[PATCH v2 46/47] mm/memory-tiers: unregister empty tiers outside memory_tier_lock

["David Hildenbrand (Arm)" <david@kernel.org>]

When offlining a node's last memory empties its tier,
clear_node_memory_tier() calls device_unregister() while holding
memory_tier_lock. A concurrent sysfs read can hold a kernfs active
reference and block in nodelist_show() on that mutex. device_unregister()
then waits in kernfs_drain() for the read to finish and release its active
reference, while the reader waits for the offlining task to release the
mutex.

Return the node's former tier from clear_node_memory_tier(), making the
helper more similar to set_node_memory_tier(), which also returns a
memory_tier pointer. Let the caller unlink an empty tier under
memory_tier_lock before rebuilding demotion targets, then unregister its
device after dropping the lock.

Fixes: 9832fb87834e ("mm/demotion: expose memory tier details via sysfs")
Cc: stable@vger.kernel.org
Reported-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/memory-tiers.c | 29 +++++++++++++----------------
 1 file changed, 13 insertions(+), 16 deletions(-)

diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
index 25e121851b58..8eda313cf619 100644
--- a/mm/memory-tiers.c
+++ b/mm/memory-tiers.c
@@ -572,21 +572,14 @@ static struct memory_tier *set_node_memory_tier(int node)
 	return memtier;
 }
 
-static void destroy_memory_tier(struct memory_tier *memtier)
+static struct memory_tier *clear_node_memory_tier(int node)
 {
-	list_del(&memtier->list);
-	device_unregister(&memtier->dev);
-}
-
-static bool clear_node_memory_tier(int node)
-{
-	bool cleared = false;
 	pg_data_t *pgdat;
 	struct memory_tier *memtier;
 
 	pgdat = NODE_DATA(node);
 	if (!pgdat)
-		return false;
+		return NULL;
 
 	/*
 	 * Make sure that anybody looking at NODE_DATA who finds
@@ -604,14 +597,10 @@ static bool clear_node_memory_tier(int node)
 		synchronize_rcu();
 		memtype = node_memory_types[node].memtype;
 		node_clear(node, memtype->nodes);
-		if (nodes_empty(memtype->nodes)) {
+		if (nodes_empty(memtype->nodes))
 			list_del_init(&memtype->tier_sibling);
-			if (list_empty(&memtier->memory_types))
-				destroy_memory_tier(memtier);
-		}
-		cleared = true;
 	}
-	return cleared;
+	return memtier;
 }
 
 static void release_memtype(struct kref *kref)
@@ -884,13 +873,21 @@ static int __meminit memtier_hotplug_callback(struct notifier_block *self,
 {
 	struct memory_tier *memtier;
 	struct node_notify *nn = _arg;
+	bool empty = false;
 
 	switch (action) {
 	case NODE_REMOVED_LAST_MEMORY:
 		mutex_lock(&memory_tier_lock);
-		if (clear_node_memory_tier(nn->nid))
+		memtier = clear_node_memory_tier(nn->nid);
+		if (memtier) {
+			empty = list_empty(&memtier->memory_types);
+			if (empty)
+				list_del(&memtier->list);
 			establish_demotion_targets();
+		}
 		mutex_unlock(&memory_tier_lock);
+		if (empty)
+			device_unregister(&memtier->dev);
 		break;
 	case NODE_ADDED_FIRST_MEMORY:
 		mutex_lock(&memory_tier_lock);

-- 
2.43.0