Re: GDB hangs with simple multi-threaded program on linux

[Tom Tromey <tromey@redhat.com>]

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>>>>> "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


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[-- Attachment #2: leader-exit-fix.patch --]
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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.  */
 

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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"