f2fs: add global mutex_lock to protect f2fs_stat_list

There is an race condition between umounting f2fs and reading f2fs/status, which
results in oops.

Fox example:
Thread A			Thread B
umount f2fs 			cat f2fs/status

f2fs_destroy_stats() {		stat_show() {
				 list_for_each_entry_safe(&f2fs_stat_list)
 list_del(&si->stat_list);
 mutex_lock(&si->stat_lock);
 si->sbi = NULL;
 mutex_unlock(&si->stat_lock);
 kfree(sbi->stat_info);
} 				 mutex_lock(&si->stat_lock) <- si is gone.
				 ...
				}

Solution with a global lock: f2fs_stat_mutex:
Thread A			Thread B
umount f2fs 			cat f2fs/status

f2fs_destroy_stats() {		stat_show() {
 mutex_lock(&f2fs_stat_mutex);
 list_del(&si->stat_list);
 mutex_unlock(&f2fs_stat_mutex);
 kfree(sbi->stat_info);		 mutex_lock(&f2fs_stat_mutex);
}				 list_for_each_entry_safe(&f2fs_stat_list)
				 ...
				 mutex_unlock(&f2fs_stat_mutex);
				}

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
[jaegeuk.kim@samsung.com: fix typos, description, and remove the existing lock]
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/debug.c

@@ -26,6 +26,7 @@
 
 static LIST_HEAD(f2fs_stat_list);
 static struct dentry *debugfs_root;
+static DEFINE_MUTEX(f2fs_stat_mutex);
 
 static void update_general_status(struct f2fs_sb_info *sbi)
 {
@@ -180,13 +181,9 @@ static int stat_show(struct seq_file *s, void *v)
 	int i = 0;
 	int j;
 
+	mutex_lock(&f2fs_stat_mutex);
 	list_for_each_entry_safe(si, next, &f2fs_stat_list, stat_list) {
 
-		mutex_lock(&si->stat_lock);
-		if (!si->sbi) {
-			mutex_unlock(&si->stat_lock);
-			continue;
-		}
 		update_general_status(si->sbi);
 
 		seq_printf(s, "\n=====[ partition info. #%d ]=====\n", i++);
@@ -286,8 +283,8 @@ static int stat_show(struct seq_file *s, void *v)
 		seq_printf(s, "\nMemory: %u KB = static: %u + cached: %u\n",
 				(si->base_mem + si->cache_mem) >> 10,
 				si->base_mem >> 10, si->cache_mem >> 10);
-		mutex_unlock(&si->stat_lock);
 	}
+	mutex_unlock(&f2fs_stat_mutex);
 	return 0;
 }
 
@@ -313,9 +310,6 @@ static int init_stats(struct f2fs_sb_info *sbi)
 		return -ENOMEM;
 
 	si = sbi->stat_info;
-	mutex_init(&si->stat_lock);
-	list_add_tail(&si->stat_list, &f2fs_stat_list);
-
 	si->all_area_segs = le32_to_cpu(raw_super->segment_count);
 	si->sit_area_segs = le32_to_cpu(raw_super->segment_count_sit);
 	si->nat_area_segs = le32_to_cpu(raw_super->segment_count_nat);
@@ -325,6 +319,11 @@ static int init_stats(struct f2fs_sb_info *sbi)
 	si->main_area_zones = si->main_area_sections /
 				le32_to_cpu(raw_super->secs_per_zone);
 	si->sbi = sbi;
+
+	mutex_lock(&f2fs_stat_mutex);
+	list_add_tail(&si->stat_list, &f2fs_stat_list);
+	mutex_unlock(&f2fs_stat_mutex);
+
 	return 0;
 }
 
@@ -347,10 +346,10 @@ void f2fs_destroy_stats(struct f2fs_sb_info *sbi)
 {
 	struct f2fs_stat_info *si = sbi->stat_info;
 
+	mutex_lock(&f2fs_stat_mutex);
 	list_del(&si->stat_list);
-	mutex_lock(&si->stat_lock);
-	si->sbi = NULL;
-	mutex_unlock(&si->stat_lock);
+	mutex_unlock(&f2fs_stat_mutex);
+
 	kfree(sbi->stat_info);
 }