_cnt - NM_I(sbi)->dirty_nat_cnt;
	up_read(&nm_i->nat_tree_lock);

Thanks,

> > Thanks,
> >
> > > Reported-by: Light Hsieh <Light.Hsieh@xxxxxxxxxxxx>
> > > Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> > > ---
> > >    fs/f2fs/node.c | 3 +--
> > >    1 file changed, 1 insertion(+), 2 deletions(-)
> > >
> > > diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
> > > index 42394de6c7eb..e8ec65e40f06 100644
> > > --- a/fs/f2fs/node.c
> > > +++ b/fs/f2fs/node.c
> > > @@ -269,11 +269,10 @@ static void __clear_nat_cache_dirty(struct f2fs_nm_info *nm_i,
> > >    {
> > >    	spin_lock(&nm_i->nat_list_lock);
> > >    	list_move_tail(&ne->list, &nm_i->nat_entries);
> > > -	spin_unlock(&nm_i->nat_list_lock);
> > > -
> > >    	set_nat_flag(ne, IS_DIRTY, false);
> > >    	set->entry_cnt--;
> > >    	nm_i->dirty_nat_cnt--;
> > > +	spin_unlock(&nm_i->nat_list_lock);
> > >    }
> > >    static unsigned int __gang_lookup_nat_set(struct f2fs_nm_info *nm_i,
> .