Re: [PATCH] writeback: check wb shutdown for bw_dwork

[Khazhy Kumykov <khazhy@google.com>]

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On Fri, Jul 29, 2022 at 6:29 PM Jan Kara <jack@suse.cz> wrote:
>
> On Fri 29-07-22 14:51:23, Khazhismel Kumykov wrote:
> > This fixes a KASAN splat in timer interrupt after removing a device
> >
> > Move wb->work_lock to be irq-safe, as complete may be called from irq
> >
> > Fixes: 45a2966fd641 ("writeback: fix bandwidth estimate for spiky workload")
> > Signed-off-by: Khazhismel Kumykov <khazhy@google.com>
>
> The patch looks good but I wish there are more details in the changelog
> because with this terse changelog I can only guess. I suppose something
> like:
Thanks for the comments, I sent an updated changelog. I hope you don't
mind me using your wording :) And I'll take note to not be shy in
messages for the future!
>
> writeback: avoid use after free after removing a device
>
> When a disk is removed, bdi_unregister() gets called to stop any further
> writeback and wait for the running one to complete. However while IO
> completes, wb_inode_writeback_end() will schedule another delayed writeback
> and by the time timer fires the bdi_writeback structure may be already
> freed. Fix the problem by checking whether bdi is still alive before
> scheduling new work in wb_inode_writeback_end(). Since this requires
> wb->work_lock and wb_inode_writeback_end() may get called from an
> interrupt, switch wb->work_lock to an irqsafe lock.
>
>                                                                 Honza
>
> > ---
> >  fs/fs-writeback.c   | 12 ++++++------
> >  mm/backing-dev.c    | 10 +++++-----
> >  mm/page-writeback.c |  6 +++++-
> >  3 files changed, 16 insertions(+), 12 deletions(-)
> >
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index 05221366a16d..08a1993ab7fd 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -134,10 +134,10 @@ static bool inode_io_list_move_locked(struct inode *inode,
> >
> >  static void wb_wakeup(struct bdi_writeback *wb)
> >  {
> > -     spin_lock_bh(&wb->work_lock);
> > +     spin_lock_irq(&wb->work_lock);
> >       if (test_bit(WB_registered, &wb->state))
> >               mod_delayed_work(bdi_wq, &wb->dwork, 0);
> > -     spin_unlock_bh(&wb->work_lock);
> > +     spin_unlock_irq(&wb->work_lock);
> >  }
> >
> >  static void finish_writeback_work(struct bdi_writeback *wb,
> > @@ -164,7 +164,7 @@ static void wb_queue_work(struct bdi_writeback *wb,
> >       if (work->done)
> >               atomic_inc(&work->done->cnt);
> >
> > -     spin_lock_bh(&wb->work_lock);
> > +     spin_lock_irq(&wb->work_lock);
> >
> >       if (test_bit(WB_registered, &wb->state)) {
> >               list_add_tail(&work->list, &wb->work_list);
> > @@ -172,7 +172,7 @@ static void wb_queue_work(struct bdi_writeback *wb,
> >       } else
> >               finish_writeback_work(wb, work);
> >
> > -     spin_unlock_bh(&wb->work_lock);
> > +     spin_unlock_irq(&wb->work_lock);
> >  }
> >
> >  /**
> > @@ -2082,13 +2082,13 @@ static struct wb_writeback_work *get_next_work_item(struct bdi_writeback *wb)
> >  {
> >       struct wb_writeback_work *work = NULL;
> >
> > -     spin_lock_bh(&wb->work_lock);
> > +     spin_lock_irq(&wb->work_lock);
> >       if (!list_empty(&wb->work_list)) {
> >               work = list_entry(wb->work_list.next,
> >                                 struct wb_writeback_work, list);
> >               list_del_init(&work->list);
> >       }
> > -     spin_unlock_bh(&wb->work_lock);
> > +     spin_unlock_irq(&wb->work_lock);
> >       return work;
> >  }
> >
> > diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> > index 95550b8fa7fe..de65cb1e5f76 100644
> > --- a/mm/backing-dev.c
> > +++ b/mm/backing-dev.c
> > @@ -260,10 +260,10 @@ void wb_wakeup_delayed(struct bdi_writeback *wb)
> >       unsigned long timeout;
> >
> >       timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
> > -     spin_lock_bh(&wb->work_lock);
> > +     spin_lock_irq(&wb->work_lock);
> >       if (test_bit(WB_registered, &wb->state))
> >               queue_delayed_work(bdi_wq, &wb->dwork, timeout);
> > -     spin_unlock_bh(&wb->work_lock);
> > +     spin_unlock_irq(&wb->work_lock);
> >  }
> >
> >  static void wb_update_bandwidth_workfn(struct work_struct *work)
> > @@ -334,12 +334,12 @@ static void cgwb_remove_from_bdi_list(struct bdi_writeback *wb);
> >  static void wb_shutdown(struct bdi_writeback *wb)
> >  {
> >       /* Make sure nobody queues further work */
> > -     spin_lock_bh(&wb->work_lock);
> > +     spin_lock_irq(&wb->work_lock);
> >       if (!test_and_clear_bit(WB_registered, &wb->state)) {
> > -             spin_unlock_bh(&wb->work_lock);
> > +             spin_unlock_irq(&wb->work_lock);
> >               return;
> >       }
> > -     spin_unlock_bh(&wb->work_lock);
> > +     spin_unlock_irq(&wb->work_lock);
> >
> >       cgwb_remove_from_bdi_list(wb);
> >       /*
> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > index 55c2776ae699..3c34db15cf70 100644
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> > @@ -2867,6 +2867,7 @@ static void wb_inode_writeback_start(struct bdi_writeback *wb)
> >
> >  static void wb_inode_writeback_end(struct bdi_writeback *wb)
> >  {
> > +     unsigned long flags;
> >       atomic_dec(&wb->writeback_inodes);
> >       /*
> >        * Make sure estimate of writeback throughput gets updated after
> > @@ -2875,7 +2876,10 @@ static void wb_inode_writeback_end(struct bdi_writeback *wb)
> >        * that if multiple inodes end writeback at a similar time, they get
> >        * batched into one bandwidth update.
> >        */
> > -     queue_delayed_work(bdi_wq, &wb->bw_dwork, BANDWIDTH_INTERVAL);
> > +     spin_lock_irqsave(&wb->work_lock, flags);
> > +     if (test_bit(WB_registered, &wb->state))
> > +             queue_delayed_work(bdi_wq, &wb->bw_dwork, BANDWIDTH_INTERVAL);
> > +     spin_unlock_irqrestore(&wb->work_lock, flags);
> >  }
> >
> >  bool __folio_end_writeback(struct folio *folio)
> > --
> > 2.37.1.455.g008518b4e5-goog
> >
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR

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