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Re: GDB hangs with simple multi-threaded program on linux
- From: Tom Tromey <tromey at redhat dot com>
- To: Thiago Jung Bauermann <bauerman at br dot ibm dot com>
- Cc: gdb at sourceware dot org
- Date: Thu, 15 Jul 2010 12:44:32 -0600
- Subject: Re: GDB hangs with simple multi-threaded program on linux
- References: <1279208729.14577.21.camel@hactar>
>>>>> "Thiago" == Thiago Jung Bauermann <bauerman@br.ibm.com> writes:
Thiago> I'm struggling with an issue which perhaps you already faced or
Thiago> thought about...
I asked around about this, and it turns out that we have a patch in the
Fedora SRPM for it.
The approach in this patch seems to be racy. Roland says we can do
better if we enable exit tracing. I see this in linux-nat.c:
/* Do not enable PTRACE_O_TRACEEXIT until GDB is more prepared to support
read-only process state. */
I wonder what that means :-)
Thiago> 1. Is it true that when the main thread exits but there are other
Thiago> threads in the thread group, then no SIGCHLD is generated to notify GDB
Thiago> that it exited (perhaps because such a SIGCHLD could be ambiguous and
Thiago> mean that the whole process exited)?
Yes, Roland said that no SIGCHLD is generated.
Thiago> 2. Is there a way for GDB to wait on just the main thread instead of on
Thiago> the whole process when it waits on a TID which is also the PID?
I guess not.
Tom
2007-07-08 Jan Kratochvil <jan.kratochvil@redhat.com>
* linux-nat.c (linux_lwp_is_zombie): New function.
(wait_lwp): Fix lockup on exit of the thread group leader.
(linux_xfer_partial): Renamed to ...
(linux_xfer_partial_lwp): ... here.
(linux_xfer_partial): New function wrapping LINUX_XFER_PARTIAL_LWP.
2008-02-24 Jan Kratochvil <jan.kratochvil@redhat.com>
Port to GDB-6.8pre.
Index: gdb-6.8.50.20081209/gdb/linux-nat.c
===================================================================
--- gdb-6.8.50.20081209.orig/gdb/linux-nat.c 2008-12-10 01:27:34.000000000 +0100
+++ gdb-6.8.50.20081209/gdb/linux-nat.c 2008-12-10 01:28:14.000000000 +0100
@@ -1981,6 +1981,31 @@ linux_handle_extended_wait (struct lwp_i
_("unknown ptrace event %d"), event);
}
+static int
+linux_lwp_is_zombie (long lwp)
+{
+ char buffer[MAXPATHLEN];
+ FILE *procfile;
+ int retval = 0;
+
+ sprintf (buffer, "/proc/%ld/status", lwp);
+ procfile = fopen (buffer, "r");
+ if (procfile == NULL)
+ {
+ warning (_("unable to open /proc file '%s'"), buffer);
+ return 0;
+ }
+ while (fgets (buffer, sizeof (buffer), procfile) != NULL)
+ if (strcmp (buffer, "State:\tZ (zombie)\n") == 0)
+ {
+ retval = 1;
+ break;
+ }
+ fclose (procfile);
+
+ return retval;
+}
+
/* Wait for LP to stop. Returns the wait status, or 0 if the LWP has
exited. */
@@ -1988,16 +2013,31 @@ static int
wait_lwp (struct lwp_info *lp)
{
pid_t pid;
- int status;
+ int status = 0;
int thread_dead = 0;
gdb_assert (!lp->stopped);
gdb_assert (lp->status == 0);
- pid = my_waitpid (GET_LWP (lp->ptid), &status, 0);
- if (pid == -1 && errno == ECHILD)
+ /* Thread group leader may have exited but we would lock up by WAITPID as it
+ waits on all its threads; __WCLONE is not applicable for the leader.
+ The thread leader restrictions is only a performance optimization here.
+ LINUX_NAT_THREAD_ALIVE cannot be used here as it requires a STOPPED
+ process; it gets ESRCH both for the zombie and for running processes. */
+ if (is_lwp (lp->ptid) && GET_PID (lp->ptid) == GET_LWP (lp->ptid)
+ && linux_lwp_is_zombie (GET_LWP (lp->ptid)))
+ {
+ thread_dead = 1;
+ if (debug_linux_nat)
+ fprintf_unfiltered (gdb_stdlog, "WL: Threads leader %s vanished.\n",
+ target_pid_to_str (lp->ptid));
+ }
+
+ if (!thread_dead)
{
- pid = my_waitpid (GET_LWP (lp->ptid), &status, __WCLONE);
+ pid = my_waitpid (GET_LWP (lp->ptid), &status, 0);
+ if (pid == -1 && errno == ECHILD)
+ pid = my_waitpid (GET_LWP (lp->ptid), &status, __WCLONE);
if (pid == -1 && errno == ECHILD)
{
/* The thread has previously exited. We need to delete it
@@ -4153,8 +4193,10 @@ linux_nat_xfer_osdata (struct target_ops
return len;
}
+/* Transfer from the specific LWP currently set by PID of INFERIOR_PTID. */
+
static LONGEST
-linux_xfer_partial (struct target_ops *ops, enum target_object object,
+linux_xfer_partial_lwp (struct target_ops *ops, enum target_object object,
const char *annex, gdb_byte *readbuf,
const gdb_byte *writebuf, ULONGEST offset, LONGEST len)
{
@@ -4201,6 +4243,45 @@ linux_xfer_partial (struct target_ops *o
offset, len);
}
+/* nptl_db expects being able to transfer memory just by specifying PID.
+ After the thread group leader exists the Linux kernel turns the task
+ into zombie no longer permitting accesses to its memory.
+ Transfer the memory from an arbitrary LWP_LIST entry in such case. */
+
+static LONGEST
+linux_xfer_partial (struct target_ops *ops, enum target_object object,
+ const char *annex, gdb_byte *readbuf,
+ const gdb_byte *writebuf, ULONGEST offset, LONGEST len)
+{
+ LONGEST xfer;
+ struct lwp_info *lp;
+ /* Not using SAVE_INFERIOR_PTID already here for better performance. */
+ struct cleanup *old_chain = NULL;
+ ptid_t inferior_ptid_orig = inferior_ptid;
+
+ errno = 0;
+ xfer = linux_xfer_partial_lwp (ops, object, annex, readbuf, writebuf,
+ offset, len);
+
+ for (lp = lwp_list; xfer == 0 && (errno == EACCES || errno == ESRCH)
+ && lp != NULL; lp = lp->next)
+ {
+ if (!is_lwp (lp->ptid) || ptid_equal (lp->ptid, inferior_ptid_orig))
+ continue;
+
+ if (old_chain == NULL)
+ old_chain = save_inferior_ptid ();
+ inferior_ptid = BUILD_LWP (GET_LWP (lp->ptid), GET_LWP (lp->ptid));
+ errno = 0;
+ xfer = linux_xfer_partial_lwp (ops, object, annex, readbuf, writebuf,
+ offset, len);
+ }
+
+ if (old_chain != NULL)
+ do_cleanups (old_chain);
+ return xfer;
+}
+
/* Create a prototype generic GNU/Linux target. The client can override
it with local methods. */
2007-07-07 Jan Kratochvil <jan.kratochvil@redhat.com>
* gdb.threads/leader-exit.c, gdb.threads/leader-exit.exp: New files.
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ ./gdb/testsuite/gdb.threads/leader-exit.c 7 Jul 2007 15:21:57 -0000
@@ -0,0 +1,47 @@
+/* Clean exit of the thread group leader should not break GDB.
+
+ Copyright 2007 Free Software Foundation, Inc.
+
+ This file is part of GDB.
+
+ This program is free software; you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 2 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program; if not, write to the Free Software
+ Foundation, Inc., 59 Temple Place - Suite 330,
+ Boston, MA 02111-1307, USA. */
+
+#include <pthread.h>
+#include <assert.h>
+#include <unistd.h>
+
+static void *start (void *arg)
+{
+ for (;;)
+ pause ();
+ /* NOTREACHED */
+ assert (0);
+ return arg;
+}
+
+int main (void)
+{
+ pthread_t thread;
+ int i;
+
+ i = pthread_create (&thread, NULL, start, NULL); /* create1 */
+ assert (i == 0);
+
+ pthread_exit (NULL);
+ /* NOTREACHED */
+ assert (0);
+ return 0;
+}
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ ./gdb/testsuite/gdb.threads/leader-exit.exp 7 Jul 2007 15:21:57 -0000
@@ -0,0 +1,64 @@
+# Copyright (C) 2007 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+
+# Exit of the thread group leader should not break GDB.
+
+# This file was written by Jan Kratochvil <jan.kratochvil@redhat.com>.
+
+if $tracelevel then {
+ strace $tracelevel
+}
+
+set testfile "leader-exit"
+set srcfile ${testfile}.c
+set binfile ${objdir}/${subdir}/${testfile}
+
+if {[gdb_compile_pthreads "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable {debug}] != "" } {
+ return -1
+}
+
+gdb_exit
+gdb_start
+gdb_reinitialize_dir $srcdir/$subdir
+gdb_load ${binfile}
+gdb_run_cmd
+
+proc stop_process { description } {
+ global gdb_prompt
+
+ # For this to work we must be sure to consume the "Continuing."
+ # message first, or GDB's signal handler may not be in place.
+ after 1000 {send_gdb "\003"}
+ gdb_expect {
+ -re "Program received signal SIGINT.*$gdb_prompt $"
+ {
+ pass $description
+ }
+ timeout
+ {
+ fail "$description (timeout)"
+ }
+ }
+}
+
+# Prevent races.
+sleep 8
+
+stop_process "Threads could be stopped"
+
+gdb_test "info threads" \
+ "\\* 2 Thread \[^\r\n\]* in \[^\r\n\]*" \
+ "Single thread has been left"