Description of problem:
Rapidly pluging and unpluggin a block device eventually leads to a kernel crash
when the device is unplugged before fully established due to a double free.

This effects RHEL5 and RHEL4u5. The fix is up at
http://xenbits.xensource.com/linux-2.6.18-xen.hg?rev/c1c57fea77e9 and/or
http://xenbits.xensource.com/kernels/rhel4x.hg?rev/5bccd45a081d

The fix depends on 
http://xenbits.xensource.com/linux-2.6.18-xen.hg?rev/11483a00c017 and/or
http://xenbits.xensource.com/kernels/rhel4x.hg?rev/156e3eaca552 which are
attached to #247265 but only applied to RHEL5, I think.

Version-Release number of selected component (if applicable):
2.6.18-8.el5 and 2.6.9-55.EL

How reproducible:
A simple loop which attaches and detaches a block device as quick as possible is
sufficient.
  
Actual results:
The backtrace on 2.6.9-55.EL:

 [<c043ea67>] softlockup_tick+0x98/0xa6

 [<c0408b7d>] timer_interrupt+0x504/0x557

 [<c043ec9b>] handle_IRQ_event+0x27/0x51

 [<c043ed58>] __do_IRQ+0x93/0xe8

 [<c040672b>] do_IRQ+0x93/0xae

 [<c053a045>] evtchn_do_upcall+0x64/0x9b

 [<c0404ec5>] hypervisor_callback+0x3d/0x48

 [<c045007b>] __pte_alloc+0x1e1/0x21a

 [<c05f4e18>] _spin_lock+0x7/0xf

 [<c05f4179>] __mutex_lock_slowpath+0x19/0x74

 [<c04d36d0>] __next_cpu+0x12/0x21

 [<c045e2ee>] kfree+0xe/0x77

 [<c05f41e3>] .text.lock.mutex+0xf/0x14

 [<c04d3e3d>] kobject_cleanup+0x4a/0x5e

 [<c04c6467>] elevator_exit+0xe/0x32

 [<c04c931c>] blk_cleanup_queue+0x2d/0x36

 [<d1029e6d>] xlvbd_del+0x4d/0x56 [xenblk]

 [<d10293f4>] blkfront_closing+0x45/0x4f [xenblk]

 [<d1029e1b>] blkif_release+0x34/0x39 [xenblk]

 [<c0468cb4>] __blkdev_put+0x50/0x123

 [<c04950d8>] register_disk+0x117/0x155

 [<c04cc39c>] add_disk+0x2b/0x36

 [<c04cbd9c>] exact_match+0x0/0x4

 [<c04cc248>] exact_lock+0x0/0xd

 [<d1029d7f>] backend_changed+0xf3/0x153 [xenblk]

 [<c053fc58>] otherend_changed+0x74/0x79

 [<c053e442>] xenwatch_handle_callback+0x12/0x44

 [<c053ee3a>] xenwatch_thread+0xf1/0x107

 [<c042cbe1>] autoremove_wake_function+0x0/0x2d

 [<c053ed49>] xenwatch_thread+0x0/0x107

 [<c042cb15>] kthread+0xc0/0xeb

 [<c042ca55>] kthread+0x0/0xeb

 [<c04029b5>] kernel_thread_helper+0x5/0xb