sigaction, fork, malloc, and futex

[Steve Beaty <beaty@emess.mscd.edu>]

	anyone have a clue on this one?  we set up a signal handler, create
	a child that sends that signal, and have the signal handler fork
	another child.  if there is a malloc(), the second child gets stuck
	in a futex(); without the malloc(), no problem.  2.4.20-30.9
	kernel.  straces at the end.  any help would be appreciated.
	thanks!

===========================================================================
#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
#include <assert.h>

/* comment out the following to show the problem */
#define WORKS

void sig_handler (int signum)
{
	int     child;

	fprintf (stderr, "Process %d received a SIGALRM signal\n", getpid());
	if ((child = fork ()) == 0)
	{
		fprintf (stderr, "%d exiting\n", getpid());
		exit (0);
	}
	fprintf (stderr, "%d waiting for %d\n", getpid(), child);
	waitpid (child, NULL, 0);
}

int
main (int argc, char **argv)
{
	int parent = getpid();
	int child;

	struct sigaction action;
	sigemptyset (&action.sa_mask);
	action.sa_handler = sig_handler;

#ifndef WORKS
	malloc (sizeof (int));
#endif

	assert (sigaction (SIGALRM, &action, NULL) == 0);

	if ((child = fork ()) == 0)
	{
		fprintf (stderr, "%d sending SIGALRM to %d\n", getpid(), parent);
		if (kill (parent, SIGALRM) == -1) perror ("kill 1");
		fprintf (stderr, "%d exiting\n", getpid());
		exit (0);
	}
	fprintf (stderr, "%d waiting for %d\n", getpid(), child);
	waitpid (child, NULL, 0);
}

/*
** if WORKS defined:

$ ./a.out
9106 sending SIGALRM to 9105
9106 exiting
Process 9105 received a SIGALRM signal
9107 exiting
9105 waiting for 9107
9105 waiting for 9106

** if WORKS undefined:

$ ./a.out
9095 sending SIGALRM to 9094
9095 exiting
Process 9094 received a SIGALRM signal

$ sh /usr/src/linux-2.4.20-30.9/scripts/ver_linux
If some fields are empty or look unusual you may have an old version.
Compare to the current minimal requirements in Documentation/Changes.
  
Linux emess0.mscd.edu 2.4.20-30.9 #1 Wed Feb 4 20:44:26 EST 2004 i686 i686
i386 GNU/Linux
  
Gnu C                  3.2.2
Gnu make               3.79.1
util-linux             2.11y
mount                  2.11y
modutils               2.4.22
e2fsprogs              1.32
jfsutils               1.0.17
reiserfsprogs          3.6.4
pcmcia-cs              3.1.31
PPP                    2.4.1
isdn4k-utils           3.1pre4
Linux C Library        2.3.2
Dynamic linker (ldd)   2.3.2
Procps                 2.0.11
Net-tools              1.60
Kbd                    1.08
Sh-utils               4.5.3
Modules Loaded         ieee1394 ide-cd cdrom i810_audio ac97_codec
soundcore mga agpgart binfmt_misc autofs e100 ipchains loop lvm-mod keybdev
mousedev hid input usb-uhci usbcore ext3 jbd

*/

#if 0

BAD:
---------------------------------------------------------------------------
$ strace ./a.out
execve("./a.out", ["./a.out"], [/* 42 vars */]) = 0
uname({sys="Linux", node="emess.mscd.edu", ...}) = 0
brk(0)                                  = 0x80499fc
open("/etc/ld.so.preload", O_RDONLY)    = -1 ENOENT (No such file or
directory)
open("/etc/ld.so.cache", O_RDONLY)      = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=105716, ...}) = 0
old_mmap(NULL, 105716, PROT_READ, MAP_PRIVATE, 3, 0) = 0x40016000
close(3)                                = 0
open("/lib/tls/libc.so.6", O_RDONLY)    = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\360W\1"..., 512) =
512
fstat64(3, {st_mode=S_IFREG|0755, st_size=1539996, ...}) = 0
old_mmap(0x42000000, 1267276, PROT_READ|PROT_EXEC, MAP_PRIVATE, 3, 0) =
0x42000000
old_mmap(0x42130000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3,
0x130000) = 0x42130000
old_mmap(0x42133000, 9804, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x42133000
close(3)                                = 0
old_mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
0) = 0x40030000
set_thread_area({entry_number:-1 -> 6, base_addr:0x400304a0, limit:1048575,
seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1,
seg_not_present:0, useable:1}) = 0
munmap(0x40016000, 105716)              = 0
getpid()                                = 9059
brk(0)                                  = 0x80499fc
brk(0x804a9fc)                          = 0x804a9fc
brk(0)                                  = 0x804a9fc
brk(0x804b000)                          = 0x804b000
rt_sigaction(SIGALRM, {0x8048568, [],
SA_RESTORER|SA_NOMASK|SA_SIGINFO|0x21328d0, 0x420277b0}, NULL, 8) = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|0x11,
<ignored>, <ignored>, 0x400304e8) = 9060
9060 sending SIGALRM to 9059
9060 exiting
--- SIGALRM (Alarm clock) @ 0 (0) ---
getpid()                                = 9059
--- SIGCHLD (Child exited) @ 0 (0) ---
write(2, "Process 9059 received a SIGALRM "..., 39Process 9059 received a
SIGALRM signal
) = 39
futex(0x421336d0, FUTEX_WAIT, 2, NULL
[control-c]
---------------------------------------------------------------------------

GOOD

---------------------------------------------------------------------------
$ strace ./a.out
execve("./a.out", ["./a.out"], [/* 42 vars */]) = 0
uname({sys="Linux", node="emess.mscd.edu", ...}) = 0
brk(0)                                  = 0x80499b8
open("/etc/ld.so.preload", O_RDONLY)    = -1 ENOENT (No such file or
directory)
open("/etc/ld.so.cache", O_RDONLY)      = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=105716, ...}) = 0
old_mmap(NULL, 105716, PROT_READ, MAP_PRIVATE, 3, 0) = 0x40016000
close(3)                                = 0
open("/lib/tls/libc.so.6", O_RDONLY)    = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\360W\1"..., 512) =
512
fstat64(3, {st_mode=S_IFREG|0755, st_size=1539996, ...}) = 0
old_mmap(0x42000000, 1267276, PROT_READ|PROT_EXEC, MAP_PRIVATE, 3, 0) =
0x42000000
old_mmap(0x42130000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3,
0x130000) = 0x42130000
old_mmap(0x42133000, 9804, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x42133000
close(3)                                = 0
old_mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
0) = 0x40030000
set_thread_area({entry_number:-1 -> 6, base_addr:0x400304a0, limit:1048575,
seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1,
seg_not_present:0, useable:1}) = 0
munmap(0x40016000, 105716)              = 0
getpid()                                = 9087
rt_sigaction(SIGALRM, {0x8048534, [],
SA_RESTORER|SA_NOMASK|SA_SIGINFO|0x21328d0, 0x420277b0}, NULL, 8) = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|0x11,
<ignored>, <ignored>, 0x400304e8) = 9088
9088 sending SIGALRM to 9087
9088 exiting
--- SIGALRM (Alarm clock) @ 0 (0) ---
getpid()                                = 9087
--- SIGCHLD (Child exited) @ 0 (0) ---
write(2, "Process 9087 received a SIGALRM "..., 39Process 9087 received a
SIGALRM signal
) = 39
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|0x11,
<ignored>, <ignored>, 0x400304e8) = 9089
getpid()                                = 9087
write(2, "9087 waiting for 9089\n", 229087 waiting for 9089
) = 22
wait4(9089, 9089 exiting
NULL, 0, NULL)              = 9089
rt_sigreturn(0x1200011)                 = 9088
getpid()                                = 9087
write(2, "9087 waiting for 9088\n", 229087 waiting for 9088
) = 22
wait4(9088, NULL, 0, NULL)              = 9088
exit_group(0)                           = ?
---------------------------------------------------------------------------
#endif
===========================================================================

-- 
Dr. Steve Beaty (B80)                                 Associate Professor
Metro State College of Denver                        beaty@emess.mscd.edu
VOX: (303) 556-5321                                 Science Building 134C
FAX: (303) 556-5381                         http://clem.mscd.edu/~beatys/