f2fs: cover free_nid management with spin_lock

After build_free_nids() searches free nid candidates from nat pages and
current journal blocks, it checks all the candidates if they are allocated
so that the nat cache has its nid with an allocated block address.

In this procedure, previously we used
    list_for_each_entry_safe(fnid, next_fnid, &nm_i->free_nid_list, list).
But, this is not covered by free_nid_list_lock, resulting in null pointer bug.

This patch moves this checking routine inside add_free_nid() in order not to use
the spin_lock.
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/node.c

@@ -1249,9 +1249,11 @@ static void __del_from_free_nid_list(struct free_nid *i)
 	kmem_cache_free(free_nid_slab, i);
 }
 
-static int add_free_nid(struct f2fs_nm_info *nm_i, nid_t nid)
+static int add_free_nid(struct f2fs_nm_info *nm_i, nid_t nid, bool build)
 {
 	struct free_nid *i;
+	struct nat_entry *ne;
+	bool allocated = false;
 
 	if (nm_i->fcnt > 2 * MAX_FREE_NIDS)
 		return -1;
@@ -1259,6 +1261,18 @@ static int add_free_nid(struct f2fs_nm_info *nm_i, nid_t nid)
 	/* 0 nid should not be used */
 	if (nid == 0)
 		return 0;
+
+	if (!build)
+		goto retry;
+
+	/* do not add allocated nids */
+	read_lock(&nm_i->nat_tree_lock);
+	ne = __lookup_nat_cache(nm_i, nid);
+	if (ne && nat_get_blkaddr(ne) != NULL_ADDR)
+		allocated = true;
+	read_unlock(&nm_i->nat_tree_lock);
+	if (allocated)
+		return 0;
 retry:
 	i = kmem_cache_alloc(free_nid_slab, GFP_NOFS);
 	if (!i) {
@@ -1309,7 +1323,7 @@ static void scan_nat_page(struct f2fs_nm_info *nm_i,
 		blk_addr = le32_to_cpu(nat_blk->entries[i].block_addr);
 		BUG_ON(blk_addr == NEW_ADDR);
 		if (blk_addr == NULL_ADDR) {
-			if (add_free_nid(nm_i, start_nid) < 0)
+			if (add_free_nid(nm_i, start_nid, true) < 0)
 				break;
 		}
 	}
@@ -1317,7 +1331,6 @@ static void scan_nat_page(struct f2fs_nm_info *nm_i,
 
 static void build_free_nids(struct f2fs_sb_info *sbi)
 {
-	struct free_nid *fnid, *next_fnid;
 	struct f2fs_nm_info *nm_i = NM_I(sbi);
 	struct curseg_info *curseg = CURSEG_I(sbi, CURSEG_HOT_DATA);
 	struct f2fs_summary_block *sum = curseg->sum_blk;
@@ -1354,22 +1367,11 @@ static void build_free_nids(struct f2fs_sb_info *sbi)
 		block_t addr = le32_to_cpu(nat_in_journal(sum, i).block_addr);
 		nid = le32_to_cpu(nid_in_journal(sum, i));
 		if (addr == NULL_ADDR)
-			add_free_nid(nm_i, nid);
+			add_free_nid(nm_i, nid, true);
 		else
 			remove_free_nid(nm_i, nid);
 	}
 	mutex_unlock(&curseg->curseg_mutex);
-
-	/* remove the free nids from current allocated nids */
-	list_for_each_entry_safe(fnid, next_fnid, &nm_i->free_nid_list, list) {
-		struct nat_entry *ne;
-
-		read_lock(&nm_i->nat_tree_lock);
-		ne = __lookup_nat_cache(nm_i, fnid->nid);
-		if (ne && nat_get_blkaddr(ne) != NULL_ADDR)
-			remove_free_nid(nm_i, fnid->nid);
-		read_unlock(&nm_i->nat_tree_lock);
-	}
 }
 
 /*
@@ -1659,7 +1661,7 @@ void flush_nat_entries(struct f2fs_sb_info *sbi)
 		}
 
 		if (nat_get_blkaddr(ne) == NULL_ADDR &&
-				add_free_nid(NM_I(sbi), nid) <= 0) {
+				add_free_nid(NM_I(sbi), nid, false) <= 0) {
 			write_lock(&nm_i->nat_tree_lock);
 			__del_from_nat_cache(nm_i, ne);
 			write_unlock(&nm_i->nat_tree_lock);