[PATCH v2 4/4] Drivers: hv: balloon: replace ha_region_mutex with spinlock

[Vitaly Kuznetsov]

lockdep reports possible circular locking dependency when udev is used
for memory onlining:

 systemd-udevd/3996 is trying to acquire lock:
  ((memory_chain).rwsem){++++.+}, at: [<ffffffff810d137e>] __blocking_notifier_call_chain+0x4e/0xc0

 but task is already holding lock:
  (&dm_device.ha_region_mutex){+.+.+.}, at: [<ffffffffa015382e>] hv_memory_notifier+0x5e/0xc0 [hv_balloon]
 ...

which is probably a false positive because we take and release
ha_region_mutex from memory notifier chain depending on the arg. No real
deadlocks were reported so far (though I'm not really sure about
preemptible kernels...) but we don't really need to hold the mutex
for so long. We use it to protect ha_region_list (and its members) and the
num_pages_onlined counter. None of these operations require us to sleep
and nothing is slow, switch to using spinlock with interrupts disabled.

While on it, replace list_for_each -> list_for_each_entry as we actually
need entries in all these cases, drop meaningless list_empty() checks.

Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
 drivers/hv/hv_balloon.c | 96 ++++++++++++++++++++++++++-----------------------
 1 file changed, 52 insertions(+), 44 deletions(-)

diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
index a8b8b9a..19498f4 100644
--- a/drivers/hv/hv_balloon.c
+++ b/drivers/hv/hv_balloon.c
@@ -547,7 +547,11 @@ struct hv_dynmem_device {
 	 */
 	struct task_struct *thread;
 
-	struct mutex ha_region_mutex;
+	/*
+	 * Protects ha_region_list, num_pages_onlined counter and individual
+	 * regions from ha_region_list.
+	 */
+	spinlock_t ha_lock;
 
 	/*
 	 * A list of hot-add regions.
@@ -571,18 +575,14 @@ static int hv_memory_notifier(struct notifier_block *nb, unsigned long val,
 			      void *v)
 {
 	struct memory_notify *mem = (struct memory_notify *)v;
+	unsigned long flags;
 
 	switch (val) {
-	case MEM_GOING_ONLINE:
-		mutex_lock(&dm_device.ha_region_mutex);
-		break;
-
 	case MEM_ONLINE:
+		spin_lock_irqsave(&dm_device.ha_lock, flags);
 		dm_device.num_pages_onlined += mem->nr_pages;
+		spin_unlock_irqrestore(&dm_device.ha_lock, flags);
 	case MEM_CANCEL_ONLINE:
-		if (val == MEM_ONLINE ||
-		    mutex_is_locked(&dm_device.ha_region_mutex))
-			mutex_unlock(&dm_device.ha_region_mutex);
 		if (dm_device.ha_waiting) {
 			dm_device.ha_waiting = false;
 			complete(&dm_device.ol_waitevent);
@@ -590,10 +590,11 @@ static int hv_memory_notifier(struct notifier_block *nb, unsigned long val,
 		break;
 
 	case MEM_OFFLINE:
-		mutex_lock(&dm_device.ha_region_mutex);
+		spin_lock_irqsave(&dm_device.ha_lock, flags);
 		dm_device.num_pages_onlined -= mem->nr_pages;
-		mutex_unlock(&dm_device.ha_region_mutex);
+		spin_unlock_irqrestore(&dm_device.ha_lock, flags);
 		break;
+	case MEM_GOING_ONLINE:
 	case MEM_GOING_OFFLINE:
 	case MEM_CANCEL_OFFLINE:
 		break;
@@ -629,9 +630,12 @@ static void hv_mem_hot_add(unsigned long start, unsigned long size,
 	unsigned long start_pfn;
 	unsigned long processed_pfn;
 	unsigned long total_pfn = pfn_count;
+	unsigned long flags;
 
 	for (i = 0; i < (size/HA_CHUNK); i++) {
 		start_pfn = start + (i * HA_CHUNK);
+
+		spin_lock_irqsave(&dm_device.ha_lock, flags);
 		has->ha_end_pfn +=  HA_CHUNK;
 
 		if (total_pfn > HA_CHUNK) {
@@ -643,11 +647,11 @@ static void hv_mem_hot_add(unsigned long start, unsigned long size,
 		}
 
 		has->covered_end_pfn +=  processed_pfn;
+		spin_unlock_irqrestore(&dm_device.ha_lock, flags);
 
 		init_completion(&dm_device.ol_waitevent);
 		dm_device.ha_waiting = !memhp_auto_online;
 
-		mutex_unlock(&dm_device.ha_region_mutex);
 		nid = memory_add_physaddr_to_nid(PFN_PHYS(start_pfn));
 		ret = add_memory(nid, PFN_PHYS((start_pfn)),
 				(HA_CHUNK << PAGE_SHIFT));
@@ -664,9 +668,10 @@ static void hv_mem_hot_add(unsigned long start, unsigned long size,
 				 */
 				do_hot_add = false;
 			}
+			spin_lock_irqsave(&dm_device.ha_lock, flags);
 			has->ha_end_pfn -= HA_CHUNK;
 			has->covered_end_pfn -=  processed_pfn;
-			mutex_lock(&dm_device.ha_region_mutex);
+			spin_unlock_irqrestore(&dm_device.ha_lock, flags);
 			break;
 		}
 
@@ -679,7 +684,6 @@ static void hv_mem_hot_add(unsigned long start, unsigned long size,
 		 */
 		if (dm_device.ha_waiting)
 			wait_for_completion_timeout(&dm_device.ol_waitevent, 5*HZ);
-		mutex_lock(&dm_device.ha_region_mutex);
 		post_status(&dm_device);
 	}
 
@@ -693,9 +697,10 @@ static void hv_online_page(struct page *pg)
 	unsigned long cur_start_pgp;
 	unsigned long cur_end_pgp;
 	bool is_gap = false;
+	unsigned long flags;
 
-	list_for_each(cur, &dm_device.ha_region_list) {
-		has = list_entry(cur, struct hv_hotadd_state, list);
+	spin_lock_irqsave(&dm_device.ha_lock, flags);
+	list_for_each_entry(has, &dm_device.ha_region_list, list) {
 		cur_start_pgp = (unsigned long)
 			pfn_to_page(has->start_pfn);
 		cur_end_pgp = (unsigned long)pfn_to_page(has->end_pfn);
@@ -736,22 +741,19 @@ static void hv_online_page(struct page *pg)
 		__online_page_free(pg);
 		break;
 	}
+	spin_unlock_irqrestore(&dm_device.ha_lock, flags);
 }
 
 static int pfn_covered(unsigned long start_pfn, unsigned long pfn_cnt)
 {
-	struct list_head *cur;
 	struct hv_hotadd_state *has;
 	struct hv_hotadd_gap *gap;
 	unsigned long residual, new_inc;
 	int ret = 0;
+	unsigned long flags;
 
-	if (list_empty(&dm_device.ha_region_list))
-		return false;
-
-	list_for_each(cur, &dm_device.ha_region_list) {
-		has = list_entry(cur, struct hv_hotadd_state, list);
-
+	spin_lock_irqsave(&dm_device.ha_lock, flags);
+	list_for_each_entry(has, &dm_device.ha_region_list, list) {
 		/*
 		 * If the pfn range we are dealing with is not in the current
 		 * "hot add block", move on.
@@ -765,8 +767,10 @@ static int pfn_covered(unsigned long start_pfn, unsigned long pfn_cnt)
 		 */
 		if (has->covered_end_pfn != start_pfn) {
 			gap = kzalloc(sizeof(struct hv_hotadd_gap), GFP_ATOMIC);
-			if (!gap)
-				return -ENOMEM;
+			if (!gap) {
+				ret = -ENOMEM;
+				break;
+			}
 
 			INIT_LIST_HEAD(&gap->list);
 			gap->start_pfn = has->covered_end_pfn;
@@ -792,10 +796,12 @@ static int pfn_covered(unsigned long start_pfn, unsigned long pfn_cnt)
 			has->end_pfn += new_inc;
 		}
 
-		return 1;
+		ret = 1;
+		break;
 	}
+	spin_unlock_irqrestore(&dm_device.ha_lock, flags);
 
-	return 0;
+	return ret;
 }
 
 static unsigned long handle_pg_range(unsigned long pg_start,
@@ -804,17 +810,13 @@ static unsigned long handle_pg_range(unsigned long pg_start,
 	unsigned long start_pfn = pg_start;
 	unsigned long pfn_cnt = pg_count;
 	unsigned long size;
-	struct list_head *cur;
 	struct hv_hotadd_state *has;
 	unsigned long pgs_ol = 0;
 	unsigned long old_covered_state;
+	unsigned long res = 0, flags;
 
-	if (list_empty(&dm_device.ha_region_list))
-		return 0;
-
-	list_for_each(cur, &dm_device.ha_region_list) {
-		has = list_entry(cur, struct hv_hotadd_state, list);
-
+	spin_lock_irqsave(&dm_device.ha_lock, flags);
+	list_for_each_entry(has, &dm_device.ha_region_list, list) {
 		/*
 		 * If the pfn range we are dealing with is not in the current
 		 * "hot add block", move on.
@@ -864,17 +866,20 @@ static unsigned long handle_pg_range(unsigned long pg_start,
 			} else {
 				pfn_cnt = size;
 			}
+			spin_unlock_irqrestore(&dm_device.ha_lock, flags);
 			hv_mem_hot_add(has->ha_end_pfn, size, pfn_cnt, has);
+			spin_lock_irqsave(&dm_device.ha_lock, flags);
 		}
 		/*
 		 * If we managed to online any pages that were given to us,
 		 * we declare success.
 		 */
-		return has->covered_end_pfn - old_covered_state;
-
+		res = has->covered_end_pfn - old_covered_state;
+		break;
 	}
+	spin_unlock_irqrestore(&dm_device.ha_lock, flags);
 
-	return 0;
+	return res;
 }
 
 static unsigned long process_hot_add(unsigned long pg_start,
@@ -884,6 +889,7 @@ static unsigned long process_hot_add(unsigned long pg_start,
 {
 	struct hv_hotadd_state *ha_region = NULL;
 	int covered;
+	unsigned long flags;
 
 	if (pfn_cnt == 0)
 		return 0;
@@ -909,12 +915,15 @@ static unsigned long process_hot_add(unsigned long pg_start,
 		INIT_LIST_HEAD(&ha_region->list);
 		INIT_LIST_HEAD(&ha_region->gap_list);
 
-		list_add_tail(&ha_region->list, &dm_device.ha_region_list);
 		ha_region->start_pfn = rg_start;
 		ha_region->ha_end_pfn = rg_start;
 		ha_region->covered_start_pfn = pg_start;
 		ha_region->covered_end_pfn = pg_start;
 		ha_region->end_pfn = rg_start + rg_size;
+
+		spin_lock_irqsave(&dm_device.ha_lock, flags);
+		list_add_tail(&ha_region->list, &dm_device.ha_region_list);
+		spin_unlock_irqrestore(&dm_device.ha_lock, flags);
 	}
 
 do_pg_range:
@@ -941,7 +950,6 @@ static void hot_add_req(struct work_struct *dummy)
 	resp.hdr.size = sizeof(struct dm_hot_add_response);
 
 #ifdef CONFIG_MEMORY_HOTPLUG
-	mutex_lock(&dm_device.ha_region_mutex);
 	pg_start = dm->ha_wrk.ha_page_range.finfo.start_page;
 	pfn_cnt = dm->ha_wrk.ha_page_range.finfo.page_cnt;
 
@@ -975,7 +983,6 @@ static void hot_add_req(struct work_struct *dummy)
 						rg_start, rg_sz);
 
 	dm->num_pages_added += resp.page_count;
-	mutex_unlock(&dm_device.ha_region_mutex);
 #endif
 	/*
 	 * The result field of the response structure has the
@@ -1520,7 +1527,7 @@ static int balloon_probe(struct hv_device *dev,
 	init_completion(&dm_device.host_event);
 	init_completion(&dm_device.config_event);
 	INIT_LIST_HEAD(&dm_device.ha_region_list);
-	mutex_init(&dm_device.ha_region_mutex);
+	spin_lock_init(&dm_device.ha_lock);
 	INIT_WORK(&dm_device.balloon_wrk.wrk, balloon_up);
 	INIT_WORK(&dm_device.ha_wrk.wrk, hot_add_req);
 	dm_device.host_specified_ha_region = false;
@@ -1639,9 +1646,9 @@ probe_error0:
 static int balloon_remove(struct hv_device *dev)
 {
 	struct hv_dynmem_device *dm = hv_get_drvdata(dev);
-	struct list_head *cur, *tmp;
-	struct hv_hotadd_state *has;
+	struct hv_hotadd_state *has, *tmp;
 	struct hv_hotadd_gap *gap, *tmp_gap;
+	unsigned long flags;
 
 	if (dm->num_pages_ballooned != 0)
 		pr_warn("Ballooned pages: %d\n", dm->num_pages_ballooned);
@@ -1656,8 +1663,8 @@ static int balloon_remove(struct hv_device *dev)
 	restore_online_page_callback(&hv_online_page);
 	unregister_memory_notifier(&hv_memory_nb);
 #endif
-	list_for_each_safe(cur, tmp, &dm->ha_region_list) {
-		has = list_entry(cur, struct hv_hotadd_state, list);
+	spin_lock_irqsave(&dm_device.ha_lock, flags);
+	list_for_each_entry_safe(has, tmp, &dm->ha_region_list, list) {
 		list_for_each_entry_safe(gap, tmp_gap, &has->gap_list, list) {
 			list_del(&gap->list);
 			kfree(gap);
@@ -1665,6 +1672,7 @@ static int balloon_remove(struct hv_device *dev)
 		list_del(&has->list);
 		kfree(has);
 	}
+	spin_unlock_irqrestore(&dm_device.ha_lock, flags);
 
 	return 0;
 }
-- 
2.7.4