: Kernel panic (handle kernel NULL pointer)occurred in NFSv4 by the nfs_update_inode()

Hi, all

I have been working on trying to evaluate the robustness of the NFS. When I run the files creating test on NFSv4, the kernel panic (handle
kernel NULL pointer) happened during the test when
the /proc/sys/sunrpc/nfs_debug was set to 65535.

The test I run is:
Mount:

      mount -t nfs4  192.168.0.21:/  /mnt

Test script:
A process creates and opens 1024 files on NFSv4 The process writes some test message to each file. The process closes all the opening files and exit. After the process exit, the test begins to umount the NFSv4
immediately.

In normal case, I haven't found the problem in the test; but when the /proc/sys/sunrpc/nfs_debug is set to debug mode(65535), the panic problem will usually happens.

The kernel panic log is as follows:

NFS: nfs_update_inode(0:18/2049980 ct=1 info=0xe)

BUG: unable to handle kernel NULL pointer dereference at virtual address
0000000c
printing eip: deb82605
*pde = 001f3067
Oops: 0000 [#1]
SMP
last sysfs file: /block/hda/removable
Modules linked in: nfs fscache nfsd ¡­. ¡­¡­.
CPU:    0
EIP:    0060:[<deb82605>]    Not tainted VLI
EFLAGS: 00010246   (2.6.18-1.2747.el5 #1)
EIP is at nfs_update_inode+0xb0/0x692 [nfs]
¡­skip¡­.
Process rpciod/0 (pid: 1865, ti=dd3ac000 task=ddd47550 task.ti=dd3ac000)
Stack: deba0609 ¡­ ¡­
Call Trace:
[<deb82c1f>] nfs_refresh_inode+0x38/0x1b0 [nfs]
[<dea9f602>] rpc_exit_task+0x1e/0x6c [sunrpc]
[<dea9f314>] __rpc_execute+0x82/0x1b3 [sunrpc]
[<c0433899>] run_workqueue+0x83/0xc5
[<c0434171>] worker_thread+0xd9/0x10c
[<c0436620>] kthread+0xc0/0xec
[<c0404d63>] kernel_thread_helper+0x7/0x10
DWARF2 unwinder stuck at kernel_thread_helper+0x7/0x10

I have investigated the problem, and found the cause was the NULL pointer i_sb->s_root in the "nfs_update_inode()" when the panic happened.

In the kernel, at the end of the file closing operation, the
nfs_file_release() will be invoked.

I have found the operation process of the kernel is as follows:
nfs_file_release()

      |-- NFS_PROTO(inode)->file_release ()
            |
          nfs_file_clear_open_context()
            |
           put_nfs_open_context()
              | -- nfs4_close_state
              |       | -- nfs4_close_ops
              |                 |
              |               nfs4_do_close()
              |                  |
              |                 nfs_update_inode()
              |                        |-- inode == inode->i_sb-

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s_root->d_inode

              |
              | -- mntput(ctx_vfsmnt)
                     |
                    atomic_dec_and_lock(&mnt->mnt_count,

&vfsmount_lock)

After the asynchronous RPC call "nfs4_close_ops" is invoked in put_nfs_open_context(), the kernel invokes the mntput(), and the mnt-

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mnt_count is decreased. In my test, after the file closing operation,

the sys_umont() was executed immediately. In normal case, the asynchronous RPC call "nfs4_close_ops" can be completed quickly, and it rarely ever happens that the sys_umount() is invoked before the end of nfs_update_inode() operation. But when the sunrpc/nfs_debug is set to debug mode, a lot of printk operations will be invoked in NFS. Due to the lots of prink operations, the RPC call "nfs4_close_ops" will easily be delayed, then it is possible that the sys_umount() is invoked before the end of nfs_update_inode() operation. In the do_umont() (Because mnt-

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mnt_count has been decreased, umount can be executed successfully), the

sb->s_root will be set to NULL in the shrink_dcache_for_umount () which is invoked by the nfs_kill_super().
Therefore, kernel panic occurred by the NULL pointer access when nfs_update_date() accessed inode->i_sb->s_root.

Because there is a possibility that sb->s_root of a super_block is set to NULL with umount when nfs4_close_ops () is not finished in the NFS. It is really necessary to check an empty pointer for the inode->i_sb-

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s_root in the nfs_update_date().

This problem still exists in the latest linux kernel.
To resolve this problem, I have made the patch below for the linux-2.6.19.2. After the patch is applied, the problem can be resolved.

Signed-off-by ShiChao <[email protected]>