infinite loop when beam.smp compiled with -O2 on debian lenny

Chetan Ahuja <>
Mon May 3 22:29:04 CEST 2010


Hi,

  We hit a bug while running rabbitmq where the beam.smp process was stuck
in a tight loop in the erts_poll_info method.
The process was eating up 100% of exactly one core (on a multi core box) and
rabbitmq was dysfunctional.  Unfortunately
I could not create a  small test case to reproduce this condition but it
would happen quite frequently while rabbitmq was in
operation.

The C code for the function didn't provide any hints on what would have been
spinning in that function
(first time looking at  this codebase though). Finally looking through the
 disassembly in gdb,  (at the point of where our process was spinning) I saw
the following  lines in the
erts_poll_info_kp method:


0x00000000004f0fe9 <erts_poll_info_kp+185>:     nopl   0x0(%rax)
0x00000000004f0ff0 <erts_poll_info_kp+192>:     jmp    0x4f0fe9
<erts_poll_info_kp+185>

(Similar assembly code  can be seen  when  the KERNEL_POLL  option is
disabled.)

 Clearly the above will trivially spin forever anytime we get into that
codepath.  The above
looks suspiciously like some code got optimized out by the compiler leaving
the crazy
loop code.

So I compiled with -O1 and then with no optimization at all.   Withe -O1, I
saw a
a weird jmp insruction jumping to it's own address:

0x0000000000517102 <erts_poll_info_kp+60>:      jmp    0x517102
<erts_poll_info_kp+60>

With no optimization,   any of those trivial spins did not exist but I
didn't analyze the unoptimized
code enough to say whether  it can be proven to have an infinite loop (i.e.,
whether the optimizing
compiler is simply doing it's job vs. this being a compiler bug).

Anyway, this problem exists at least since  erlang-base_12.b.3-dfsg debian
package version and has been
verified to exists in the  github version as of today.


 Her'es the gcc  and  debian version info:
 $ gcc --version
gcc-4.3.real (Debian 4.3.2-1.1) 4.3.2
Copyright (C) 2008 Free Software Foundation, Inc.

$ cat /etc/debian_version
5.0


I'd be happy to provide any other info as needed.

Thanks
Chetan Ahuja


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