Re: [9fans] tip 9vx segfault on Ubuntu in Xen

["Tom Lieber" <alltom@gmail.com>]

On Tue, Jul 1, 2008 at 10:13 AM, Russ Cox <rsc@swtch.com> wrote:
> When it stops, then can should attach gdb by
> looking up the pid in ps and running
>
>        gdb 9vx pid
>        thread apply all where 20
>
> That's usually safer than running 9vx under
> gdb from the start.  9vx is a multithreaded
> program, so it would help to know what the
> other threads are doing, not just the X11 thread.

Attaching doesn't seem to work in this case:

Attaching to program: /home/tom/vx32/src/9vx/9vx, process 20840
/build/buildd/gdb-6.6.dfsg/gdb/linux-nat.c:1026: internal-error:
linux_nat_attach: Assertion `pid == GET_PID (inferior_ptid) &&
WIFSTOPPED (status) && WSTOPSIG (status) == SIGSTOP' failed.
A problem internal to GDB has been detected

Searching for this error yielded an e-mail saying, "When Xorg hangs
inside a signal handler, gdb is unable to attach to it." And "thread
apply all where 20" has no output.

However, if I run it from within gdb with -F, and ignoring SIGSEGV I get this:

Thread 4 (Thread 1654795152 (LWP 20864)):
#0  0xffffe410 in __kernel_vsyscall ()
#1  0x4011e676 in pthread_cond_wait@@GLIBC_2.3.2 () from
/lib/tls/i686/cmov/libpthread.so.0
#2  0x08053990 in runproc () at 9vx/sched.c:237
#3  0x0808f1cb in sched () at 9vx/a/proc.c:165
#4  0x08052842 in squidboy (v=0x828e8d0) at 9vx/main.c:723
#5  0x4011a46b in start_thread () from /lib/tls/i686/cmov/libpthread.so.0
#6  0x4020073e in clone () from /lib/tls/i686/cmov/libc.so.6

Thread 3 (Thread 1645087632 (LWP 20863)):
#0  0xffffe410 in __kernel_vsyscall ()
#1  0x4012237a in do_sigwait () from /lib/tls/i686/cmov/libpthread.so.0
#2  0x4012241f in sigwait () from /lib/tls/i686/cmov/libpthread.so.0
#3  0x0805675d in timerkproc (v=0x0) at 9vx/time.c:168
#4  0x0805723b in linkproc () at 9vx/trap.c:484
#5  0x00000000 in ?? ()

Thread 2 (Thread 1635380112 (LWP 20862)):
#0  0xffffe410 in __kernel_vsyscall ()
#1  0x401f6647 in poll () from /lib/tls/i686/cmov/libc.so.6
#2  0x40061839 in ?? () from /usr/lib/libX11.so.6
#3  0x08272970 in ?? ()
#4  0x00000001 in ?? ()
#5  0xffffffff in ?? ()
#6  0x4006a67f in _X11TransWrite () from /usr/lib/libX11.so.6
#7  0x40061c2f in _XRead () from /usr/lib/libX11.so.6
#8  0x4006347b in _XReadEvents () from /usr/lib/libX11.so.6
#9  0x4004f6eb in XNextEvent () from /usr/lib/libX11.so.6
#10 0x080a1f37 in _xproc (v=0x0) at 9vx/x11/x11-kernel.c:141
#11 0x0805723b in linkproc () at 9vx/trap.c:484
#12 0x00000000 in ?? ()

Thread 1 (Thread 1076381360 (LWP 20859)):
#0  sigsegv (signo=11, info=0x8254eac, v=0x8254f2c) at 9vx/main.c:500
#1  <signal handler called>
#2  sigsegv (signo=11, info=0x825522c, v=0x82552ac) at 9vx/main.c:500
#3  <signal handler called>
#4  sigsegv (signo=11, info=0x82555ac, v=0x825562c) at 9vx/main.c:500
#5  <signal handler called>
#6  sigsegv (signo=11, info=0x825592c, v=0x82559ac) at 9vx/main.c:500
#7  <signal handler called>
#8  sigsegv (signo=11, info=0x8255cac, v=0x8255d2c) at 9vx/main.c:500
#9  <signal handler called>
#10 sigsegv (signo=11, info=0x825602c, v=0x82560ac) at 9vx/main.c:500
#11 <signal handler called>
#12 sigsegv (signo=11, info=0x82563ac, v=0x825642c) at 9vx/main.c:500
#13 <signal handler called>
#14 sigsegv (signo=11, info=0x825672c, v=0x82567ac) at 9vx/main.c:500
#15 <signal handler called>
#16 sigsegv (signo=11, info=0x8256aac, v=0x8256b2c) at 9vx/main.c:500
#17 <signal handler called>
#18 sigsegv (signo=11, info=0x8256e2c, v=0x8256eac) at 9vx/main.c:500
#19 <signal handler called>
(More stack frames follow...)
#0  0xffffe410 in __kernel_vsyscall ()

Is the last one legitimate?

--
Tom Lieber
http://AllTom.com/