[PATCH 06/11] bdi: Shutdown writeback on all cgwbs in cgwb_bdi_destroy()

[Jan Kara]

Currently we waited for all cgwbs to get freed in cgwb_bdi_destroy()
which also means that writeback has been shutdown on them. Since this
wait is going away, directly shutdown writeback on cgwbs from
cgwb_bdi_destroy() to avoid live writeback structures after
bdi_unregister() has finished. To make that safe with concurrent
shutdown from cgwb_release_workfn(), we also have to make sure
wb_shutdown() returns only after the bdi_writeback structure is really
shutdown.

Acked-by: Tejun Heo <t...@kernel.org>
Signed-off-by: Jan Kara <j...@suse.cz>
---
 include/linux/backing-dev-defs.h |  1 +
 mm/backing-dev.c                 | 22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 8fb3dcdebc80..8af720f22a2d 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -21,6 +21,7 @@ struct dentry;
  */
 enum wb_state {
        WB_registered,          /* bdi_register() was done */
+       WB_shutting_down,       /* wb_shutdown() in progress */
        WB_writeback_running,   /* Writeback is in progress */
        WB_has_dirty_io,        /* Dirty inodes on ->b_{dirty|io|more_io} */
 };
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index e3d56dba4da8..b67be4fc12c4 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -356,8 +356,15 @@ static void wb_shutdown(struct bdi_writeback *wb)
        spin_lock_bh(&wb->work_lock);
        if (!test_and_clear_bit(WB_registered, &wb->state)) {
                spin_unlock_bh(&wb->work_lock);
+               /*
+                * 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.
+                */
+               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);
@@ -369,6 +376,12 @@ 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));
+       /*
+        * Make sure bit gets cleared after shutdown is finished. Matches with
+        * the barrier provided by test_and_clear_bit() above.
+        */
+       smp_wmb();
+       clear_bit(WB_shutting_down, &wb->state);
 }
 
 static void wb_exit(struct bdi_writeback *wb)
@@ -699,12 +712,21 @@ static void cgwb_bdi_destroy(struct backing_dev_info *bdi)
 {
        struct radix_tree_iter iter;
        void **slot;
+       struct bdi_writeback *wb;
 
        WARN_ON(test_bit(WB_registered, &bdi->wb.state));
 
        spin_lock_irq(&cgwb_lock);
        radix_tree_for_each_slot(slot, &bdi->cgwb_tree, &iter, 0)
                cgwb_kill(*slot);
+
+       while (!list_empty(&bdi->wb_list)) {
+               wb = list_first_entry(&bdi->wb_list, struct bdi_writeback,
+                                     bdi_node);
+               spin_unlock_irq(&cgwb_lock);
+               wb_shutdown(wb);
+               spin_lock_irq(&cgwb_lock);
+       }
        spin_unlock_irq(&cgwb_lock);
 
        /*
-- 
2.10.2