Re: [RFC] Is it correctly that the usage for spin_{lock|unlock}_irq in clear_page_dirty_for_io

[Greg Thelen <gthelen@google.com>]

On Tue, Apr 3, 2018 at 5:03 AM Michal Hocko <mhocko@kernel.org> wrote:

> On Mon 02-04-18 19:50:50, Wang Long wrote:
> >
> > Hi,  Johannes Weiner and Tejun Heo
> >
> > I use linux-4.4.y to test the new cgroup controller io and the current
> > stable kernel linux-4.4.y has the follow logic
> >
> >
> > int clear_page_dirty_for_io(struct page *page){
> > ...
> > ...
> >                 memcg = mem_cgroup_begin_page_stat(page); ----------(a)
> >                 wb = unlocked_inode_to_wb_begin(inode, &locked);
---------(b)
> >                 if (TestClearPageDirty(page)) {
> >                         mem_cgroup_dec_page_stat(memcg,
MEM_CGROUP_STAT_DIRTY);
> >                         dec_zone_page_state(page, NR_FILE_DIRTY);
> >                         dec_wb_stat(wb, WB_RECLAIMABLE);
> >                         ret =1;
> >                 }
> >                 unlocked_inode_to_wb_end(inode, locked); -----------(c)
> >                 mem_cgroup_end_page_stat(memcg); -------------(d)
> >                 return ret;
> > ...
> > ...
> > }
> >
> >
> > when memcg is moving, and I_WB_SWITCH flags for inode is set. the logic
> > is the following:
> >
> >
> > spin_lock_irqsave(&memcg->move_lock, flags); -------------(a)
> >         spin_lock_irq(&inode->i_mapping->tree_lock); ------------(b)
> >         spin_unlock_irq(&inode->i_mapping->tree_lock); -----------(c)
> > spin_unlock_irqrestore(&memcg->move_lock, flags); -----------(d)
> >
> >
> > after (c) , the local irq is enabled. I think it is not correct.
> >
> > We get a deadlock backtrace after (c), the cpu get an softirq and in the
> > irq it also call mem_cgroup_begin_page_stat to lock the same
> > memcg->move_lock.
> >
> > Since the conditions are too harsh, this scenario is difficult to
> > reproduce.  But it really exists.
> >
> > So how about change (b) (c) to spin_lock_irqsave/spin_lock_irqrestore?

> Yes, it seems we really need this even for the current tree. Please note
> that At least clear_page_dirty_for_io doesn't lock memcg anymore.
> __cancel_dirty_page still uses lock_page_memcg though (former
> mem_cgroup_begin_page_stat).
> --
> Michal Hocko
> SUSE Labs

I agree the issue looks real in 4.4 stable and upstream.  It seems like
unlocked_inode_to_wb_begin/_end should use spin_lock_irqsave/restore.

I'm testing a little patch now.