f2fs: fix to avoid race during access gc_thread pointer

Thread A			Thread B
- f2fs_remount
 - stop_gc_thread
				- f2fs_sbi_store
   sbi->gc_thread = NULL;
				  access sbi->gc_thread->gc_*

Previously, we allocate memory for sbi->gc_thread based on background
gc thread mount option, the memory can be released if we turn off
that mount option, but still there are several places access gc_thread
pointer without considering race condition, result in NULL point
dereference.

In order to fix this issue, use sb->s_umount to exclude those operations.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/sysfs.c

@@ -165,7 +165,7 @@ static ssize_t f2fs_sbi_show(struct f2fs_attr *a,
 	return snprintf(buf, PAGE_SIZE, "%u\n", *ui);
 }
 
-static ssize_t f2fs_sbi_store(struct f2fs_attr *a,
+static ssize_t __f2fs_sbi_store(struct f2fs_attr *a,
 			struct f2fs_sb_info *sbi,
 			const char *buf, size_t count)
 {
@@ -278,6 +278,23 @@ out:
 	return count;
 }
 
+static ssize_t f2fs_sbi_store(struct f2fs_attr *a,
+			struct f2fs_sb_info *sbi,
+			const char *buf, size_t count)
+{
+	ssize_t ret;
+	bool gc_entry = (!strcmp(a->attr.name, "gc_urgent") ||
+					a->struct_type == GC_THREAD);
+
+	if (gc_entry)
+		down_read(&sbi->sb->s_umount);
+	ret = __f2fs_sbi_store(a, sbi, buf, count);
+	if (gc_entry)
+		up_read(&sbi->sb->s_umount);
+
+	return ret;
+}
+
 static ssize_t f2fs_attr_show(struct kobject *kobj,
 				struct attribute *attr, char *buf)
 {