crash in dummynet, , si->sched

[Andriy Gapon]

Unfortunately, all we have is some information from a ddb text dump.  We do not
have a vmcore and we do not have a way to re-create the crash.  It happened just
once on a production system.

So, the information follows.

dn_enqueue fs 0 si 0, dropping


Fatal trap 12: page fault while in kernel mode
cpuid = 0; apic id = 00
fault virtual address   = 0x60
fault code              = supervisor read data, page not present
instruction pointer     = 0x20:0xffffffff8077bdff
stack pointer           = 0x28:0xfffffe1096343910
frame pointer           = 0x28:0xfffffe1096343920
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 0 (dummynet)

db:3:psinfo> bt
Tracing pid 0 tid 100248 td 0xfffff8002829d4d0
 stack1 drain_scheduler_cb+0x1f drain_scheduler_sch_cb+0x25
dn_ht_scan_bucket+0x7a dn_drain_scheduler+0x20 dummynet_task+0x219
taskqueue_run_locked+0x71 taskqueue_thread_loop+0x56 fork_exit+0x121
fork_trampoline+0xe

drain_scheduler_cb+0x1f:        movq    0x60(%rdx),%rax

Here is disassembly of the function with some notes of mine:
   0xffffffff8077bde0 <+0>:     push   %rbp
   0xffffffff8077bde1 <+1>:     testb  $0x20,0x90(%rdi) // test DN_ACTIVE
   0xffffffff8077bde8 <+8>:     mov    %rsp,%rbp
   0xffffffff8077bdeb <+11>:    jne    0xffffffff8077bdf4 <drain_scheduler_cb+20>
   0xffffffff8077bded <+13>:    cmpq   $0x0,0x78(%rdi)
   0xffffffff8077bdf2 <+18>:    je     0xffffffff8077bdf8 <drain_scheduler_cb+24>
   0xffffffff8077bdf4 <+20>:    leaveq
   0xffffffff8077bdf5 <+21>:    xor    %eax,%eax
   0xffffffff8077bdf7 <+23>:    retq
   0xffffffff8077bdf8 <+24>:    mov    0x88(%rdi),%rdx // rdx = si->sched
   0xffffffff8077bdff <+31>:    mov    0x60(%rdx),%rax // rax = si->sched->fp
   0xffffffff8077be03 <+35>:    testb  $0x1,0x10(%rax)

So, it seems that dummynet ran into dn_sch_inst with sched field being NULL.

I am not sure how that could be possible.
Also, I am not sure if that "dn_enqueue ..." message is related to the crash.

Does anyone have any ideas?
Thank you.

P.S.
I found a somewhat similar but different and very old report:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=166937
It seems that it was not really root-caused and fixed, but marked as fixed
because of a chance that it could have been caused by flaky hardware.

-- 
Andriy Gapon