Public bug reported:

Scheduler deadlocks have been observed on c1.xlarge EC2 instances
running 10.04.3 LTS with the  3.0.0-20-virtual Oneiric backport kernel.
The symptoms appear similar to bug 929941, where multiple CPUs are
waiting on scheduler runqueue locks. But in this case, only a few CPUs
are stuck.

A typical set of stack traces from the guest state looks like:

VCPU0
rip: ffffffff810013aa hypercall_page+0x3aa
flags: 00001202 i nz
rsp: ffff8801b3c27910
rax: 0000000000000000   rcx: ffffffff810013aa   rdx: ffff8801b3c27954
rbx: ffff88000265cb30   rsi: ffff8801b3c27938   rdi: 0000000000000003
rbp: ffff8801b3c27958    r8: 0000000000000001    r9: 0000000000000001
r10: 0000000000000000   r11: 0000000000000202   r12: 0000000000000011
r13: 0000000000000001   r14: 0000000000000001   r15: 0000000000000000
 cs: e033        ss: e02b        ds: 0000        es: 0000
 fs: 0000 @ 00007f4ce223f700
 gs: 0000 @ ffff8801bfed4000/0000000000000000

cr0: 80050033
cr2: 0061ade0
cr3: 0e93d000
cr4: 00002660

dr0: 00000000
dr1: 00000000
dr2: 00000000
dr3: 00000000
dr6: ffff0ff0
dr7: 00000400
Code (instr addr ffffffff810013aa)
cc cc cc cc cc cc cc cc cc cc cc 51 41 53 b8 1d 00 00 00 0f 05 <41> 5b 59 c3 cc 
cc cc cc cc cc cc


Stack:
 0000000000000246 0000000000000000 ffffffff81394b42 ffff8801b3c27938
 0000000000000000 ffff8801b3c27954 ffffffff00000001 0000000000000000
 0000000481394ad6 ffff8801b3c27968 ffffffff81394b60 ffff8801b3c279b8
 ffffffff8100933f ffff8801b3c27a48 0000000000000000 ffff8801b3c27998

Call Trace:
  [<ffffffff810013aa>] hypercall_page+0x3aa  <--
  [<ffffffff81394b42>] xen_poll_irq_timeout+0x42
  [<ffffffff81394b60>] xen_poll_irq+0x10
  [<ffffffff8100933f>] xen_spin_lock_slow+0x7f
  [<ffffffff81009435>] xen_spin_lock_flags+0x75
  [<ffffffff8160365f>] _raw_spin_lock_irqsave+0x2f
  [<ffffffff8104ee50>] task_rq_lock+0x40
  [<ffffffff8104f069>] task_sched_runtime+0x29
  [<ffffffff81085f38>] thread_group_cputime+0x88
  [<ffffffff812c4dc9>] apparmor_ptrace_access_check+0x39
  [<ffffffff81051593>] thread_group_times+0x33
  [<ffffffff811d4192>] do_task_stat+0x6d2
  [<ffffffff8160361e>] _raw_spin_lock+0xe
  [<ffffffff8119165f>] seq_open+0x4f
  [<ffffffff811d0380>] sched_autogroup_show+0x70
  [<ffffffff811d0380>] sched_autogroup_show+0x70
  [<ffffffff8119173a>] single_open+0x7a
  [<ffffffff811cdbf0>] sched_open+0x20
  [<ffffffff811cdc0b>] proc_single_open+0x1b
  [<ffffffff8118e460>] mntput_no_expire+0x60
  [<ffffffff8118e5ad>] mntput+0x1d
  [<ffffffff811d4624>] proc_tgid_stat+0x14
  [<ffffffff811d03e1>] proc_single_show+0x61
  [<ffffffff81191be2>] seq_read+0xf2
  [<ffffffff8116fe35>] vfs_read+0xc5
  [<ffffffff81170001>] sys_read+0x51
  [<ffffffff8160ba02>] system_call_fastpath+0x16


VCPU1
rip: ffffffff8105a777 try_to_wake_up+0xd7
flags: 00001202 i nz
rsp: ffff8801bfef28f0
rax: 0000000000000003   rcx: 0000000000000000   rdx: 0000000000000001
rbx: 0000000000012980   rsi: ffff8801b1990078   rdi: 0000000000000000
rbp: ffff8801bfef2950    r8: 0000000000000000    r9: 0000000000000000
r10: 0000000000000000   r11: 00000000fb981853   r12: ffff88000265c530
r13: 0000000000000000   r14: ffff88000265cb30   r15: 0000000000000000
 cs: e033        ss: e02b        ds: 0000        es: 0000
 fs: 0000 @ 00007ff68926c700
 gs: 0000 @ ffff8801bfeef000/0000000000000000

cr0: 8005003b
cr2: 00441d80
cr3: 1174cb000
cr4: 00002660

dr0: 00000000
dr1: 00000000
dr2: 00000000
dr3: 00000000
dr6: ffff0ff0
dr7: 00000400
Code (instr addr ffffffff8105a777)
00 00 eb 0c 66 2e 0f 1f 84 00 00 00 00 00 f3 90 41 8b 54 24 28 <85> d2 75 f5 49 
8b 14 24 31 c0 83


Stack:
 0000000000000000 ffff8801b41d2858 ffff8801bfef2950 ffffffff8153e51e
 0000000300000004 ffff8801b1990078 ffff8801bfef2930 ffff8800026f6c18
 0000000000000001 ffff8800026f6c30 0000000000000000 0000000000000000
 ffff8801bfef2960 ffffffff8105a962 ffff8801bfef29b0 ffffffff81049709

Call Trace:
  [<ffffffff8105a777>] try_to_wake_up+0xd7  <--
  [<ffffffff8153e51e>] ip_finish_output+0x16e
  [<ffffffff8105a962>] default_wake_function+0x12
  [<ffffffff81049709>] __wake_up_common+0x59
  [<ffffffff81049758>] __wake_up_locked+0x18
  [<ffffffff811afcf4>] ep_poll_callback+0xa4
  [<ffffffff81049709>] __wake_up_common+0x59
  [<ffffffff8104ed53>] __wake_up_sync_key+0x53
  [<ffffffff814f3d2e>] sock_def_readable+0x3e
  [<ffffffff8155255a>] tcp_rcv_established+0x26a
  [<ffffffff8100742d>] xen_force_evtchn_callback+0xd
  [<ffffffff81007b72>] check_events+0x12
  [<ffffffff8155a9a5>] tcp_v4_do_rcv+0x125
  [<ffffffff8155c169>] tcp_v4_rcv+0x5a9
  [<ffffffff8153854d>] ip_local_deliver_finish+0xdd
  [<ffffffff81538790>] ip_local_deliver+0x80
  [<ffffffff81537db9>] ip_rcv_finish+0x119
  [<ffffffff815383a8>] ip_rcv+0x228
  [<ffffffff815d0afd>] packet_rcv_spkt+0x4d
  [<ffffffff815036b0>] __netif_receive_skb+0x1e0
  [<ffffffff81506790>] netif_receive_skb+0x80
  [<ffffffff814614f4>] handle_incoming_queue+0x134
  [<ffffffff81461fd7>] xennet_poll+0x277
  [<ffffffff81506ff8>] net_rx_action+0x108
  [<ffffffff8160361e>] _raw_spin_lock+0xe
  [<ffffffff8106889f>] __do_softirq+0xbf
  [<ffffffff810d3f7d>] handle_edge_irq+0x9d
  [<ffffffff8160cc1c>] call_softirq+0x1c
  [<ffffffff8100d3d5>] do_softirq+0x65
  [<ffffffff8106869d>] irq_exit+0xbd
  [<ffffffff81394ee5>] xen_evtchn_do_upcall+0x35
  [<ffffffff8160cc6e>] xen_do_hypervisor_callback+0x1e


VCPU2
rip: ffffffff810013aa hypercall_page+0x3aa
flags: 00001202 i nz
rsp: ffff8801bff0da00
rax: 0000000000000000   rcx: ffffffff810013aa   rdx: ffff8801bff0da44
rbx: ffff8800026f6c00   rsi: ffff8801bff0da28   rdi: 0000000000000003
rbp: ffff8801bff0da48    r8: 00000000000000c3    r9: 000000000000c110
r10: 0000000000100000   r11: 0000000000000202   r12: 000000000000001d
r13: 0000000000000001   r14: 0000000000000001   r15: 0000000000000000
 cs: e033        ss: e02b        ds: 0000        es: 0000
 fs: 0000 @ 00007f408128b700
 gs: 0000 @ ffff8801bff0a000/0000000000000000

cr0: 80050033
cr2: 0061ade0
cr3: 289f2000
cr4: 00002660

dr0: 00000000
dr1: 00000000
dr2: 00000000
dr3: 00000000
dr6: ffff0ff0
dr7: 00000400
Code (instr addr ffffffff810013aa)
cc cc cc cc cc cc cc cc cc cc cc 51 41 53 b8 1d 00 00 00 0f 05 <41> 5b 59 c3 cc 
cc cc cc cc cc cc


Stack:
 0000000000112000 0000000000000000 ffffffff81394b42 ffff8801bff0da28
 0000000000000000 ffff8801bff0da44 ffffffff00000001 0000000000000000
 0000001081394ad6 ffff8801bff0da58 ffffffff81394b60 ffff8801bff0daa8
 ffffffff8100933f ffff8801bff0da88 0000000000000000 ffff8801b3d14530

Call Trace:
  [<ffffffff810013aa>] hypercall_page+0x3aa  <--
  [<ffffffff81394b42>] xen_poll_irq_timeout+0x42
  [<ffffffff81394b60>] xen_poll_irq+0x10
  [<ffffffff8100933f>] xen_spin_lock_slow+0x7f
  [<ffffffff81009435>] xen_spin_lock_flags+0x75
  [<ffffffff8160365f>] _raw_spin_lock_irqsave+0x2f
  [<ffffffff811afca9>] ep_poll_callback+0x59
  [<ffffffff816036ae>] _raw_spin_unlock_irqrestore+0x1e
  [<ffffffff81049709>] __wake_up_common+0x59
  [<ffffffff8104ed53>] __wake_up_sync_key+0x53
  [<ffffffff814f3d2e>] sock_def_readable+0x3e
  [<ffffffff8154ef50>] tcp_data_queue+0x300
  [<ffffffff81552639>] tcp_rcv_established+0x349
  [<ffffffff8155a9a5>] tcp_v4_do_rcv+0x125
  [<ffffffff8155c169>] tcp_v4_rcv+0x5a9
  [<ffffffff81054e18>] enqueue_sleeper+0x188
  [<ffffffff8153854d>] ip_local_deliver_finish+0xdd
  [<ffffffff81538790>] ip_local_deliver+0x80
  [<ffffffff81537db9>] ip_rcv_finish+0x119
  [<ffffffff815383a8>] ip_rcv+0x228
  [<ffffffff815036b0>] __netif_receive_skb+0x1e0
  [<ffffffff81007b5f>] xen_restore_fl_direct_reloc+0x4
  [<ffffffff810d7af4>] rcu_enter_nohz+0x44
  [<ffffffff81503aeb>] process_backlog+0x10b
  [<ffffffff810d10b5>] handle_irq_event_percpu+0xb5
  [<ffffffff81506ff8>] net_rx_action+0x108
  [<ffffffff8100742d>] xen_force_evtchn_callback+0xd
  [<ffffffff8106889f>] __do_softirq+0xbf
  [<ffffffff81007b5f>] xen_restore_fl_direct_reloc+0x4
  [<ffffffff8160cc1c>] call_softirq+0x1c


VCPU3
rip: ffffffff810013aa hypercall_page+0x3aa
flags: 00001202 i nz
rsp: ffff8801b1993b10
rax: 0000000000000000   rcx: ffffffff810013aa   rdx: ffff8801b1993b54
rbx: ffff8801bff01980   rsi: ffff8801b1993b38   rdi: 0000000000000003
rbp: ffff8801b1993b58    r8: ffff8801bf004760    r9: 0000000000000040
r10: 000000000000001d   r11: 0000000000000202   r12: 0000000000000023
r13: 0000000000000001   r14: 0000000000000001   r15: 0000000000000000
 cs: e033        ss: e02b        ds: 0000        es: 0000
 fs: 0000 @ 00007f1a15c06700
 gs: 0000 @ ffff8801bff25000/0000000000000000

cr0: 80050033
cr2: 7f78b037de20
cr3: 0e9c9000
cr4: 00002660

dr0: 00000000
dr1: 00000000
dr2: 00000000
dr3: 00000000
dr6: ffff0ff0
dr7: 00000400
Code (instr addr ffffffff810013aa)
cc cc cc cc cc cc cc cc cc cc cc 51 41 53 b8 1d 00 00 00 0f 05 <41> 5b 59 c3 cc 
cc cc cc cc cc cc


Stack:
 0000000000000001 0000000000000000 ffffffff81394b42 ffff8801b1993b38
 0000000000000000 ffff8801b1993b54 ffffffff00000001 0000000000000000
 0000001681394ad6 ffff8801b1993b68 ffffffff81394b60 ffff8801b1993bb8
 ffffffff8100933f 0000000000000000 0000000000000000 0000000000000000

Call Trace:
  [<ffffffff810013aa>] hypercall_page+0x3aa  <--
  [<ffffffff81394b42>] xen_poll_irq_timeout+0x42
  [<ffffffff81394b60>] xen_poll_irq+0x10
  [<ffffffff8100933f>] xen_spin_lock_slow+0x7f
  [<ffffffff81009496>] xen_spin_lock+0x56
  [<ffffffff8160361e>] _raw_spin_lock+0xe
  [<ffffffff8104d90d>] double_rq_lock+0x2d
  [<ffffffff81059f15>] load_balance+0x1f5
  [<ffffffff81601576>] __schedule+0x7e6
  [<ffffffff8160165f>] schedule+0x3f
  [<ffffffff816025b5>] schedule_hrtimeout_range_clock+0xc5
  [<ffffffff810881c0>] update_rmtp+0x80
  [<ffffffff81089524>] hrtimer_start_range_ns+0x14
  [<ffffffff81602663>] schedule_hrtimeout_range+0x13
  [<ffffffff811af9f8>] ep_poll+0x2d8
  [<ffffffff8105a950>] try_to_wake_up+0x2b0
  [<ffffffff81290c2b>] security_file_permission+0x8b
  [<ffffffff8116fee0>] vfs_read+0x170
  [<ffffffff811afb35>] sys_epoll_wait+0xc5
  [<ffffffff8160ba02>] system_call_fastpath+0x16


VCPU4
rip: ffffffff810013aa hypercall_page+0x3aa
flags: 00001246 i z p
rsp: ffff8801b45f5ed8
rax: 0000000000000000   rcx: ffffffff810013aa   rdx: 0000000000000000
rbx: ffff8801b45f4010   rsi: 0000000000000000   rdi: 0000000000000001
rbp: ffff8801b45f5ef0    r8: 0000000000000000    r9: 0000000000000000
r10: 0000000000000000   r11: 0000000000000246   r12: 0000000000000004
r13: 0000000000000000   r14: 0000000000000000   r15: 0000000000000000
 cs: e033        ss: e02b        ds: 002b        es: 002b
 fs: 0000 @ 00007fe085364700
 gs: 0000 @ ffff8801bff40000/0000000000000000

cr0: 8005003b
cr2: 7f7127c10000
cr3: 29c76000
cr4: 00002660

dr0: 00000000
dr1: 00000000
dr2: 00000000
dr3: 00000000
dr6: ffff0ff0
dr7: 00000400
Code (instr addr ffffffff810013aa)
cc cc cc cc cc cc cc cc cc cc cc 51 41 53 b8 1d 00 00 00 0f 05 <41> 5b 59 c3 cc 
cc cc cc cc cc cc


Stack:
 0000000000000000 0000000000000000 ffffffff81007490 ffff8801b45f5f10
 ffffffff81013c3d ffff8801b45f4010 ffffffff81cc7f48 ffff8801b45f5f40
 ffffffff8100b0c7 ffffffff81007b19 6811f30c74562efe 0000000000000000
 0000000000000000 ffff8801b45f5f50 ffffffff815f523d 0000000000000000

Call Trace:
  [<ffffffff810013aa>] hypercall_page+0x3aa  <--
  [<ffffffff81007490>] xen_safe_halt+0x10
  [<ffffffff81013c3d>] default_idle+0x5d
  [<ffffffff8100b0c7>] cpu_idle+0xb7
  [<ffffffff81007b19>] xen_irq_enable_direct_reloc+0x4
  [<ffffffff815f523d>] cpu_bringup_and_idle+0xe


VCPU5
rip: ffffffff810013aa hypercall_page+0x3aa
flags: 00001246 i z p
rsp: ffff8801b45f7ed8
rax: 0000000000000000   rcx: ffffffff810013aa   rdx: 0000000000000000
rbx: ffff8801b45f6010   rsi: 0000000000000000   rdi: 0000000000000001
rbp: ffff8801b45f7ef0    r8: 0000000000000000    r9: 0000000000000000
r10: 0000000000000000   r11: 0000000000000246   r12: 0000000000000005
r13: 0000000000000000   r14: 0000000000000000   r15: 0000000000000000
 cs: e033        ss: e02b        ds: 002b        es: 002b
 fs: 0000 @ 00007fcf9cf7a700
 gs: 0000 @ ffff8801bff5b000/0000000000000000

cr0: 8005003b
cr2: 7faf54aac360
cr3: 325e8000
cr4: 00002660

dr0: 00000000
dr1: 00000000
dr2: 00000000
dr3: 00000000
dr6: ffff0ff0
dr7: 00000400
Code (instr addr ffffffff810013aa)
cc cc cc cc cc cc cc cc cc cc cc 51 41 53 b8 1d 00 00 00 0f 05 <41> 5b 59 c3 cc 
cc cc cc cc cc cc


Stack:
 0000000000000000 0000000000000000 ffffffff81007490 ffff8801b45f7f10
 ffffffff81013c3d ffff8801b45f6010 ffffffff81cc7f48 ffff8801b45f7f40
 ffffffff8100b0c7 ffffffff81007b19 afec8dc6140892fe 0000000000000000
 0000000000000000 ffff8801b45f7f50 ffffffff815f523d 0000000000000000

Call Trace:
  [<ffffffff810013aa>] hypercall_page+0x3aa  <--
  [<ffffffff81007490>] xen_safe_halt+0x10
  [<ffffffff81013c3d>] default_idle+0x5d
  [<ffffffff8100b0c7>] cpu_idle+0xb7
  [<ffffffff81007b19>] xen_irq_enable_direct_reloc+0x4
  [<ffffffff815f523d>] cpu_bringup_and_idle+0xe


VCPU6
rip: ffffffff810013aa hypercall_page+0x3aa
flags: 00001246 i z p
rsp: ffff8801b4601ed8
rax: 0000000000000000   rcx: ffffffff810013aa   rdx: 0000000000000000
rbx: ffff8801b4600010   rsi: 0000000000000000   rdi: 0000000000000001
rbp: ffff8801b4601ef0    r8: 0000000000000000    r9: 0000000000000000
r10: 0000000000000000   r11: 0000000000000246   r12: 0000000000000006
r13: 0000000000000000   r14: 0000000000000000   r15: 0000000000000000
 cs: e033        ss: e02b        ds: 002b        es: 002b
 fs: 0000 @ 00007ff5deb9b700
 gs: 0000 @ ffff8801bff76000/0000000000000000

cr0: 8005003b
cr2: 7faf5345dff8
cr3: 10160000
cr4: 00002660

dr0: 00000000
dr1: 00000000
dr2: 00000000
dr3: 00000000
dr6: ffff0ff0
dr7: 00000400
Code (instr addr ffffffff810013aa)
cc cc cc cc cc cc cc cc cc cc cc 51 41 53 b8 1d 00 00 00 0f 05 <41> 5b 59 c3 cc 
cc cc cc cc cc cc


Stack:
 0000000000000000 0000000000000000 ffffffff81007490 ffff8801b4601f10
 ffffffff81013c3d ffff8801b4600010 ffffffff81cc7f48 ffff8801b4601f40
 ffffffff8100b0c7 ffffffff81007b19 f6fcdef4e1be86f4 0000000000000000
 0000000000000000 ffff8801b4601f50 ffffffff815f523d 0000000000000000

Call Trace:
  [<ffffffff810013aa>] hypercall_page+0x3aa  <--
  [<ffffffff81007490>] xen_safe_halt+0x10
  [<ffffffff81013c3d>] default_idle+0x5d
  [<ffffffff8100b0c7>] cpu_idle+0xb7
  [<ffffffff81007b19>] xen_irq_enable_direct_reloc+0x4
  [<ffffffff815f523d>] cpu_bringup_and_idle+0xe


VCPU7
rip: ffffffff810013aa hypercall_page+0x3aa
flags: 00001246 i z p
rsp: ffff8801b4603ed8
rax: 0000000000000000   rcx: ffffffff810013aa   rdx: 0000000000000000
rbx: ffff8801b4602010   rsi: 0000000000000000   rdi: 0000000000000001
rbp: ffff8801b4603ef0    r8: 0000000000000000    r9: 0000000000000000
r10: 0000000000000000   r11: 0000000000000246   r12: 0000000000000007
r13: 0000000000000000   r14: 0000000000000000   r15: 0000000000000000
 cs: e033        ss: e02b        ds: 002b        es: 002b
 fs: 0000 @ 00007fe081751700
 gs: 0000 @ ffff8801bff91000/0000000000000000

cr0: 8005003b
cr2: 7faf54bdfde0
cr3: 29c76000
cr4: 00002660

dr0: 00000000
dr1: 00000000
dr2: 00000000
dr3: 00000000
dr6: ffff0ff0
dr7: 00000400
Code (instr addr ffffffff810013aa)
cc cc cc cc cc cc cc cc cc cc cc 51 41 53 b8 1d 00 00 00 0f 05 <41> 5b 59 c3 cc 
cc cc cc cc cc cc


Stack:
 0000000000000000 0000000000000000 ffffffff81007490 ffff8801b4603f10
 ffffffff81013c3d ffff8801b4602010 ffffffff81cc7f48 ffff8801b4603f40
 ffffffff8100b0c7 ffffffff81007b19 0c1d63da394a80a3 0000000000000000
 0000000000000000 ffff8801b4603f50 ffffffff815f523d 0000000000000000

Call Trace:
  [<ffffffff810013aa>] hypercall_page+0x3aa  <--
  [<ffffffff81007490>] xen_safe_halt+0x10
  [<ffffffff81013c3d>] default_idle+0x5d
  [<ffffffff8100b0c7>] cpu_idle+0xb7
  [<ffffffff81007b19>] xen_irq_enable_direct_reloc+0x4
  [<ffffffff815f523d>] cpu_bringup_and_idle+0xe

** Affects: linux-lts-backport-oneiric (Ubuntu)
     Importance: Undecided
         Status: New

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Title:
  Scheduler deadlock running 3.0.0-20-virtual on c1.xlarge EC2 instance

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