Received-SPF: pass (google.com: best guess record for domain of linux-ext4-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
From:   Meng Xu <mengxu.gatech@gmail.com>
Date:   Thu, 28 Nov 2019 12:03:04 -0500
Message-ID: <CAAwBoOLoHTZGWFw5y_3MoMgZDQ3gCUQrsAO8Z=U4RwV9KyA_fA@mail.gmail.com>
Subject: potential data race on ext_inode_hdr(inode)->eh_depth,
 ext_inode_hdr(inode)->eh_max between a creat and unlink syscall
To:     linux-ext4@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Sender: linux-ext4-owner@vger.kernel.org
Precedence: bulk

Hi Ext4 Developers,

I notice a potential data race on ext_inode_hdr(inode)->eh_depth,
ext_inode_hdr(inode)->eh_max between a create and unlink syscall.
Following is the trace:

[Setup]
mkdir("foo", 511) = 0;
open("foo", 65536, 511) = 3;
create("bar", 511) = 4;
symlink("foo", "sym_foo") = 0;
open("sym_foo", 65536, 511) = 5;

[Thread 1]
create("bar", 438);

__do_sys_creat
  ksys_open
    do_filp_open
      path_openat
        do_last
          handle_truncate
            do_truncate
              notify_change
                ext4_setattr
                  ext4_truncate
                    ext4_ext_truncate
                      ext4_ext _remove_space
                        [WRITE, 2 bytes] ext_inode_hdr(inode)->eh_depth = 0;
                        [WRITE, 2 bytes] ext_inode_hdr(inode)->eh_max
= cpu_to_le16(ext4_ext_space_root(inode, 0));

[Thread 2]
unlink("sym_foo");

__do_sys_unlink
  do_unlinkat
    iput
      iput_final
        evict
          ext4_evict_inode
            ext4_orphan_del
              ext4_mark_iloc_dirty
                ext4_do_update_inode
                  [READ, 4 bytes] raw_inode->i_block[block] = ei->i_data[block];


I could observe that the order between the READ and WRITE is not
deterministic and I was curious what will happen if the READ takes
place in the middle of the two WRITES? Does it cause any damages or
violations?

Best Regards,
Meng