dm-delay: fix bugs introduced by kthread mode

This commit fixes the following bugs introduced by commit 70bbeb29
("dm delay: for short delays, use kthread instead of timers and wq"):

* the function flush_worker_fn has no exit path - on unload, this
  function will just loop and consume 100% CPU without any progress

* the wake-up mechanism in flush_worker_fn is racy - a wake up will be
  missed if the process adds entries to the delayed_bios list just
  before set_current_state(TASK_INTERRUPTIBLE)

* flush_delayed_bios_fast submits a bio while holding a global mutex;
  this may deadlock if we have multiple stacked dm-delay devices and
  the underlying device attempts to acquire the mutex too

* if the target constructor fails, it will call delay_dtr. delay_dtr
  would attempt to free dc->timer_lock without it being initialized by
  the constructor.

* if the target constructor's kthread allocation fails, delay_dtr
  would crash trying to dereference dc->worker because it is non-NULL
  due to ERR_PTR.

Fixes: 70bbeb29 ("dm delay: for short delays, use kthread instead of timers and wq")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-delay.c

@@ -73,9 +73,23 @@ static inline bool delay_is_fast(struct delay_c *dc)
 	return !!dc->worker;
 }
 
+static void flush_bios(struct bio *bio)
+{
+	struct bio *n;
+
+	while (bio) {
+		n = bio->bi_next;
+		bio->bi_next = NULL;
+		dm_submit_bio_remap(bio, NULL);
+		bio = n;
+	}
+}
+
 static void flush_delayed_bios_fast(struct delay_c *dc, bool flush_all)
 {
 	struct dm_delay_info *delayed, *next;
+	struct bio_list flush_bio_list;
+	bio_list_init(&flush_bio_list);
 
 	mutex_lock(&delayed_bios_lock);
 	list_for_each_entry_safe(delayed, next, &dc->delayed_bios, list) {
@@ -83,47 +97,42 @@ static void flush_delayed_bios_fast(struct delay_c *dc, bool flush_all)
 			struct bio *bio = dm_bio_from_per_bio_data(delayed,
 						sizeof(struct dm_delay_info));
 			list_del(&delayed->list);
-			dm_submit_bio_remap(bio, NULL);
+			bio_list_add(&flush_bio_list, bio);
 			delayed->class->ops--;
 		}
 	}
 	mutex_unlock(&delayed_bios_lock);
+
+	flush_bios(bio_list_get(&flush_bio_list));
 }
 
 static int flush_worker_fn(void *data)
 {
 	struct delay_c *dc = data;
 
-	while (1) {
+	while (!kthread_should_stop()) {
 		flush_delayed_bios_fast(dc, false);
+		mutex_lock(&delayed_bios_lock);
 		if (unlikely(list_empty(&dc->delayed_bios))) {
 			set_current_state(TASK_INTERRUPTIBLE);
+			mutex_unlock(&delayed_bios_lock);
 			schedule();
-		} else
+		} else {
+			mutex_unlock(&delayed_bios_lock);
 			cond_resched();
+		}
 	}
 
 	return 0;
 }
 
-static void flush_bios(struct bio *bio)
-{
-	struct bio *n;
-
-	while (bio) {
-		n = bio->bi_next;
-		bio->bi_next = NULL;
-		dm_submit_bio_remap(bio, NULL);
-		bio = n;
-	}
-}
-
-static struct bio *flush_delayed_bios(struct delay_c *dc, bool flush_all)
+static void flush_delayed_bios(struct delay_c *dc, bool flush_all)
 {
 	struct dm_delay_info *delayed, *next;
 	unsigned long next_expires = 0;
 	unsigned long start_timer = 0;
-	struct bio_list flush_bios = { };
+	struct bio_list flush_bio_list;
+	bio_list_init(&flush_bio_list);
 
 	mutex_lock(&delayed_bios_lock);
 	list_for_each_entry_safe(delayed, next, &dc->delayed_bios, list) {
@@ -131,7 +140,7 @@ static struct bio *flush_delayed_bios(struct delay_c *dc, bool flush_all)
 			struct bio *bio = dm_bio_from_per_bio_data(delayed,
 						sizeof(struct dm_delay_info));
 			list_del(&delayed->list);
-			bio_list_add(&flush_bios, bio);
+			bio_list_add(&flush_bio_list, bio);
 			delayed->class->ops--;
 			continue;
 		}
@@ -147,7 +156,7 @@ static struct bio *flush_delayed_bios(struct delay_c *dc, bool flush_all)
 	if (start_timer)
 		queue_timeout(dc, next_expires);
 
-	return bio_list_get(&flush_bios);
+	flush_bios(bio_list_get(&flush_bio_list));
 }
 
 static void flush_expired_bios(struct work_struct *work)
@@ -158,7 +167,7 @@ static void flush_expired_bios(struct work_struct *work)
 	if (delay_is_fast(dc))
 		flush_delayed_bios_fast(dc, false);
 	else
-		flush_bios(flush_delayed_bios(dc, false));
+		flush_delayed_bios(dc, false);
 }
 
 static void delay_dtr(struct dm_target *ti)
@@ -177,8 +186,7 @@ static void delay_dtr(struct dm_target *ti)
 	if (dc->worker)
 		kthread_stop(dc->worker);
 
-	if (!delay_is_fast(dc))
-		mutex_destroy(&dc->timer_lock);
+	mutex_destroy(&dc->timer_lock);
 
 	kfree(dc);
 }
@@ -236,6 +244,7 @@ static int delay_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 
 	ti->private = dc;
 	INIT_LIST_HEAD(&dc->delayed_bios);
+	mutex_init(&dc->timer_lock);
 	dc->may_delay = true;
 	dc->argc = argc;
 
@@ -282,12 +291,12 @@ static int delay_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 					    "dm-delay-flush-worker");
 		if (IS_ERR(dc->worker)) {
 			ret = PTR_ERR(dc->worker);
+			dc->worker = NULL;
 			goto bad;
 		}
 	} else {
 		timer_setup(&dc->delay_timer, handle_delayed_timer, 0);
 		INIT_WORK(&dc->flush_expired_bios, flush_expired_bios);
-		mutex_init(&dc->timer_lock);
 		dc->kdelayd_wq = alloc_workqueue("kdelayd", WQ_MEM_RECLAIM, 0);
 		if (!dc->kdelayd_wq) {
 			ret = -EINVAL;
@@ -345,11 +354,11 @@ static void delay_presuspend(struct dm_target *ti)
 	dc->may_delay = false;
 	mutex_unlock(&delayed_bios_lock);
 
-	if (delay_is_fast(dc))
+	if (delay_is_fast(dc)) {
 		flush_delayed_bios_fast(dc, true);
-	else {
+	} else {
 		del_timer_sync(&dc->delay_timer);
-		flush_bios(flush_delayed_bios(dc, true));
+		flush_delayed_bios(dc, true);
 	}
 }