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Subjectpossible recursive locking in udp4_lib_rcv
Hi!
I noticed the following warnings when running on 2.6.27-rc2
with lockdep checker enabled:

[ 2912.004106] Initializing XFRM netlink socket
[ 2922.009629]
[ 2922.009632] =============================================
[ 2922.009640] [ INFO: possible recursive locking detected ]
[ 2922.009643] 2.6.27-rc2-mst-suspend #32
[ 2922.009645] ---------------------------------------------
[ 2922.009648] Xorg/5958 is trying to acquire lock:
[ 2922.009650] (slock-AF_INET/1){-+..}, at: [<c02f105a>] __udp4_lib_rcv+0x34a/0x920
[ 2922.009660]
[ 2922.009661] but task is already holding lock:
[ 2922.009663] (slock-AF_INET/1){-+..}, at: [<c02f105a>] __udp4_lib_rcv+0x34a/0x920
[ 2922.009669]
[ 2922.009669] other info that might help us debug this:
[ 2922.009672] 4 locks held by Xorg/5958:
[ 2922.009674] #0: (rcu_read_lock){..--}, at: [<c02af476>] netif_receive_skb+0x66/0x390
[ 2922.009681] #1: (rcu_read_lock){..--}, at: [<c02d1cc0>] ip_local_deliver_finish+0x30/0x1f0
[ 2922.009688] #2: (slock-AF_INET/1){-+..}, at: [<c02f105a>] __udp4_lib_rcv+0x34a/0x920
[ 2922.009694] #3: (rcu_read_lock){..--}, at: [<c02d1cc0>] ip_local_deliver_finish+0x30/0x1f0
[ 2922.009700]
2922.009701] stack backtrace:
[ 2922.009704] Pid: 5958, comm: Xorg Not tainted 2.6.27-rc2-mst-suspend #32
[ 2922.009707] [<c014acc6>] __lock_acquire+0xee6/0x1130
[ 2922.009713] [<c014af71>] lock_acquire+0x61/0x80
[ 2922.009717] [<c02f105a>] ? __udp4_lib_rcv+0x34a/0x920
[ 2922.009721] [<c033008a>] _spin_lock_nested+0x2a/0x40
[ 2922.009726] [<c02f105a>] ? __udp4_lib_rcv+0x34a/0x920
[ 2922.009731] [<c02f105a>] __udp4_lib_rcv+0x34a/0x920
[ 2922.009738] [<f8c81360>] ? ipv4_confirm+0x0/0xe0 [nf_conntrack_ipv4]
[ 2922.009746] [<c02cb549>] ? nf_iterate+0x59/0x80
[ 2922.009751] [<c02f1642>] udp_rcv+0x12/0x20
[ 2922.009755] [<c02d1d3a>] ip_local_deliver_finish+0xaa/0x1f0
[ 2922.009758] [<c02d1cc0>] ? ip_local_deliver_finish+0x30/0x1f0
[ 2922.009763] [<c02d2250>] ip_local_deliver+0x30/0xa0
[ 2922.009766] [<c02d1c90>] ? ip_local_deliver_finish+0x0/0x1f0
[ 2922.009770] [<c030943b>] xfrm4_transport_finish+0x6b/0xf0
[ 2922.009776] [<c0309170>] ? xfrm4_rcv_encap_finish+0x0/0x60
[ 2922.009780] [<c031099d>] xfrm_input+0x22d/0x360
[ 2922.009784] [<c03091f3>] xfrm4_rcv_encap+0x23/0x30
[ 2922.009788] [<c030939b>] xfrm4_udp_encap_rcv+0x16b/0x1a0
[ 2922.009792] [<c02f0bc4>] udp_queue_rcv_skb+0x174/0x2c0
[ 2922.009795] [<c0330091>] ? _spin_lock_nested+0x31/0x40
[ 2922.009800] [<c02f148e>] __udp4_lib_rcv+0x77e/0x920
[ 2922.009805] [<f8c81360>] ? ipv4_confirm+0x0/0xe0 [nf_conntrack_ipv4]
[ 2922.009811] [<c02cb549>] ? nf_iterate+0x59/0x80
[ 2922.009816] [<c02f1642>] udp_rcv+0x12/0x20
[ 2922.009819] [<c02d1d3a>] ip_local_deliver_finish+0xaa/0x1f0
[ 2922.009822] [<c02d1cc0>] ? ip_local_deliver_finish+0x30/0x1f0
[ 2922.009827] [<c02d2250>] ip_local_deliver+0x30/0xa0
[ 2922.009830] [<c02d1c90>] ? ip_local_deliver_finish+0x0/0x1f0
[ 2922.009834] [<c02d1a3e>] ip_rcv_finish+0xfe/0x350
[ 2922.009838] [<c02cb719>] ? nf_hook_slow+0xd9/0x100
[ 2922.009842] [<c02d1940>] ? ip_rcv_finish+0x0/0x350
[ 2922.009846] [<c02d2136>] ip_rcv+0x1a6/0x290
[ 2922.009849] [<c02d1940>] ? ip_rcv_finish+0x0/0x350
[ 2922.009853] [<c02d1f90>] ? ip_rcv+0x0/0x290
[ 2922.009857] [<c02af638>] netif_receive_skb+0x228/0x390
[ 2922.009860] [<c02af476>] ? netif_receive_skb+0x66/0x390
[ 2922.009865] [<f8cb9550>] ? packet_rcv_spkt+0x0/0x110 [af_packet]
[ 2922.009872] [<c02b1fcc>] process_backlog+0x7c/0xe0
[ 2922.009876] [<c02b1887>] net_rx_action+0xa7/0x150
[ 2922.009879] [<c012ca47>] __do_softirq+0x87/0x100
[ 2922.009883] [<c012cb17>] do_softirq+0x57/0x60
[ 2922.009887] [<c012cea7>] irq_exit+0x77/0x90
[ 2922.009890] [<c01060c5>] do_IRQ+0x45/0x80
[ 2922.009894] [<c01498f4>] ? trace_hardirqs_on_caller+0xc4/0x150
[ 2922.009899] [<c010463c>] common_interrupt+0x28/0x30
[ 2922.009903] =======================
[ 2922.277171] PPP generic driver version 2.4.2

(I'm not sure what I did, something related to ipsec and/or ppp).

I have not seen this with v2.6.26 or earlier.
Any idea whether this is a real bug or lockdep false positive?

--
MST


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