Re: [PATCH] bdi: Fix another oops in wb_workfn()

Tetsuo Handa
2018-06-13T10:43:47Z

On 2018/06/13 0:57, Jan Kara wrote:
> On Mon 11-06-18 10:20:53, Tejun Heo wrote:
>> Hello,
>>
>> On Mon, Jun 11, 2018 at 06:29:20PM +0200, Jan Kara wrote:
>>>> Would something like the following work or am I missing the point
>>>> entirely?
>>>
>>> I was pondering the same solution for a while but I think it won't work.
>>> The problem is that e.g. wb_memcg_offline() could have already removed
>>> wb from the radix tree but it is still pending in bdi->wb_list
>>> (wb_shutdown() has not run yet) and so we'd drop reference we didn't get.
>>
>> Yeah, right, so the root cause is that we're walking the wb_list while
>> holding lock and expecting the object to stay there even after lock is
>> released. Hmm... we can use a mutex to synchronize the two
>> destruction paths. It's not like they're hot paths anyway.
>
> Hmm, do you mean like having a per-bdi or even a global mutex that would
> protect whole wb_shutdown()? Yes, that should work and we could get rid of
> WB_shutting_down bit as well with that. Just it seems a bit strange to
> introduce a mutex only to synchronize these two shutdown paths - usually
> locks protect data structures and in this case we have cgwb_lock for
> that so it looks like a duplication from a first look.
>

Can't we utilize RCU grace period (like shown below) ?

If wb_shutdown(wb) by cgwb_release_workfn() was faster than wb_shutdown(wb) by cgwb_bdi_unregister():

cgwb_bdi_unregister(bdi) cgwb_release_workfn(work)

wb = container_of(work, struct bdi_writeback, release_work);
spin_lock_irq(&cgwb_lock);
wb = list_first_entry(&bdi->wb_list, struct bdi_writeback, bdi_node); /* Same wb here */
rcu_read_lock(); /* Prevent kfree_rcu() from invoking kfree() */
spin_unlock_irq(&cgwb_lock);
wb_shutdown(wb);
spin_lock_bh(&wb->work_lock);
!test_and_clear_bit(WB_registered, &wb->state) is "false".
set_bit(WB_shutting_down, &wb->state);
spin_unlock_bh(&wb->work_lock);
mod_delayed_work(bdi_wq, &wb->dwork, 0);
flush_delayed_work(&wb->dwork);
cgwb_remove_from_bdi_list(wb);
spin_lock_irq(&cgwb_lock);
list_del_rcu(&wb->bdi_node);
spin_unlock_irq(&cgwb_lock);
wb_exit(wb);
kfree_rcu(wb, rcu); /* Won't call kfree() because of rcu_read_lock() */
wb_shutdown(wb);
spin_lock_bh(&wb->work_lock); /* Safe to access because kfree() cannot be called */
!test_and_clear_bit(WB_registered, &wb->state) is "true".
spin_unlock_bh(&wb->work_lock);
rcu_read_unlock();
kfree(wb);
schedule_timeout_uninterruptible(HZ / 10); /* Try to wait in case list_del_rcu() is not yet called */
spin_lock_irq(&cgwb_lock);
wb = list_first_entry(&bdi->wb_list, struct bdi_writeback, bdi_node); /* Different wb if list_del_rcu() was already called, same wb otherwise */
rcu_read_lock(); /* Prevent kfree_rcu() from invoking kfree() if still same wb here */
spin_unlock_irq(&cgwb_lock);

If wb_shutdown(wb) by cgwb_bdi_unregister() was faster than wb_shutdown(wb) by cgwb_release_workfn():

cgwb_bdi_unregister(bdi) cgwb_release_workfn(work)

wb = container_of(work, struct bdi_writeback, release_work);
spin_lock_irq(&cgwb_lock);
wb = list_first_entry(&bdi->wb_list, struct bdi_writeback, bdi_node); /* Same wb here */
rcu_read_lock();
spin_unlock_irq(&cgwb_lock);
wb_shutdown(wb);
spin_lock_bh(&wb->work_lock);
!test_and_clear_bit(WB_registered, &wb->state) is "false".
set_bit(WB_shutting_down, &wb->state);
spin_unlock_bh(&wb->work_lock);
rcu_read_unlock();
mod_delayed_work(bdi_wq, &wb->dwork, 0);
flush_delayed_work(&wb->dwork);
cgwb_remove_from_bdi_list(wb);
spin_lock_irq(&cgwb_lock);
list_del_rcu(&wb->bdi_node);
spin_unlock_irq(&cgwb_lock);
spin_lock_irq(&cgwb_lock);
wb = list_first_entry(&bdi->wb_list, struct bdi_writeback, bdi_node); /* Different wb here */
rcu_read_lock();
spin_unlock_irq(&cgwb_lock);
wb_shutdown(wb);
spin_lock_bh(&wb->work_lock); /* Safe to access because kfree() cannot be called */
!test_and_clear_bit(WB_registered, &wb->state) is "true".
spin_unlock_bh(&wb->work_lock);
wb_exit(wb);
kfree_rcu(wb, rcu);

mm/backing-dev.c | 24 +++++++++++++++++++-----
1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 347cc83..6886cea 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -353,13 +353,24 @@ static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi,
/*
* Remove bdi from the global list and shutdown any threads we have running
*/
-static void wb_shutdown(struct bdi_writeback *wb)
+static void wb_shutdown(struct bdi_writeback *wb, const bool in_rcu)
{
/* Make sure nobody queues further work */
spin_lock_bh(&wb->work_lock);
if (!test_and_clear_bit(WB_registered, &wb->state)) {
spin_unlock_bh(&wb->work_lock);
/*
+ * Try to wait for cgwb_remove_from_bdi_list() without
+ * using wait_on_bit(), for kfree_rcu(wb, rcu) from
+ * cgwb_release_workfn() might invoke kfree() before we
+ * recheck WB_shutting_down bit.
+ */
+ if (in_rcu) {
+ rcu_read_unlock();
+ schedule_timeout_uninterruptible(HZ / 10);
+ return;
+ }
+ /*
* Wait for wb shutdown to finish if someone else is just
* running wb_shutdown(). Otherwise we could proceed to wb /
* bdi destruction before wb_shutdown() is finished.
@@ -369,8 +380,9 @@ static void wb_shutdown(struct bdi_writeback *wb)
}
set_bit(WB_shutting_down, &wb->state);
spin_unlock_bh(&wb->work_lock);
+ if (in_rcu)
+ rcu_read_unlock();

- cgwb_remove_from_bdi_list(wb);
/*
* Drain work list and shutdown the delayed_work. !WB_registered
* tells wb_workfn() that @wb is dying and its work_list needs to
@@ -379,6 +391,7 @@ static void wb_shutdown(struct bdi_writeback *wb)
mod_delayed_work(bdi_wq, &wb->dwork, 0);
flush_delayed_work(&wb->dwork);
WARN_ON(!list_empty(&wb->work_list));
+ cgwb_remove_from_bdi_list(wb);
/*
* Make sure bit gets cleared after shutdown is finished. Matches with
* the barrier provided by test_and_clear_bit() above.
@@ -508,7 +521,7 @@ static void cgwb_release_workfn(struct work_struct *work)
struct bdi_writeback *wb = container_of(work, struct bdi_writeback,
release_work);

- wb_shutdown(wb);
+ wb_shutdown(wb, false);

css_put(wb->memcg_css);
css_put(wb->blkcg_css);
@@ -721,8 +734,9 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
while (!list_empty(&bdi->wb_list)) {
wb = list_first_entry(&bdi->wb_list, struct bdi_writeback,
bdi_node);
+ rcu_read_lock();
spin_unlock_irq(&cgwb_lock);
- wb_shutdown(wb);
+ wb_shutdown(wb, true);
spin_lock_irq(&cgwb_lock);
}
spin_unlock_irq(&cgwb_lock);
@@ -944,7 +958,7 @@ void bdi_unregister(struct backing_dev_info *bdi)
{
/* make sure nobody finds us on the bdi_list anymore */
bdi_remove_from_list(bdi);
- wb_shutdown(&bdi->wb);
+ wb_shutdown(&bdi->wb, false);
cgwb_bdi_unregister(bdi);

if (bdi->dev) {
--

Or, equivalent one but easier to read:

mm/backing-dev.c | 42 +++++++++++++++++++++++++++++++-----------
1 file changed, 31 insertions(+), 11 deletions(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 347cc83..77088a3 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -350,15 +350,25 @@ static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi,

static void cgwb_remove_from_bdi_list(struct bdi_writeback *wb);

-/*
- * Remove bdi from the global list and shutdown any threads we have running
- */
-static void wb_shutdown(struct bdi_writeback *wb)
+static bool wb_start_shutdown(struct bdi_writeback *wb)
{
/* Make sure nobody queues further work */
spin_lock_bh(&wb->work_lock);
if (!test_and_clear_bit(WB_registered, &wb->state)) {
spin_unlock_bh(&wb->work_lock);
+ return false;
+ }
+ set_bit(WB_shutting_down, &wb->state);
+ spin_unlock_bh(&wb->work_lock);
+ return true;
+}
+
+/*
+ * Remove bdi from the global list and shutdown any threads we have running
+ */
+static void wb_shutdown(struct bdi_writeback *wb, const bool started)
+{
+ if (!started && !wb_start_shutdown(wb)) {
/*
* Wait for wb shutdown to finish if someone else is just
* running wb_shutdown(). Otherwise we could proceed to wb /
@@ -367,10 +377,6 @@ static void wb_shutdown(struct bdi_writeback *wb)
wait_on_bit(&wb->state, WB_shutting_down, TASK_UNINTERRUPTIBLE);
return;
}
- set_bit(WB_shutting_down, &wb->state);
- spin_unlock_bh(&wb->work_lock);
-
- cgwb_remove_from_bdi_list(wb);
/*
* Drain work list and shutdown the delayed_work. !WB_registered
* tells wb_workfn() that @wb is dying and its work_list needs to
@@ -379,6 +385,7 @@ static void wb_shutdown(struct bdi_writeback *wb)
mod_delayed_work(bdi_wq, &wb->dwork, 0);
flush_delayed_work(&wb->dwork);
WARN_ON(!list_empty(&wb->work_list));
+ cgwb_remove_from_bdi_list(wb);
/*
* Make sure bit gets cleared after shutdown is finished. Matches with
* the barrier provided by test_and_clear_bit() above.
@@ -508,7 +515,7 @@ static void cgwb_release_workfn(struct work_struct *work)
struct bdi_writeback *wb = container_of(work, struct bdi_writeback,
release_work);

- wb_shutdown(wb);
+ wb_shutdown(wb, false);

css_put(wb->memcg_css);
css_put(wb->blkcg_css);
@@ -711,6 +718,7 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
struct radix_tree_iter iter;
void **slot;
struct bdi_writeback *wb;
+ bool started;

WARN_ON(test_bit(WB_registered, &bdi->wb.state));

@@ -721,8 +729,20 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
while (!list_empty(&bdi->wb_list)) {
wb = list_first_entry(&bdi->wb_list, struct bdi_writeback,
bdi_node);
+ rcu_read_lock();
spin_unlock_irq(&cgwb_lock);
- wb_shutdown(wb);
+ started = wb_start_shutdown(wb);
+ rcu_read_unlock();
+ if (started)
+ wb_shutdown(wb, true);
+ else
+ /*
+ * Try to wait for cgwb_remove_from_bdi_list() without
+ * using wait_on_bit(), for kfree_rcu(wb, rcu) from
+ * cgwb_release_workfn() might invoke kfree() before we
+ * recheck WB_shutting_down bit.
+ */
+ schedule_timeout_uninterruptible(HZ / 10);
spin_lock_irq(&cgwb_lock);
}
spin_unlock_irq(&cgwb_lock);
@@ -944,7 +964,7 @@ void bdi_unregister(struct backing_dev_info *bdi)
{
/* make sure nobody finds us on the bdi_list anymore */
bdi_remove_from_list(bdi);
- wb_shutdown(&bdi->wb);
+ wb_shutdown(&bdi->wb, false);
cgwb_bdi_unregister(bdi);

if (bdi->dev) {
--

Or, toggling a dedicated flag using test_and_change_bit():

include/linux/backing-dev-defs.h | 1 +
mm/backing-dev.c | 8 +++++++-
2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 0bd432a..93ff83c 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -26,6 +26,7 @@ enum wb_state {
WB_writeback_running, /* Writeback is in progress */
WB_has_dirty_io, /* Dirty inodes on ->b_{dirty|io|more_io} */
WB_start_all, /* nr_pages == 0 (all) work pending */
+ WB_postpone_kfree, /* cgwb_bdi_unregister() will access later */
};

enum wb_congested_state {
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 347cc83..422d7a7 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -516,7 +516,10 @@ static void cgwb_release_workfn(struct work_struct *work)
fprop_local_destroy_percpu(&wb->memcg_completions);
percpu_ref_exit(&wb->refcnt);
wb_exit(wb);
- kfree_rcu(wb, rcu);
+ spin_lock_irq(&cgwb_lock);
+ if (!test_and_change_bit(WB_postpone_kfree, &wb->state))
+ kfree_rcu(wb, rcu);
+ spin_unlock_irq(&cgwb_lock);
}

static void cgwb_release(struct percpu_ref *refcnt)
@@ -721,9 +724,12 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
while (!list_empty(&bdi->wb_list)) {
wb = list_first_entry(&bdi->wb_list, struct bdi_writeback,
bdi_node);
+ set_bit(WB_postpone_kfree, &wb->state);
spin_unlock_irq(&cgwb_lock);
wb_shutdown(wb);
spin_lock_irq(&cgwb_lock);
+ if (!test_and_change_bit(WB_postpone_kfree, &wb->state))
+ kfree_rcu(wb, rcu);
}
spin_unlock_irq(&cgwb_lock);
}
--