dm-delay: change locking to avoid contention

The delayed_bios list is protected by one mutex shared by all dm-delay
devices. This mutex must be held whenever a bio is added or expired bios
are removed from the list.  Since a large number of expired bios could
be on the list, flush_delayed_bios() can schedule while holding the
mutex. This means a flush_delayed_bios() call on any dm-delay device can
slow down delay_map() calls on any other dm-delay device.

To keep dm-delay devices from slowing each other down and keep
processing delay bios from slowing adding delayed bios, the global mutex
has been removed, and each dm-delay device now has two locks.
delayed_bios_lock is a spinlock that must be held whenever the
delayed_bios list is accessed. process_bios_lock is a mutex that must be
held whenever a process has temporarily pulled bios off the delayed_bios
list to check which ones should be processed. It must be held until all
the bios that won't be processed are returned to the list. This is what
flush_delayed_bios() now does. The mutex is necessary to guarantee that
delay_presuspend() sees the entire list of delayed bios when it calls
flush_delayed_bios().

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

Author: bmarzins

drivers/md/dm-delay.c

@@ -29,6 +29,8 @@ struct delay_class {
 struct delay_c {
 	struct timer_list delay_timer;
 	struct mutex timer_lock;
+	struct mutex process_bios_lock; /* hold while removing bios to be processed from list */
+	spinlock_t delayed_bios_lock; /* hold on all accesses to delayed_bios list */
 	struct workqueue_struct *kdelayd_wq;
 	struct work_struct flush_expired_bios;
 	struct list_head delayed_bios;
@@ -49,8 +51,6 @@ struct dm_delay_info {
 	unsigned long expires;
 };
 
-static DEFINE_MUTEX(delayed_bios_lock);
-
 static void handle_delayed_timer(struct timer_list *t)
 {
 	struct delay_c *dc = from_timer(dc, t, delay_timer);
@@ -89,12 +89,16 @@ static void flush_delayed_bios(struct delay_c *dc, bool flush_all)
 {
 	struct dm_delay_info *delayed, *next;
 	struct bio_list flush_bio_list;
+	LIST_HEAD(local_list);
 	unsigned long next_expires = 0;
 	bool start_timer = false;
 	bio_list_init(&flush_bio_list);
 
-	mutex_lock(&delayed_bios_lock);
-	list_for_each_entry_safe(delayed, next, &dc->delayed_bios, list) {
+	mutex_lock(&dc->process_bios_lock);
+	spin_lock(&dc->delayed_bios_lock);
+	list_replace_init(&dc->delayed_bios, &local_list);
+	spin_unlock(&dc->delayed_bios_lock);
+	list_for_each_entry_safe(delayed, next, &local_list, list) {
 		cond_resched();
 		if (flush_all || time_after_eq(jiffies, delayed->expires)) {
 			struct bio *bio = dm_bio_from_per_bio_data(delayed,
@@ -114,7 +118,10 @@ static void flush_delayed_bios(struct delay_c *dc, bool flush_all)
 			}
 		}
 	}
-	mutex_unlock(&delayed_bios_lock);
+	spin_lock(&dc->delayed_bios_lock);
+	list_splice(&local_list, &dc->delayed_bios);
+	spin_unlock(&dc->delayed_bios_lock);
+	mutex_unlock(&dc->process_bios_lock);
 
 	if (start_timer)
 		queue_timeout(dc, next_expires);
@@ -128,13 +135,13 @@ static int flush_worker_fn(void *data)
 
 	while (!kthread_should_stop()) {
 		flush_delayed_bios(dc, false);
-		mutex_lock(&delayed_bios_lock);
+		spin_lock(&dc->delayed_bios_lock);
 		if (unlikely(list_empty(&dc->delayed_bios))) {
 			set_current_state(TASK_INTERRUPTIBLE);
-			mutex_unlock(&delayed_bios_lock);
+			spin_unlock(&dc->delayed_bios_lock);
 			schedule();
 		} else {
-			mutex_unlock(&delayed_bios_lock);
+			spin_unlock(&dc->delayed_bios_lock);
 			cond_resched();
 		}
 	}
@@ -168,6 +175,7 @@ static void delay_dtr(struct dm_target *ti)
 	if (dc->worker)
 		kthread_stop(dc->worker);
 
+	mutex_destroy(&dc->process_bios_lock);
 	mutex_destroy(&dc->timer_lock);
 
 	kfree(dc);
@@ -227,6 +235,8 @@ 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);
+	mutex_init(&dc->process_bios_lock);
+	spin_lock_init(&dc->delayed_bios_lock);
 	dc->may_delay = true;
 	dc->argc = argc;
 
@@ -309,14 +319,14 @@ static int delay_bio(struct delay_c *dc, struct delay_class *c, struct bio *bio)
 	delayed->context = dc;
 	delayed->expires = expires = jiffies + msecs_to_jiffies(c->delay);
 
-	mutex_lock(&delayed_bios_lock);
+	spin_lock(&dc->delayed_bios_lock);
 	if (unlikely(!dc->may_delay)) {
-		mutex_unlock(&delayed_bios_lock);
+		spin_unlock(&dc->delayed_bios_lock);
 		return DM_MAPIO_REMAPPED;
 	}
 	c->ops++;
 	list_add_tail(&delayed->list, &dc->delayed_bios);
-	mutex_unlock(&delayed_bios_lock);
+	spin_unlock(&dc->delayed_bios_lock);
 
 	if (delay_is_fast(dc))
 		wake_up_process(dc->worker);
@@ -330,9 +340,9 @@ static void delay_presuspend(struct dm_target *ti)
 {
 	struct delay_c *dc = ti->private;
 
-	mutex_lock(&delayed_bios_lock);
+	spin_lock(&dc->delayed_bios_lock);
 	dc->may_delay = false;
-	mutex_unlock(&delayed_bios_lock);
+	spin_unlock(&dc->delayed_bios_lock);
 
 	if (!delay_is_fast(dc))
 		timer_delete(&dc->delay_timer);