backing-dev: fix wakeup timer races with bdi_unregister()

While 7a401a9 ("backing-dev: ensure wakeup_timer is deleted") addressed the problem of the bdi being freed with a queued wakeup timer, there are other races that could happen if the wakeup timer expires after/during bdi_unregister(), before bdi_destroy() is called. wakeup_timer_fn() could attempt to wakeup a task which has already has been freed, or could access a NULL bdi->dev via the wake_forker_thread tracepoint. Cc: <[email protected]> Cc: Jens Axboe <[email protected]> Reported-by: Chanho Min <[email protected]> Reviewed-by: Namjae Jeon <[email protected]> Signed-off-by: Rabin Vincent <[email protected]> Signed-off-by: Wu Fengguang <[email protected]>
kyagi · Feb 1, 2012 · 2673b4c · 2673b4c
1 parent 62aa2b5
commit 2673b4c
Showing 1 changed file with 18 additions and 5 deletions.
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
@@ -318,7 +318,7 @@ static void wakeup_timer_fn(unsigned long data)
 	if (bdi->wb.task) {
 		trace_writeback_wake_thread(bdi);
 		wake_up_process(bdi->wb.task);
-	} else {
+	} else if (bdi->dev) {
 		/*
 		 * When bdi tasks are inactive for long time, they are killed.
 		 * In this case we have to wake-up the forker thread which
@@ -584,6 +584,8 @@ EXPORT_SYMBOL(bdi_register_dev);
  */
 static void bdi_wb_shutdown(struct backing_dev_info *bdi)
 {
+	struct task_struct *task;
+
 	if (!bdi_cap_writeback_dirty(bdi))
 		return;
 
@@ -602,8 +604,13 @@ static void bdi_wb_shutdown(struct backing_dev_info *bdi)
 	 * Finally, kill the kernel thread. We don't need to be RCU
 	 * safe anymore, since the bdi is gone from visibility.
 	 */
-	if (bdi->wb.task)
-		kthread_stop(bdi->wb.task);
+	spin_lock_bh(&bdi->wb_lock);
+	task = bdi->wb.task;
+	bdi->wb.task = NULL;
+	spin_unlock_bh(&bdi->wb_lock);
+
+	if (task)
+		kthread_stop(task);
 }
 
 /*
@@ -623,7 +630,9 @@ static void bdi_prune_sb(struct backing_dev_info *bdi)
 
 void bdi_unregister(struct backing_dev_info *bdi)
 {
-	if (bdi->dev) {
+	struct device *dev = bdi->dev;
+
+	if (dev) {
 		bdi_set_min_ratio(bdi, 0);
 		trace_writeback_bdi_unregister(bdi);
 		bdi_prune_sb(bdi);
@@ -632,8 +641,12 @@ void bdi_unregister(struct backing_dev_info *bdi)
 		if (!bdi_cap_flush_forker(bdi))
 			bdi_wb_shutdown(bdi);
 		bdi_debug_unregister(bdi);
-		device_unregister(bdi->dev);
+
+		spin_lock_bh(&bdi->wb_lock);
 		bdi->dev = NULL;
+		spin_unlock_bh(&bdi->wb_lock);
+
+		device_unregister(dev);
 	}
 }
 EXPORT_SYMBOL(bdi_unregister);