kernel panic in skb_copy_bits

[Joe Jin <joe.jin@oracle.com>]

Hi,

When we do fail over test with iscsi + multipath by reset the switches
on OVM(2.6.39) we hit the panic:

BUG: unable to handle kernel paging request at ffff88006d9e8d48
IP: [<ffffffff812605bb>] memcpy+0xb/0x120
PGD 1798067 PUD 1fd2067 PMD 213f067 PTE 0
Oops: 0000 [#1] SMP 
CPU 7 
Modules linked in: dm_nfs tun nfs fscache auth_rpcgss nfs_acl xen_blkback xen_netback xen_gntdev xen_evtchn lockd sunrpc bridge stp llc bonding be2iscsi iscsi_boot_sysfs ib_iser rdma_cm ib_cm iw_cm ib_sa ib_mad ib_core ib_addr iscsi_tcp bnx2i cnic uio ipv6 cxgb3i libcxgbi cxgb3 mdio dm_round_robin dm_multipath libiscsi_tcp libiscsi scsi_transport_iscsi xenfs xen_privcmd video sbs sbshc acpi_memhotplug acpi_ipmi ipmi_msghandler parport_pc lp parport ixgbe dca sr_mod cdrom bnx2 radeon ttm drm_kms_helper drm snd_seq_dummy i2c_algo_bit i2c_core snd_seq_oss snd_seq_midi_event snd_seq snd_seq_device snd_pcm_oss snd_mixer_oss serio_raw snd_pcm snd_timer snd soundcore snd_page_alloc iTCO_wdt pcspkr iTCO_vendor_support pata_acpi dcdbas i5k_amb ata_generic hwmon floppy ghes i5000_edac edac_core hed 
 dm_snapshot dm_zero dm_mirror dm_region_hash dm_log dm_mod usb_storage lpfc scsi_transport_fc scsi_tgt ata_piix sg shpchp mptsas mptscsih mptbase scsi_transport_sas sd_mod crc_t10dif ext3 j!
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Pid: 0, comm: swapper Tainted: G        W   2.6.39-300.32.1.el5uek #1 Dell Inc. PowerEdge 2950/0DP246
RIP: e030:[<ffffffff812605bb>]  [<ffffffff812605bb>] memcpy+0xb/0x120
RSP: e02b:ffff8801003c3d58  EFLAGS: 00010246
RAX: ffff880076b9e280 RBX: ffff8800714d2c00 RCX: 0000000000000057
RDX: 0000000000000000 RSI: ffff88006d9e8d48 RDI: ffff880076b9e280
RBP: ffff8801003c3dc0 R08: 00000000000bf723 R09: 0000000000000000
R10: 0000000000000000 R11: 000000000000000a R12: 0000000000000034
R13: 0000000000000034 R14: 00000000000002b8 R15: 00000000000005a8
FS:  00007fc1e852a6e0(0000) GS:ffff8801003c0000(0000) knlGS:0000000000000000
CS:  e033 DS: 002b ES: 002b CR0: 000000008005003b
CR2: ffff88006d9e8d48 CR3: 000000006370b000 CR4: 0000000000002660
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process swapper (pid: 0, threadinfo ffff880077ac0000, task ffff880077abe240)
Stack:
 ffffffff8142db21 0000000000000000 ffff880076b9e280 ffff8800637097f0
 000002ec00000000 00000000000002b8 ffff880077ac0000 0000000000000000
 ffff8800637097f0 ffff880066c9a7c0 00000000fffffdb4 000000000000024c
Call Trace:
 <IRQ> 
 [<ffffffff8142db21>] ? skb_copy_bits+0x1c1/0x2e0
 [<ffffffff8142f173>] skb_copy+0xf3/0x120
 [<ffffffff81447fbc>] neigh_timer_handler+0x1ac/0x350
 [<ffffffff810573fe>] ? account_idle_ticks+0xe/0x10
 [<ffffffff81447e10>] ? neigh_alloc+0x180/0x180
 [<ffffffff8107dbaa>] call_timer_fn+0x4a/0x110
 [<ffffffff81447e10>] ? neigh_alloc+0x180/0x180
 [<ffffffff8107f82a>] run_timer_softirq+0x13a/0x220
 [<ffffffff81075c39>] __do_softirq+0xb9/0x1d0
 [<ffffffff810d9678>] ? handle_percpu_irq+0x48/0x70
 [<ffffffff81511d3c>] call_softirq+0x1c/0x30
 [<ffffffff810172e5>] do_softirq+0x65/0xa0
 [<ffffffff8107656b>] irq_exit+0xab/0xc0
 [<ffffffff812f97d5>] xen_evtchn_do_upcall+0x35/0x50
 [<ffffffff81511d8e>] xen_do_hypervisor_callback+0x1e/0x30
 <EOI> 
 [<ffffffff810013aa>] ? xen_hypercall_sched_op+0xa/0x20
 [<ffffffff810013aa>] ? xen_hypercall_sched_op+0xa/0x20
 [<ffffffff8100a0b0>] ? xen_safe_halt+0x10/0x20
 [<ffffffff8101dfeb>] ? default_idle+0x5b/0x170
 [<ffffffff81014ac6>] ? cpu_idle+0xc6/0xf0
 [<ffffffff8100a8c9>] ? xen_irq_enable_direct_reloc+0x4/0x4
 [<ffffffff814f7bbe>] ? cpu_bringup_and_idle+0xe/0x10
Code: 01 c6 43 4c 04 19 c0 4c 8b 65 f0 4c 8b 6d f8 83 e0 fc 83 c0 08 88 43 4d 48 8b 5d e8 c9 c3 90 90 48 89 f8 89 d1 c1 e9 03 83 e2 07 <f3> 48 a5 89 d1 f3 a4 c3 20 48 83 ea 20 4c 8b 06 4c 8b 4e 08 4c 
RIP  [<ffffffff812605bb>] memcpy+0xb/0x120
 RSP <ffff8801003c3d58>
CR2: ffff88006d9e8d48

Reviewed vmcore I found the skb->users is 1 at the moment, checked network neighbour 
history I found skb_get() be replaced by skb_copy by commit 7e36763b2c:

commit 7e36763b2c204d59de4e88087f84a2c0c8421f25                               
Author: Frank Blaschka <frank.blaschka@de.ibm.com>                            
Date:   Mon Mar 3 12:16:04 2008 -0800

    [NET]: Fix race in generic address resolution.                            
        
    neigh_update sends skb from neigh->arp_queue while neigh_timer_handler    
    has increased skbs refcount and calls solicit with the                    
    skb. neigh_timer_handler should not increase skbs refcount but make a     
    copy of the skb and do solicit with the copy.                             
    
    Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>                 
    Signed-off-by: David S. Miller <davem@davemloft.net>                      

So can you please give some details of the race? per vmcore seems like the skb data
be freed, I suspected skb_get() lost at somewhere?
I reverted above commit the panic not occurred during our testing.

Any input will appreciate!

Best Regards,
Joe