[RFC PATCH v2 1/3] mm/memory-tiers: change mutex to rw semaphore

[Gregory Price]

Tiers will have externally readable information, such as weights,
which may change at runtime. This information is expected to be
used by task threads during memory allocation so it cannot be
protected by hard mutual exclusion.

To support this, change the tiering mutex to a rw semaphore.

Signed-off-by: Gregory Price <gregory.price@xxxxxxxxxxxx>
---
 mm/memory-tiers.c | 39 ++++++++++++++++++++-------------------
 1 file changed, 20 insertions(+), 19 deletions(-)

diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
index 37a4f59d9585..0a3241a2cadc 100644
--- a/mm/memory-tiers.c
+++ b/mm/memory-tiers.c
@@ -5,6 +5,7 @@
 #include <linux/kobject.h>
 #include <linux/memory.h>
 #include <linux/memory-tiers.h>
+#include <linux/rwsem.h>
 
 #include "internal.h"
 
@@ -33,7 +34,7 @@ struct node_memory_type_map {
 	int map_count;
 };
 
-static DEFINE_MUTEX(memory_tier_lock);
+static DECLARE_RWSEM(memory_tier_sem);
 static LIST_HEAD(memory_tiers);
 static struct node_memory_type_map node_memory_types[MAX_NUMNODES];
 static struct memory_dev_type *default_dram_type;
@@ -137,10 +138,10 @@ static ssize_t nodelist_show(struct device *dev,
 	int ret;
 	nodemask_t nmask;
 
-	mutex_lock(&memory_tier_lock);
+	down_read(&memory_tier_sem);
 	nmask = get_memtier_nodemask(to_memory_tier(dev));
 	ret = sysfs_emit(buf, "%*pbl\n", nodemask_pr_args(&nmask));
-	mutex_unlock(&memory_tier_lock);
+	up_read(&memory_tier_sem);
 	return ret;
 }
 static DEVICE_ATTR_RO(nodelist);
@@ -167,7 +168,7 @@ static struct memory_tier *find_create_memory_tier(struct memory_dev_type *memty
 	int adistance = memtype->adistance;
 	unsigned int memtier_adistance_chunk_size = MEMTIER_CHUNK_SIZE;
 
-	lockdep_assert_held_once(&memory_tier_lock);
+	lockdep_assert_held_write(&memory_tier_sem);
 
 	adistance = round_down(adistance, memtier_adistance_chunk_size);
 	/*
@@ -230,12 +231,12 @@ static struct memory_tier *__node_get_memory_tier(int node)
 	if (!pgdat)
 		return NULL;
 	/*
-	 * Since we hold memory_tier_lock, we can avoid
+	 * Since we hold memory_tier_sem, we can avoid
 	 * RCU read locks when accessing the details. No
 	 * parallel updates are possible here.
 	 */
 	return rcu_dereference_check(pgdat->memtier,
-				     lockdep_is_held(&memory_tier_lock));
+				     lockdep_is_held(&memory_tier_sem));
 }
 
 #ifdef CONFIG_MIGRATION
@@ -335,7 +336,7 @@ static void disable_all_demotion_targets(void)
 	for_each_node_state(node, N_MEMORY) {
 		node_demotion[node].preferred = NODE_MASK_NONE;
 		/*
-		 * We are holding memory_tier_lock, it is safe
+		 * We are holding memory_tier_sem, it is safe
 		 * to access pgda->memtier.
 		 */
 		memtier = __node_get_memory_tier(node);
@@ -364,7 +365,7 @@ static void establish_demotion_targets(void)
 	int distance, best_distance;
 	nodemask_t tier_nodes, lower_tier;
 
-	lockdep_assert_held_once(&memory_tier_lock);
+	lockdep_assert_held_write(&memory_tier_sem);
 
 	if (!node_demotion)
 		return;
@@ -479,7 +480,7 @@ static struct memory_tier *set_node_memory_tier(int node)
 	pg_data_t *pgdat = NODE_DATA(node);
 
 
-	lockdep_assert_held_once(&memory_tier_lock);
+	lockdep_assert_held_write(&memory_tier_sem);
 
 	if (!node_state(node, N_MEMORY))
 		return ERR_PTR(-EINVAL);
@@ -569,15 +570,15 @@ EXPORT_SYMBOL_GPL(put_memory_type);
 void init_node_memory_type(int node, struct memory_dev_type *memtype)
 {
 
-	mutex_lock(&memory_tier_lock);
+	down_write(&memory_tier_sem);
 	__init_node_memory_type(node, memtype);
-	mutex_unlock(&memory_tier_lock);
+	up_write(&memory_tier_sem);
 }
 EXPORT_SYMBOL_GPL(init_node_memory_type);
 
 void clear_node_memory_type(int node, struct memory_dev_type *memtype)
 {
-	mutex_lock(&memory_tier_lock);
+	down_write(&memory_tier_sem);
 	if (node_memory_types[node].memtype == memtype)
 		node_memory_types[node].map_count--;
 	/*
@@ -588,7 +589,7 @@ void clear_node_memory_type(int node, struct memory_dev_type *memtype)
 		node_memory_types[node].memtype = NULL;
 		put_memory_type(memtype);
 	}
-	mutex_unlock(&memory_tier_lock);
+	up_write(&memory_tier_sem);
 }
 EXPORT_SYMBOL_GPL(clear_node_memory_type);
 
@@ -607,17 +608,17 @@ static int __meminit memtier_hotplug_callback(struct notifier_block *self,
 
 	switch (action) {
 	case MEM_OFFLINE:
-		mutex_lock(&memory_tier_lock);
+		down_write(&memory_tier_sem);
 		if (clear_node_memory_tier(arg->status_change_nid))
 			establish_demotion_targets();
-		mutex_unlock(&memory_tier_lock);
+		up_write(&memory_tier_sem);
 		break;
 	case MEM_ONLINE:
-		mutex_lock(&memory_tier_lock);
+		down_write(&memory_tier_sem);
 		memtier = set_node_memory_tier(arg->status_change_nid);
 		if (!IS_ERR(memtier))
 			establish_demotion_targets();
-		mutex_unlock(&memory_tier_lock);
+		up_write(&memory_tier_sem);
 		break;
 	}
 
@@ -638,7 +639,7 @@ static int __init memory_tier_init(void)
 				GFP_KERNEL);
 	WARN_ON(!node_demotion);
 #endif
-	mutex_lock(&memory_tier_lock);
+	down_write(&memory_tier_sem);
 	/*
 	 * For now we can have 4 faster memory tiers with smaller adistance
 	 * than default DRAM tier.
@@ -661,7 +662,7 @@ static int __init memory_tier_init(void)
 			break;
 	}
 	establish_demotion_targets();
-	mutex_unlock(&memory_tier_lock);
+	up_write(&memory_tier_sem);
 
 	hotplug_memory_notifier(memtier_hotplug_callback, MEMTIER_HOTPLUG_PRI);
 	return 0;
-- 
2.39.1