I'm running a linux-2.6.26.1 kernel with the real-time
patch-2.6.26-rt1 plus most of the patches discussed on the
linux-rt-users list since rt1. (except ppc patches, and not Gregory
Haskins experimental stuff)
The above info is mostly just full exclosure, I'm sure that this bug
exists in plain linux-2.6.26 + real-time patch-2.6.26-rt1 too.

BUG: using smp_processor_id() in preemptible [00000000] code: firefox-bin/4091
caller is __qdisc_run+0x160/0x1e9
Pid: 4091, comm: firefox-bin Tainted: G        W 2.6.26.1-rt1.jk #4

Call Trace:
 [<ffffffff803468ac>] debug_smp_processor_id+0xe4/0xf4
 [<ffffffff8040673f>] __qdisc_run+0x160/0x1e9
 [<ffffffff803f406c>] dev_queue_xmit+0x1b3/0x2ee
 [<ffffffff8041abec>] ip_finish_output+0x2a6/0x2ef
 [<ffffffff8041ad18>] ip_output+0xe3/0xec
 [<ffffffff80419c54>] ip_local_out+0x25/0x29
 [<ffffffff8041a4c6>] ip_queue_xmit+0x2ce/0x35e
 [<ffffffff8042e274>] ? __tcp_push_pending_frames+0x74a/0x860
 [<ffffffff8042e274>] ? __tcp_push_pending_frames+0x74a/0x860
 [<ffffffff80286265>] ? trace_preempt_on+0x1f/0x105
 [<ffffffff8042b6b2>] ? tcp_transmit_skb+0x72a/0x78f
 [<ffffffff8042e274>] ? __tcp_push_pending_frames+0x74a/0x860
 [<ffffffff8042b6d8>] tcp_transmit_skb+0x750/0x78f
 [<ffffffff8042e274>] __tcp_push_pending_frames+0x74a/0x860
 [<ffffffff802b53b3>] ? __kmalloc_node+0x48/0x4a
 [<ffffffff803ee44d>] ? __alloc_skb+0x70/0x136
 [<ffffffff8042e518>] tcp_send_fin+0x18e/0x19a
 [<ffffffff80420358>] tcp_close+0x1cc/0x413
 [<ffffffff8043d1a6>] inet_release+0x55/0x5c
 [<ffffffff803e78af>] sock_release+0x1f/0xb2
 [<ffffffff803e797b>] sock_close+0x39/0x3f
 [<ffffffff802bb11b>] __fput+0xca/0x18d
 [<ffffffff802bb1f7>] fput+0x19/0x1b
 [<ffffffff802b8120>] filp_close+0x6b/0x76
 [<ffffffff802b81d5>] sys_close+0xaa/0xe9
 [<ffffffff80226c07>] sysenter_do_call+0x8c/0x149
 [<ffffffff8046ca94>] ? trace_hardirqs_on_thunk+0x3a/0x3c

---------------------------
| preempt count: 00000001 ]
| 1-level deep critical section nesting:
----------------------------------------
.. [<ffffffff80346859>] .... debug_smp_processor_id+0x91/0xf4
.....[<ffffffff8040673f>] ..   ( <= __qdisc_run+0x160/0x1e9)

BUG: firefox-bin:4091 task might have lost a preemption check!
Pid: 4091, comm: firefox-bin Tainted: G        W 2.6.26.1-rt1.jk #4

Call Trace:
 [<ffffffff804708c8>] ? sub_preempt_count+0xd1/0xe6
 [<ffffffff80235c41>] preempt_enable_no_resched+0x5c/0x5e
 [<ffffffff803468b1>] debug_smp_processor_id+0xe9/0xf4
 [<ffffffff8040673f>] __qdisc_run+0x160/0x1e9
 [<ffffffff803f406c>] dev_queue_xmit+0x1b3/0x2ee
 [<ffffffff8041abec>] ip_finish_output+0x2a6/0x2ef
 [<ffffffff8041ad18>] ip_output+0xe3/0xec
 [<ffffffff80419c54>] ip_local_out+0x25/0x29
 [<ffffffff8041a4c6>] ip_queue_xmit+0x2ce/0x35e
 [<ffffffff8042e274>] ? __tcp_push_pending_frames+0x74a/0x860
 [<ffffffff8042e274>] ? __tcp_push_pending_frames+0x74a/0x860
 [<ffffffff80286265>] ? trace_preempt_on+0x1f/0x105
 [<ffffffff8042b6b2>] ? tcp_transmit_skb+0x72a/0x78f
 [<ffffffff8042e274>] ? __tcp_push_pending_frames+0x74a/0x860
 [<ffffffff8042b6d8>] tcp_transmit_skb+0x750/0x78f
 [<ffffffff8042e274>] __tcp_push_pending_frames+0x74a/0x860
 [<ffffffff802b53b3>] ? __kmalloc_node+0x48/0x4a
 [<ffffffff803ee44d>] ? __alloc_skb+0x70/0x136
 [<ffffffff8042e518>] tcp_send_fin+0x18e/0x19a
 [<ffffffff80420358>] tcp_close+0x1cc/0x413
 [<ffffffff8043d1a6>] inet_release+0x55/0x5c
 [<ffffffff803e78af>] sock_release+0x1f/0xb2
 [<ffffffff803e797b>] sock_close+0x39/0x3f
 [<ffffffff802bb11b>] __fput+0xca/0x18d
 [<ffffffff802bb1f7>] fput+0x19/0x1b
 [<ffffffff802b8120>] filp_close+0x6b/0x76
 [<ffffffff802b81d5>] sys_close+0xaa/0xe9
 [<ffffffff80226c07>] sysenter_do_call+0x8c/0x149
 [<ffffffff8046ca94>] ? trace_hardirqs_on_thunk+0x3a/0x3c

---------------------------
| preempt count: 00000000 ]
| 0-level deep critical section nesting:

__qdisc_run() calls qdisc_restart() which calls
handle_dev_cpu_collision(skb, dev, q); and then the problem shows up
here:
__get_cpu_var(netdev_rx_stat).cpu_collision++;

The solution is to disable interrupts around the above increment. Here
is an attached patch to do so. (Thank's to Peter Zijlstra for help in
the analysis and dropping the answer in my lap, so if I got it right
it is due to his help, but if I messed it up, then I did that part all
by myself.)

Unless there are objections, please apply.