bdi: Shutdown writeback on all cgwbs in cgwb_bdi_destroy()

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 <tj@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 22 insertions, 0 deletions

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);
 
         /*