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Subject[PATCH v3] f2fs: fix to avoid race during access gc_thread pointer
Date
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>
---
v3:
- fix missing 'gc_urgent' case
fs/f2fs/sysfs.c | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/fs/f2fs/sysfs.c b/fs/f2fs/sysfs.c
index dd940d156af6..ac3ea6044936 100644
--- 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 @@ static ssize_t f2fs_sbi_store(struct f2fs_attr *a,
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)
{
--
2.17.0.391.g1f1cddd558b5
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