[erlang-patches] Per-thread FPE pointer not being initialized

[David Reiss]

> Where did the spurious FP exception get detected, i.e.,
> what runtime system procedure does 0x4aaded correspond to?
It looks like it is dec_term (and gdb reports it as line 1206 of
beam/external.c, which is a call to sys_chars_to_double).  When I
first detected the problem, it was in the context of a larger
application.  The reported address was 0x51d507, which is in
sys_chars_to_double.  The boundaries get a bit blurred in optimized
builds. :)

--David



On Wed, Jul 1, 2009 at 8:16 AM, Raimo
Niskanen<raimo+erlang-patches@REDACTED> wrote:
> On Wed, Jul 01, 2009 at 01:55:39PM +0200, Mikael Pettersson wrote:
>> Raimo Niskanen writes:
>>  > On Wed, Jul 01, 2009 at 12:03:04PM +0200, Mikael Pettersson wrote:
>>  > > David Reiss writes:
>>  > >  > The per-thread last floating point exception pointer is not initialized to
>>  > >  > NULL when it is allocated.  As a result, spurious floating point
>>  > >  > exceptions are reported.
>>  > >  >
>>  > >  > This is 100% reproducible for me on CentOS 5.1 for x86-64, with Erlang/OTP
>>  > >  > R13B built from source with gcc 2.5 and gcc 4.1.2.  It doesn't happen with
>>  > >  > R13B01, but, based on the nature of the bug, I think that is most likely
>>  > >  > due to chance.
>>  > >  >
>>  > >  > Steps to reproduce:
>>  > >  >
>>  > >  > 1/ Start up a node with
>>  > >  >   dreiss@REDACTED:~$ erl -noshell -name bob@`hostname`
>>  > >  >
>>  > >  > 2/ Build and compile test program
>>  > >  >   dreiss@REDACTED:~$ cat test.erl
>>  > >  >   -module(test).
>>  > >  >   -export([start/1]).
>>  > >  >   start([Remote]) ->
>>  > >  >     rpc:call(Remote, erlang, is_float, [750.0]).
>>  > >  >   dreiss@REDACTED:~$ erlc test.erl
>>  > >  >
>>  > >  > 3/ Run test program
>>  > >  >   dreiss@REDACTED:~$ erl -noshell -name alice@`hostname` -pz . -s test
>>  > >  > start bob@`hostname` -s init stop
>>  > >  >
>>  > >  > The "alice" node exits with no output.  After running the "alice" command,
>>  > >  > the "bob" node prints the following
>>  > >  >
>>  > >  >   ERTS_FP_CHECK_INIT at 0x4aaded: detected unhandled FPE at 0x2
>>  > >  >
>>  > >  > Patch:
>>  > >  >
>>  > >  > diff otp_src_R13B/erts/emulator/sys/unix/sys_float.c
>>  > >  > otp_src_R13B/sys/unix/sys_float.c/sys/unix/sys_float.c
>>  > >  > --- otp_src_R13B/erts/emulator/sys/unix/sys_float.c   2009-06-30
>>  > >  > 17:53:31.000000000 -0700
>>  > >  > +++ otp_src_R13B/erts/emulator/sys/unix/sys_float.c   2009-06-30
>>  > >  > 17:53:31.000000000 -0700
>>  > >  > @@ -57,7 +57,8 @@
>>  > >  >  void erts_thread_init_fp_exception(void)
>>  > >  >  {
>>  > >  >      unsigned long *fpe = erts_alloc(ERTS_ALC_T_FP_EXCEPTION, sizeof(*fpe));
>>  > >  > +    *fpe = 0;
>>  > >  >      erts_tsd_set(fpe_key, fpe);
>>  > >  >  }
>>  > >
>>  > > Thanks, that bug was mine :-( Your fix is correct.
>>  >
>>  > Would your patch in http://www.erlang.org/cgi-bin/ezmlm-cgi/2/1380
>>  > also solve this problem? Or are both needed?
>>  >
>>  > --- otp-0622/erts/emulator/sys/unix/sys_float.c.~1~        2009-02-26 11:45:48.000000000 +0100
>>  > +++ otp-0622/erts/emulator/sys/unix/sys_float.c    2009-06-26 11:14:15.000000000 +0200
>>  > @@ -792,6 +792,8 @@ sys_chars_to_double(char* buf, double* f
>>  >  int
>>  >  matherr(struct exception *exc)
>>  >  {
>>  > +#if !defined(NO_FPE_SIGNALS)
>>  >      set_current_fp_exception((unsigned long)__builtin_return_address(0));
>>  > +#endif
>>  >      return 1;
>>  >  }
>>
>> The two patches fix independent issues. The #if !defined change in
>> sys_chars_to_double() fixes a compile/link error with !gcc.
>
> Sorry I misread the patch. I accidentally saw it add
> set_current_fp_exception(0) to matherr(exc) but it
> only added preprocessor directives...
>
>>
>> /Mikael
>>
>> ________________________________________________________________
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>> erlang-patches (at) erlang.org
>
> --
>
> / Raimo Niskanen, Erlang/OTP, Ericsson AB
>
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