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Subject[BUG] 2.6.38-rc2: Circular Locking Dependency
As I was hunting something different I found the following (potential)
problem on an openSuSE 11.3 system with kernel 2.6.38-rc2.
The message is triggerd by smpppd starting a dsl connection.

Knut


NET: Registered protocol family 24

=======================================================
[ INFO: possible circular locking dependency detected ]
2.6.38-rc2-kape #7
-------------------------------------------------------
pppd/2529 is trying to acquire lock:
(&(&pch->downl)->rlock){+.....}, at: [<f814a634>] ppp_push+0x59/0x4a8
[ppp_generic]
but task is already holding lock:
(&(&ppp->wlock)->rlock){+.-...}, at: [<f814ae1b>]
ppp_xmit_process+0x19/0x451 [ppp_generic]
which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (&(&ppp->wlock)->rlock){+.-...}:
[<c01462b2>] lock_acquire+0x47/0x5e
[<c0471de1>] _raw_spin_lock_bh+0x2a/0x39
[<f814ae1b>] ppp_xmit_process+0x19/0x451 [ppp_generic]
[<f814b39f>] ppp_start_xmit+0x14c/0x165 [ppp_generic]
[<c04179ae>] dev_hard_start_xmit+0x3b1/0x489
[<c0424a53>] sch_direct_xmit+0x55/0x1b1
[<c0417cf9>] dev_queue_xmit+0x273/0x4dd
[<c0434d31>] ip_finish_output+0x2b9/0x31f
[<c043578c>] ip_output+0xe0/0xfb
[<c0432c33>] ip_forward_finish+0x7b/0xa1
[<c0432ede>] ip_forward+0x285/0x313
[<c0431a30>] ip_rcv_finish+0x2b4/0x30f
[<c0431ef6>] ip_rcv+0x21c/0x242
[<c0415090>] __netif_receive_skb+0x34a/0x388
[<c04151f7>] netif_receive_skb+0x32/0x35
[<c0415218>] napi_skb_finish+0x1e/0x34
[<c0415d72>] napi_gro_receive+0xbf/0xc7
[<c035f25a>] sky2_poll+0x66e/0x92f
[<c04153eb>] net_rx_action+0x3f/0xfe
[<c0128563>] __do_softirq+0x76/0xfd
-> #1 (_xmit_NETROM){+.-...}:
[<c01462b2>] lock_acquire+0x47/0x5e
[<c0471c9c>] _raw_spin_lock_irqsave+0x2e/0x3e
[<c040ed60>] skb_dequeue+0x12/0x4a
[<f814c237>] ppp_channel_push+0x2e/0x94 [ppp_generic]
[<f814c33f>] ppp_write+0xa2/0xac [ppp_generic]
[<c0188e50>] vfs_write+0x8c/0x120
[<c018909d>] sys_write+0x3b/0x60
[<c010274c>] sysenter_do_call+0x12/0x32
-> #0 (&(&pch->downl)->rlock){+.....}:
[<c014594c>] __lock_acquire+0xe23/0x13ad
[<c01462b2>] lock_acquire+0x47/0x5e
[<c0471de1>] _raw_spin_lock_bh+0x2a/0x39
[<f814a634>] ppp_push+0x59/0x4a8 [ppp_generic]
[<f814b1d9>] ppp_xmit_process+0x3d7/0x451 [ppp_generic]
[<f814c336>] ppp_write+0x99/0xac [ppp_generic]
[<c0188e50>] vfs_write+0x8c/0x120
[<c018909d>] sys_write+0x3b/0x60
[<c010274c>] sysenter_do_call+0x12/0x32
other info that might help us debug this:

1 lock held by pppd/2529:
#0: (&(&ppp->wlock)->rlock){+.-...}, at: [<f814ae1b>]
ppp_xmit_process+0x19/0x451 [ppp_generic]
stack backtrace:
Pid: 2529, comm: pppd Not tainted 2.6.38-rc2-kape #7
Call Trace:
[<c0143c54>] ? print_circular_bug+0x93/0x9f
[<c014594c>] ? __lock_acquire+0xe23/0x13ad
[<c014599a>] ? __lock_acquire+0xe71/0x13ad
[<c01462b2>] ? lock_acquire+0x47/0x5e
[<f814a634>] ? ppp_push+0x59/0x4a8 [ppp_generic]
[<c0471de1>] ? _raw_spin_lock_bh+0x2a/0x39
[<f814a634>] ? ppp_push+0x59/0x4a8 [ppp_generic]
[<f814a634>] ? ppp_push+0x59/0x4a8 [ppp_generic]
[<c014685c>] ? mark_held_locks+0x41/0x5d
[<c0472236>] ? _raw_spin_unlock_irqrestore+0x36/0x59
[<c0146958>] ? trace_hardirqs_on_caller+0xe0/0x11a
[<c0472242>] ? _raw_spin_unlock_irqrestore+0x42/0x59
[<c040ed91>] ? skb_dequeue+0x43/0x4a
[<f814b1d9>] ? ppp_xmit_process+0x3d7/0x451 [ppp_generic]
[<c0474723>] ? sub_preempt_count+0x81/0x8e
[<c040ec5e>] ? skb_queue_tail+0x2d/0x32
[<f814c336>] ? ppp_write+0x99/0xac [ppp_generic]
[<c0188e50>] ? vfs_write+0x8c/0x120
[<f814c29d>] ? ppp_write+0x0/0xac [ppp_generic]
[<c018909d>] ? sys_write+0x3b/0x60
[<c010274c>] ? sysenter_do_call+0x12/0x32



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