Description of problem:
This is related to the following kernel.org bugzillas:

http://bugzilla.kernel.org/show_bug.cgi?id=7903
http://bugzilla.kernel.org/show_bug.cgi?id=8077

A change was introduced in Febuary to fix hangs in i_size_read caused by calling
i_size_write without holding i_mutex. The fix for this worked out in KBZ#7903
was to use the i_lock spinlock to synchronize cifs' use of i_size_xxxx(). This
introduces another deadlock:

cifs_get_inode_info_unix()
  spin_lock(&inode->i_lock);
  if (is_size_safe_to_change(cifsInfo, end_of_file)) {
    [...]
  }
}

is_size_safe_to_change() can end up sleeping via the following sequence of calls:

is_size_safe_to_change+0x24/0x90 [cifs]
find_writable_file+0xe4/0x184 [cifs]
cifs_reopen_file+0x2e4/0x524 [cifs]
CIFSSMBOpen+0x2d8/0x518 [cifs]
SendReceive+0x2dc/0x598 [cifs]
wait_for_response+0xe8/0x1bc [cifs]
schedule+0xa98/0xbf4

If we wind up with one thread sleeping in this code path it is possible for
the system to deadlock should all other CPUs enter CIFS and attempt to take
this inode's i_lock. This has been seen on 2 CPU power5 systems.

Version-Release number of selected component (if applicable):
2.6.18-32.el5

How reproducible:
Moderate - stress test of several hours duration required.

Steps to Reproduce:
1. Run LTP locktest case (testcases/network/nfsv4/locks/) for several hours:

http://ltp.cvs.sourceforge.net/ltp/ltp/testcases/network/nfsv4/locks/


Actual results:
After some time the system deadlocks and reports soft lockups pointing at the
above CIFS code:

<3>BUG: soft lockup detected on CPU#1!.
<4>Call Trace:.
<4>[C0000000E4D3F290] [C00000000000FFDC] .show_stack+0x68/0x1b0 (unreliable).
<4>[C0000000E4D3F330] [C0000000000A68C4] .softlockup_tick+0xf0/0x13c.
<4>[C0000000E4D3F3E0] [C000000000075BF8] .run_local_timers+0x1c/0x30.
<4>[C0000000E4D3F460] [C0000000000235F0] .timer_interrupt+0xa8/0x498.
<4>[C0000000E4D3F540] [C0000000000034F4] decrementer_common+0xf4/0x100.
<4>--- Exception: 901 at ._spin_lock+0x40/0x88.
<4>    LR = .cifs_get_inode_info_unix+0x784/0x9c0 [cifs].
<4>[C0000000E4D3F830] [C0000000EA44EE40] 0xc0000000ea44ee40 (unreliable).
<4>[C0000000E4D3F8B0] [D000000000B4BE78] .cifs_get_inode_info_unix+0x784/0x9c0
[cifs].
<4>[C0000000E4D3FA20] [D000000000B4A908] .cifs_open+0x72c/0x8d0 [cifs].
<4>[C0000000E4D3FB30] [C0000000000E8754] .__dentry_open+0x13c/0x2bc.
<4>[C0000000E4D3FBE0] [C0000000000E8A48] .do_filp_open+0x50/0x70.
<4>[C0000000E4D3FD00] [C0000000000E8ADC] .do_sys_open+0x74/0x130.
<4>[C0000000E4D3FDB0] [C000000000128210] .compat_sys_open+0x24/0x38.
<4>[C0000000E4D3FE30] [C0000000000086A4] syscall_exit+0x0/0x40.
<3>BUG: soft lockup detected on CPU#0!.
<4>Call Trace:.
<4>[C0000000B307B290] [C00000000000FFDC] .show_stack+0x68/0x1b0 (unreliable).
<4>[C0000000B307B330] [C0000000000A68C4] .softlockup_tick+0xf0/0x13c.
<4>[C0000000B307B3E0] [C000000000075BF8] .run_local_timers+0x1c/0x30.
<4>[C0000000B307B460] [C0000000000235F0] .timer_interrupt+0xa8/0x498.
<4>[C0000000B307B540] [C0000000000034F4] decrementer_common+0xf4/0x100.
<4>--- Exception: 901 at ._spin_lock+0x50/0x88.
<4>    LR = .cifs_get_inode_info_unix+0x784/0x9c0 [cifs].
<4>[C0000000B307B830] [D000000000B889F8] 0xd000000000b889f8 (unreliable).
<4>[C0000000B307B8B0] [D000000000B4BE78] .cifs_get_inode_info_unix+0x784/0x9c0
[cifs].
<4>[C0000000B307BA20] [D000000000B4A908] .cifs_open+0x72c/0x8d0 [cifs].
<4>[C0000000B307BB30] [C0000000000E8754] .__dentry_open+0x13c/0x2bc.
<4>[C0000000B307BBE0] [C0000000000E8A48] .do_filp_open+0x50/0x70.
<4>[C0000000B307BD00] [C0000000000E8ADC] .do_sys_open+0x74/0x130.
<4>[C0000000B307BDB0] [C000000000128210] .compat_sys_open+0x24/0x38.
<4>[C0000000B307BE30] [C0000000000086A4] syscall_exit+0x0/0x40.

Expected results:
No deadlock, fs test runs to completion.

Additional info:
The original kernel.org bugzilla was triggered via multiple instances of cp/find
etc. It's not clear yet if this problem can also be triggered by these simple
tests but is reported to be reliably triggered by the LTP testcase.