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Subject[RFC][PATCH 04/16] writeback: fix possible race when shutting down bdi
Date
From: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

Current bdi code has the following race between 'bdi_wb_shutdown()'
and 'bdi_forker_thread()'.

Initial condition: BDI_pending is cleaned, bdi has no writeback thread,
because it was inactive and exited, 'bdi_wb_shutdown()' and
'bdi_forker_thread()' are executed concurrently.

1. bdi_wb_shutdown() executes wait_on_bit(), tests the BDI_pending bit,
it is clean, so it does not wait for anything.

2. 'bdi_forker_thread()' takes the 'bdi_lock', finds out that bdi has
work to do, takes it out of the 'bdi_list', sets the BDI_pending flag,
unlocks the 'bdi_lock' lock

3. 'bdi_wb_shutdown()' takes the lock, and nasty things start happening:
a) it removes the bdi from bdi->bdi_list, but the bdi is not in any
list
b) it starts deleting the bdi, but 'bdi_forker_thread()' is still working
with it.

Note, it is very difficult to hit this race, and I never observed it, so it
is quite theoretical, but it is still a race. Also note, this race exist
without my previous clean-ups as well.

This patch fixes this race by making 'bdi_wb_shutdown()' first search for
the bdi in the 'bdi_list', and only if it is there, remove it from 'bdi_list'
and destroy. But if it is not there, assume it is in transit and re-try
waiting on the BDI_pending bit.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
---
mm/backing-dev.c | 41 ++++++++++++++++++++++++++++-------------
1 files changed, 28 insertions(+), 13 deletions(-)
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index b34c12a..a445ff0 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -456,15 +456,26 @@ void static bdi_add_default_flusher_thread(struct backing_dev_info *bdi)
}

/*
- * Remove bdi from bdi_list, and ensure that it is no longer visible
+ * Look up for bdi in the bdi_list. If found, remove it, ensure that it is
+ * no longer visible, and return 0. If not found, return 1.
*/
-static void bdi_remove_from_list(struct backing_dev_info *bdi)
+static int bdi_remove_from_list(struct backing_dev_info *me)
{
+ struct backing_dev_info *bdi;
+
spin_lock_bh(&bdi_lock);
- list_del_rcu(&bdi->bdi_list);
+ list_for_each_entry(bdi, &bdi_list, bdi_list) {
+ if (bdi == me) {
+ list_del_rcu(&me->bdi_list);
+ spin_unlock_bh(&bdi_lock);
+ synchronize_rcu();
+ return 0;
+ }
+
+ }
spin_unlock_bh(&bdi_lock);
+ return 1;

- synchronize_rcu();
}

int bdi_register(struct backing_dev_info *bdi, struct device *parent,
@@ -532,16 +543,20 @@ static void bdi_wb_shutdown(struct backing_dev_info *bdi)
if (!bdi_cap_writeback_dirty(bdi))
return;

- /*
- * If setup is pending, wait for that to complete first
- */
- wait_on_bit(&bdi->state, BDI_pending, bdi_sched_wait,
- TASK_UNINTERRUPTIBLE);
+ do {
+ /*
+ * If setup is pending, wait for that to complete first
+ */
+ wait_on_bit(&bdi->state, BDI_pending, bdi_sched_wait,
+ TASK_UNINTERRUPTIBLE);

- /*
- * Make sure nobody finds us on the bdi_list anymore
- */
- bdi_remove_from_list(bdi);
+ /*
+ * Make sure nobody finds us on the bdi_list anymore. However,
+ * bdi may be temporary be not in the bdi_list but be in transit
+ * in bdi_forker_thread. Namely, this may happen if we race
+ * with the forker thread.
+ */
+ } while (bdi_remove_from_list(bdi));

/*
* Finally, kill the kernel thread. We don't need to be RCU
--
1.7.1.1


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