PR_SET_PTRACER does not work from a thread

Linux ubuntu 2.6.35-22-generic #33-Ubuntu SMP Sun Sep 19 20:34:50 UTC 2010 i686 GNU/Linux

The program below reproduces the problem.
If you launch it without argument, the thread will call PR_SET_PTRACER.
Then the child process cannot ptrace its parent.
If it is the main thread which is calling PR_SET_PTRACER, then it works properly.
Just give an argument to the program to have the main thread calling PR_SET_PTRACER

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <sys/ptrace.h>
#include <sys/prctl.h>
int main_does_prctl;
int ret;

pid_t child;

static void *thr_fn(void *v)
{
   if (!main_does_prctl) {
      ret = prctl (PR_SET_PTRACER, child, 0, 0, 0);
      printf("thread prtctl result %d \n", ret);
   }
   printf("thread sleeping\n");
   sleep(100);
}

int main(int argc, char*argv[])
{
   pthread_t thr;
   pid_t parent = getpid();
   long ptrace_result;

   child = fork();
   if (child == 0) {
      /* This is the child, sleep to let our parent set_ptracer
         us. */
      printf("child sleeping\n");
      sleep (5);
      printf("will ptrace my parent %d\n", parent);
      ptrace_result = ptrace (PTRACE_ATTACH, parent, NULL, NULL);
      if (ptrace_result == 0)
         printf ("ptrace attach successful\n");
      else
         perror ("ptrace attach has failed");
      printf("killing parent\n");
      ptrace_result = ptrace (PTRACE_KILL, parent, NULL, NULL);
      if (ptrace_result == 0)
         printf ("ptrace kill successful\n");
      else
         perror ("ptrace kill has failed");
      exit(0);
   }

   main_does_prctl = argc > 1;

   pthread_create(&thr, NULL, thr_fn, NULL);

   if (main_does_prctl) {
      ret = prctl (PR_SET_PTRACER, child, 0, 0, 0);
      printf("main prtctl result %d \n", ret);
   }
   printf("main sleeping\n");
   sleep(100);
}

ProblemType: Bug
DistroRelease: Ubuntu 10.10
Package: linux-image-2.6.35-22-generic 2.6.35-22.33
Regression: No
Reproducible: Yes
ProcVersionSignature: Ubuntu 2.6.35-22.33-generic 2.6.35.4
Uname: Linux 2.6.35-22-generic i686
AlsaVersion: Advanced Linux Sound Architecture Driver Version 1.0.23.
AplayDevices:
 **** List of PLAYBACK Hardware Devices ****
 card 0: Intel [HDA Intel], device 0: ALC268 Analog [ALC268 Analog]
   Subdevices: 1/1
   Subdevice #0: subdevice #0
Architecture: i386
ArecordDevices:
 **** List of CAPTURE Hardware Devices ****
 card 0: Intel [HDA Intel], device 0: ALC268 Analog [ALC268 Analog]
   Subdevices: 1/1
   Subdevice #0: subdevice #0
AudioDevicesInUse:
 Cannot stat file /proc/4717/fd/40: Stale NFS file handle
                      USER PID ACCESS COMMAND
 /dev/snd/controlC0: ubuntu 3262 F.... pulseaudio
CRDA: Error: [Errno 2] No such file or directory
Card0.Amixer.info:
 Card hw:0 'Intel'/'HDA Intel at 0xd4600000 irq 45'
   Mixer name : 'Realtek ALC268'
   Components : 'HDA:10ec0268,1179ff66,00100003 HDA:11c11040,11790001,00100200'
   Controls : 13
   Simple ctrls : 8
Date: Sat Mar 5 20:00:32 2011
LiveMediaBuild: Ubuntu 10.10 "Maverick Meerkat" - Release i386 (20101007)
MachineType: TOSHIBA Satellite L300
ProcCmdLine: file=/cdrom/preseed/hostname.seed boot=casper initrd=/casper/initrd.lz quiet splash -- maybe-ubiquity
ProcEnviron:
 LANG=en_US.UTF-8
 SHELL=/bin/bash
RelatedPackageVersions: linux-firmware 1.38
RfKill:
 0: phy0: Wireless LAN
  Soft blocked: no
  Hard blocked: no
SourcePackage: linux
dmi.bios.date: 10/09/2008
dmi.bios.vendor: INSYDE
dmi.bios.version: 1.30
dmi.board.asset.tag: Base Board Asset Tag
dmi.board.name: Portable PC
dmi.board.vendor: TOSHIBA
dmi.board.version: Base Board Version
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 10
dmi.chassis.vendor: Chassis Manufacturer
dmi.chassis.version: Chassis Version
dmi.modalias: dmi:bvnINSYDE:bvr1.30:bd10/09/2008:svnTOSHIBA:pnSatelliteL300:pvrPSLB8E-01200DBT:rvnTOSHIBA:rnPortablePC:rvrBaseBoardVersion:cvnChassisManufacturer:ct10:cvrChassisVersion:
dmi.product.name: Satellite L300
dmi.product.version: PSLB8E-01200DBT
dmi.sys.vendor: TOSHIBA