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>

diff --git a/fs/f2fs/sysfs.c b/fs/f2fs/sysfs.c
index dd940d156af62ee54c8c047a19a29aa7768497fb..ac3ea604493687e2b19b419b7fd09f6dd6000dc0 100644 (file)
--- a/fs/f2fs/sysfs.c
+++ b/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)
 {