Red Hat Bugzilla – Bug 225515
[patch]Kernel panic (handle kernel NULLpointer) occurred in NFSv4
Last modified: 2007-11-30 17:07:41 EST
Description of problem:
If the /proc/sys/sunrpc/nfs_debug was set to 65535, when closing many
opening files in NFSv4, the kernel panic (handle kernel NULL pointer) usually
kernel in RHEL5Beta2
Steps to Reproduce:
1. Mount: mount -t nfs4 192.168.0.21:/ /mnt
2. A process creates and opens 1024 files on NFSv4
3. The process writes some test message to each file.
4. The process closes all the opening files and exit.
5. After the process exit, the test begins to umount the NFSv4
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 happen.
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
printing eip: deb82605
*pde = 001f3067
Oops: 0000 [#1]
last sysfs file: /block/hda/removable
Modules linked in: nfs fscache nfsd ¡. ¡¡.
EIP: 0060:[<deb82605>] Not tainted VLI
EFLAGS: 00010246 (2.6.18-1.2747.el5 #1)
EIP is at nfs_update_inode+0xb0/0x692 [nfs]
Process rpciod/0 (pid: 1865, ti=dd3ac000 task=ddd47550 task.ti=dd3ac000)
Stack: deba0609 ¡ ¡
[<deb82c1f>] nfs_refresh_inode+0x38/0x1b0 [nfs]
[<dea9f602>] rpc_exit_task+0x1e/0x6c [sunrpc]
[<dea9f314>] __rpc_execute+0x82/0x1b3 [sunrpc]
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
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_PROTO(inode)->file_release ()
| -- nfs4_close_state
| | -- nfs4_close_ops
| |-- inode == inode->i_sb-
| -- mntput(ctx_vfsmnt)
After the asynchronous RPC call "nfs4_close_ops" is invoked in
put_nfs_open_context(), the kernel invokes the mntput(), and the mnt-
>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-
>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-
>s_root in the nfs_update_date().
To resolve this problem, I have made the patch attachment for the
kernel. After the patch is applied, the problem can be resolved in my test.
Created attachment 146986 [details]
This request was evaluated by Red Hat Product Management for inclusion in a Red
Hat Enterprise Linux maintenance release. Product Management has requested
further review of this request by Red Hat Engineering, for potential
inclusion in a Red Hat Enterprise Linux Update release for currently deployed
products. This request is not yet committed for inclusion in an Update
QE ack for RHEL 5.1
Created attachment 156385 [details]
Proposed Upstream patch.
Would it be possible to test this upstream patch?
Unfortunately I'm having no success in reproducing
this oops, so if you could verify that this patch
fixes the race condition that would be very helpful.
I am interested in testing this patch, but it will be some time before I can do
so, since I'm in the process of some major server work.
Will this patch also apply to the Fedora Core 6 kernel sources? I had this
problem also on FC6. (not sure about FC7, haven't had a chance to try nfsv4 on it).
Yes... it should apply to an FC6 kernel... Please let me know if there is a
You can download this test kernel from http://people.redhat.com/dzickus/el5
An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on the solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.