Re: [PATCH] fs: prevent data-race due to missing inode_lock when calling vfs_getattr
From: Jeongjun Park
Date: Mon Nov 18 2024 - 01:01:06 EST
Hello,
> Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Nov 18, 2024 at 01:37:19AM +0900, Jeongjun Park wrote:
>> Many filesystems lock inodes before calling vfs_getattr, so there is no
>> data-race for inodes. However, some functions in fs/stat.c that call
>> vfs_getattr do not lock inodes, so the data-race occurs.
>>
>> Therefore, we need to apply a patch to remove the long-standing data-race
>> for inodes in some functions that do not lock inodes.
>
> Why do we care? Slapping even a shared lock on a _very_ hot path, with
> possible considerable latency, would need more than "theoretically it's
> a data race".
All the functions that added lock in this patch are called only via syscall,
so in most cases there will be no noticeable performance issue. And
this data-race is not a problem that only occurs in theory. It is
a bug that syzbot has been reporting for years. Many file systems that
exist in the kernel lock inode_lock before calling vfs_getattr, so
data-race does not occur, but only fs/stat.c has had a data-race
for years. This alone shows that adding inode_lock to some
functions is a good way to solve the problem without much
performance degradation.
Regards,
Jeongjun Park