Merge tag 'block-5.16-2021-12-17' of git://git.kernel.dk/linux-block

Pull block fixes from Jens Axboe:

 - Fix for hammering on the delayed run queue timer (me)

 - bcache regression fix for this merge window (Lin)

 - Fix a divide-by-zero in the blk-iocost code (Tejun)

* tag 'block-5.16-2021-12-17' of git://git.kernel.dk/linux-block:
  bcache: fix NULL pointer reference in cached_dev_detach_finish
  block: reduce kblockd_mod_delayed_work_on() CPU consumption
  iocost: Fix divide-by-zero on donation from low hweight cgroup

Author: torvalds

block/blk-core.c

@@ -1484,6 +1484,8 @@ EXPORT_SYMBOL(kblockd_schedule_work);
 int kblockd_mod_delayed_work_on(int cpu, struct delayed_work *dwork,
 				unsigned long delay)
 {
+	if (!delay)
+		return queue_work_on(cpu, kblockd_workqueue, &dwork->work);
 	return mod_delayed_work_on(cpu, kblockd_workqueue, dwork, delay);
 }
 EXPORT_SYMBOL(kblockd_mod_delayed_work_on);

block/blk-iocost.c

@@ -2311,7 +2311,14 @@ static void ioc_timer_fn(struct timer_list *timer)
 			hwm = current_hweight_max(iocg);
 			new_hwi = hweight_after_donation(iocg, old_hwi, hwm,
 							 usage, &now);
-			if (new_hwi < hwm) {
+			/*
+			 * Donation calculation assumes hweight_after_donation
+			 * to be positive, a condition that a donor w/ hwa < 2
+			 * can't meet. Don't bother with donation if hwa is
+			 * below 2. It's not gonna make a meaningful difference
+			 * anyway.
+			 */
+			if (new_hwi < hwm && hwa >= 2) {
 				iocg->hweight_donating = hwa;
 				iocg->hweight_after_donation = new_hwi;
 				list_add(&iocg->surplus_list, &surpluses);

drivers/md/bcache/super.c

@@ -1139,6 +1139,7 @@ static void cancel_writeback_rate_update_dwork(struct cached_dev *dc)
 static void cached_dev_detach_finish(struct work_struct *w)
 {
 	struct cached_dev *dc = container_of(w, struct cached_dev, detach);
+	struct cache_set *c = dc->disk.c;
 
 	BUG_ON(!test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags));
 	BUG_ON(refcount_read(&dc->count));
@@ -1156,7 +1157,7 @@ static void cached_dev_detach_finish(struct work_struct *w)
 
 	bcache_device_detach(&dc->disk);
 	list_move(&dc->list, &uncached_devices);
-	calc_cached_dev_sectors(dc->disk.c);
+	calc_cached_dev_sectors(c);
 
 	clear_bit(BCACHE_DEV_DETACHING, &dc->disk.flags);
 	clear_bit(BCACHE_DEV_UNLINK_DONE, &dc->disk.flags);