Re: panic with tcp timers

 Hi,

On 6/20/16 11:55 AM, Julien Charbon wrote:
> On 6/20/16 9:39 AM, Gleb Smirnoff wrote:
>> On Fri, Jun 17, 2016 at 11:27:39AM +0200, Julien Charbon wrote:
>> J> > Comparing stable/10 and head, I see two changes that could
>> J> > affect that:
>> J> > 
>> J> > - callout_async_drain
>> J> > - switch to READ lock for inp info in tcp timers
>> J> > 
>> J> > That's why you are in To, Julien and Hans :)
>> J> > 
>> J> > We continue investigating, and I will keep you updated.
>> J> > However, any help is welcome. I can share cores.
>>
>> Now, spending some time with cores and adding a bunch of
>> extra CTRs, I have a sequence of events that lead to the
>> panic. In short, the bug is in the callout system. It seems
>> to be not relevant to the callout_async_drain, at least for
>> now. The transition to READ lock unmasked the problem, that's
>> why NetflixBSD 10 doesn't panic.
>>
>> The panic requires heavy contention on the TCP info lock.
>>
>> [CPU 1] the callout fires, tcp_timer_keep entered
>> [CPU 1] blocks on INP_INFO_RLOCK(&V_tcbinfo);
>> [CPU 2] schedules the callout
>> [CPU 2] tcp_discardcb called
>> [CPU 2] callout successfully canceled
>> [CPU 2] tcpcb freed
>> [CPU 1] unblocks... panic
>>
>> When the lock was WLOCK, all contenders were resumed in a
>> sequence they came to the lock. Now, that they are readers,
>> once the lock is released, readers are resumed in a "random"
>> order, and this allows tcp_discardcb to go before the old
>> running callout, and this unmasks the panic.
> 
>  Highly interesting.  I should be able to reproduce that (will be useful
> for testing the corresponding fix).

 Finally, I was able to reproduce it (without glebius fix).   The trick
was to really lower TCP keep timer expiration:

$ sysctl -a | grep tcp.keep
net.inet.tcp.keepidle: 7200000
net.inet.tcp.keepintvl: 75000
net.inet.tcp.keepinit: 75000
net.inet.tcp.keepcnt: 8
$ sudo bash -c "sysctl net.inet.tcp.keepidle=10 && sysctl
net.inet.tcp.keepintvl=50 && sysctl net.inet.tcp.keepinit=10"
Password:
net.inet.tcp.keepidle: 7200000 -> 10
net.inet.tcp.keepintvl: 75000 -> 50
net.inet.tcp.keepinit: 75000 -> 10

 Note: It will certainly close all your ssh connections to the tested
server.

 Now I will test in order:

#1. glebius fix
https://svnweb.freebsd.org/base?view=revision&revision=302350

#2. rss extra fix
https://reviews.freebsd.org/D7135

#3. rrs TCP Timer cleanup
https://reviews.freebsd.org/D7136

 My panic for reference:

Fatal trap 9: general protection fault while in kernel mode
cpuid = 10; apic id = 28
[root_at_atlas-dl360-4 ~]# instruction pointer     = 0x20:0xffffffff80c346f1
stack pointer           = 0x28:0xfffffe1f29b848b0
frame pointer           = 0x28:0xfffffe1f29b848e0
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         = 12 (swi4: clock (4))
trap number             = 9
panic: general protection fault
cpuid = 10
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame
0xfffffe1f29b844a0
vpanic() at vpanic+0x182/frame 0xfffffe1f29b84520
panic() at panic+0x43/frame 0xfffffe1f29b84580
trap_fatal() at trap_fatal+0x351/frame 0xfffffe1f29b845e0
trap() at trap+0x820/frame 0xfffffe1f29b847f0
calltrap() at calltrap+0x8/frame 0xfffffe1f29b847f0
--- trap 0x9, rip = 0xffffffff80c346f1, rsp = 0xfffffe1f29b848c0, rbp =
0xfffffe1f29b848e0 ---
tcp_timer_keep() at tcp_timer_keep+0x51/frame 0xfffffe1f29b848e0
softclock_call_cc() at softclock_call_cc+0x19c/frame 0xfffffe1f29b849c0
softclock() at softclock+0x47/frame 0xfffffe1f29b849e0
intr_event_execute_handlers() at intr_event_execute_handlers+0x96/frame
0xfffffe1f29b84a20
ithread_loop() at ithread_loop+0xa6/frame 0xfffffe1f29b84a70
fork_exit() at fork_exit+0x84/frame 0xfffffe1f29b84ab0
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe1f29b84ab0
--- trap 0, rip = 0, rsp = 0, rbp = 0 ---

--
Julien