Add support for LWP-based threads on FreeBSD.

Older versions of FreeBSD supported userland threading via a pure
user-space threading library (N threads scheduled on 1 process) and
a N:M model (N threads scheduled on M LWPs).  However, modern FreeBSD
versions only support a M:M threading model where each user thread is
backed by a dedicated LWP.  This thread target only supports this
threading model.  It also uses ptrace to query and alter LWP state
directly rather than using libthread_db to simplify the implementation.

FreeBSD recently gained support for reporting LWP events (birth and death
of LWPs).  GDB will use LWP events when present.  For older systems it
fetches the list of LWPs in the to_update_thread_list target op to update
the list of threads on each stop.

This target supports scheduler locking by using ptrace to suspend
individual LWPs as necessary before resuming a process.

gdb/ChangeLog:

	* configure.ac: Check for support for LWP names on FreeBSD.
	* fbsd-nat.c [PT_LWPINFO] New variable debug_fbsd_lwp.
	[TDP_RFPPWAIT || HAVE_STRUCT_PTRACE_LWPINFO_PL_TDNAME]
	(fbsd_fetch_kinfo_proc): Move function earlier.
	[PT_LWPINFO] (fbsd_thread_alive): New function.
	[PT_LWPINFO] (fbsd_pid_to_str): New function.
	[HAVE_STRUCT_PTRACE_LWPINFO_PL_TDNAME] (fbsd_thread_name): New function.
	[PT_LWP_EVENTS] (fbsd_enable_lwp_events): New function.
	[PT_LWPINFO] (fbsd_add_threads): New function.
	[PT_LWPINFO] (fbsd_update_thread_list): New function.
	[PT_LWPINFO] New variable super_resume.
	[PT_LWPINFO] (resume_one_thread_cb): New function.
	[PT_LWPINFO] (resume_all_threads_cb): New function.
	[PT_LWPINFO] (fbsd_resume): New function.
	(fbsd_remember_child): Save full ptid instead of plain pid.
	(fbsd_is_child_pending): Return ptid of saved child process.
	(fbsd_wait): Include lwp in returned ptid and switch to LWP ptid on
	first stop.
	[PT_LWP_EVENTS] Handle LWP events.
	[TDP_RFPPWAIT] Include LWP in child ptid.
	(fbsd_post_startup_inferior) [PT_LWP_EVENTS]: Enable LWP events.
	(fbsd_post_attach) [PT_LWP_EVENTS]: Enable LWP events.
	Add threads for existing processes.
	(fbsd_nat_add_target) [PT_LWPINFO]: Set "to_thread_alive" to
	"fbsd_thread_alive".
	Set "to_pid_to_str" to "fbsd_pid_to_str".
	[HAVE_STRUCT_PTRACE_LWPINFO_PL_TDNAME]: Set "to_thread_name" to
	"fbsd_thread_name".
	[PT_LWPINFO]: Set "to_update_thread_list" to "fbsd_update_thread_list".
	Set "to_has_thread_control" to "tc_schedlock".
	Set "to_resume" to "fbsd_resume".
	(_initialize_fbsd_nat): New function.
	* configure: Regenerate.
	* config.in: Regenerate.

gdb/doc/ChangeLog:

	* gdb.texinfo (Debugging Output): Document "set/show debug fbsd-lwp".

8 files changed, 469 insertions, 32 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 0d9a7fb05dc..2cdf2fa8e27 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,42 @@
 2016-01-19  John Baldwin  <jhb@FreeBSD.org>
 
+	* configure.ac: Check for support for LWP names on FreeBSD.
+	* fbsd-nat.c [PT_LWPINFO] New variable debug_fbsd_lwp.
+	[TDP_RFPPWAIT || HAVE_STRUCT_PTRACE_LWPINFO_PL_TDNAME]
+	(fbsd_fetch_kinfo_proc): Move function earlier.
+	[PT_LWPINFO] (fbsd_thread_alive): New function.
+	[PT_LWPINFO] (fbsd_pid_to_str): New function.
+	[HAVE_STRUCT_PTRACE_LWPINFO_PL_TDNAME] (fbsd_thread_name): New function.
+	[PT_LWP_EVENTS] (fbsd_enable_lwp_events): New function.
+	[PT_LWPINFO] (fbsd_add_threads): New function.
+	[PT_LWPINFO] (fbsd_update_thread_list): New function.
+	[PT_LWPINFO] New variable super_resume.
+	[PT_LWPINFO] (resume_one_thread_cb): New function.
+	[PT_LWPINFO] (resume_all_threads_cb): New function.
+	[PT_LWPINFO] (fbsd_resume): New function.
+	(fbsd_remember_child): Save full ptid instead of plain pid.
+	(fbsd_is_child_pending): Return ptid of saved child process.
+	(fbsd_wait): Include lwp in returned ptid and switch to LWP ptid on
+	first stop.
+	[PT_LWP_EVENTS] Handle LWP events.
+	[TDP_RFPPWAIT] Include LWP in child ptid.
+	(fbsd_post_startup_inferior) [PT_LWP_EVENTS]: Enable LWP events.
+	(fbsd_post_attach) [PT_LWP_EVENTS]: Enable LWP events.
+	Add threads for existing processes.
+	(fbsd_nat_add_target) [PT_LWPINFO]: Set "to_thread_alive" to
+	"fbsd_thread_alive".
+	Set "to_pid_to_str" to "fbsd_pid_to_str".
+	[HAVE_STRUCT_PTRACE_LWPINFO_PL_TDNAME]: Set "to_thread_name" to
+	"fbsd_thread_name".
+	[PT_LWPINFO]: Set "to_update_thread_list" to "fbsd_update_thread_list".
+	Set "to_has_thread_control" to "tc_schedlock".
+	Set "to_resume" to "fbsd_resume".
+	(_initialize_fbsd_nat): New function.
+	* configure: Regenerate.
+	* config.in: Regenerate.
+
+2016-01-19  John Baldwin  <jhb@FreeBSD.org>
+
 	* amd64bsd-nat.c (amd64bsd_fetch_inferior_registers): Use
 	get_ptrace_pid.
 	(amd64bsd_store_inferior_registers): Use get_ptrace_pid.
diff --git a/gdb/NEWS b/gdb/NEWS
index f4ba65dfa9d..43121174860 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -3,6 +3,8 @@
 
 *** Changes since GDB 7.10
 
+* GDB now supports debugging kernel-based threads on FreeBSD.
+
 * Per-inferior thread numbers
 
   Thread numbers are now per inferior instead of global.  If you're
@@ -100,6 +102,10 @@ set debug bfd-cache
 show debug bfd-cache
   Control display of debugging info regarding bfd caching.
 
+set debug fbsd-lwp
+show debug fbsd-lwp
+  Control display of debugging info regarding FreeBSD threads.
+
 set remote multiprocess-extensions-packet
 show remote multiprocess-extensions-packet
   Set/show the use of the remote protocol multiprocess extensions.
diff --git a/gdb/config.in b/gdb/config.in
index 6196fa00526..dc9da0a7361 100644
--- a/gdb/config.in
+++ b/gdb/config.in
@@ -450,6 +450,9 @@
 /* Define to 1 if your system has struct lwp. */
 #undef HAVE_STRUCT_LWP
 
+/* Define to 1 if `struct ptrace_lwpinfo' is a member of `pl_tdname'. */
+#undef HAVE_STRUCT_PTRACE_LWPINFO_PL_TDNAME
+
 /* Define to 1 if your system has struct reg in <machine/reg.h>. */
 #undef HAVE_STRUCT_REG
 
diff --git a/gdb/configure b/gdb/configure
index 11f91e31cbf..701cfcb0239 100755
--- a/gdb/configure
+++ b/gdb/configure
@@ -12965,6 +12965,22 @@ $as_echo "#define HAVE_PT_GETXMMREGS 1" >>confdefs.h
 
 fi
 
+# See if <sys/ptrace.h> supports LWP names on FreeBSD
+# Older FreeBSD versions don't have the pl_tdname member of
+# `struct ptrace_lwpinfo'.
+ac_fn_c_check_member "$LINENO" "struct ptrace_lwpinfo" "pl_tdname" "ac_cv_member_struct_ptrace_lwpinfo_pl_tdname" "#include <sys/ptrace.h>
+"
+if test "x$ac_cv_member_struct_ptrace_lwpinfo_pl_tdname" = x""yes; then :
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_STRUCT_PTRACE_LWPINFO_PL_TDNAME 1
+_ACEOF
+
+
+fi
+
+
+
 # Detect which type of /proc is in use, such as for Solaris.
 
 if test "${target}" = "${host}"; then
diff --git a/gdb/configure.ac b/gdb/configure.ac
index 8f9486e2025..a7b7421ced0 100644
--- a/gdb/configure.ac
+++ b/gdb/configure.ac
@@ -1511,6 +1511,13 @@ if test $gdb_cv_have_pt_getxmmregs = yes; then
   [Define if sys/ptrace.h defines the PT_GETXMMREGS request.])
 fi
 
+# See if <sys/ptrace.h> supports LWP names on FreeBSD
+# Older FreeBSD versions don't have the pl_tdname member of
+# `struct ptrace_lwpinfo'.
+AC_CHECK_MEMBERS([struct ptrace_lwpinfo.pl_tdname], [], [],
+                 [#include <sys/ptrace.h>])
+
+
 # Detect which type of /proc is in use, such as for Solaris.
 
 if test "${target}" = "${host}"; then
diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog
index 59a8e0acabd..990f2ec2563 100644
--- a/gdb/doc/ChangeLog
+++ b/gdb/doc/ChangeLog
@@ -1,3 +1,7 @@
+2016-01-19  John Baldwin  <jhb@FreeBSD.org>
+
+	* gdb.texinfo (Debugging Output): Document "set/show debug fbsd-lwp".
+
 2016-01-18  Pedro Alves  <palves@redhat.com>
 
 	* gdb.texinfo (Threads): Mention that GDB displays the ID and name
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index f996c8d1915..cd311db5240 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -23608,6 +23608,11 @@ expression parsing.  The default is off.
 @item show debug expression
 Displays the current state of displaying debugging info about
 @value{GDBN} expression parsing.
+@item set debug fbsd-lwp
+@cindex FreeBSD LWP debug messages
+Turns on or off debugging messages from the FreeBSD LWP debug support.
+@item show debug fbsd-lwp
+Show the current state of FreeBSD LWP debugging messages.
 @item set debug frame
 @cindex frame debugging info
 Turns on or off display of @value{GDBN} frame debugging info.  The
diff --git a/gdb/fbsd-nat.c b/gdb/fbsd-nat.c
index 702e51d9474..d2ec5274abd 100644
--- a/gdb/fbsd-nat.c
+++ b/gdb/fbsd-nat.c
@@ -22,6 +22,7 @@
 #include "inferior.h"
 #include "regcache.h"
 #include "regset.h"
+#include "gdbcmd.h"
 #include "gdbthread.h"
 #include "gdb_wait.h"
 #include <sys/types.h>
@@ -204,11 +205,283 @@ fbsd_find_memory_regions (struct target_ops *self,
 #endif
 
 #ifdef PT_LWPINFO
+static int debug_fbsd_lwp;
+
 static ptid_t (*super_wait) (struct target_ops *,
 			     ptid_t,
 			     struct target_waitstatus *,
 			     int);
 
+static void
+show_fbsd_lwp_debug (struct ui_file *file, int from_tty,
+		     struct cmd_list_element *c, const char *value)
+{
+  fprintf_filtered (file, _("Debugging of FreeBSD lwp module is %s.\n"), value);
+}
+
+#if defined(TDP_RFPPWAIT) || defined(HAVE_STRUCT_PTRACE_LWPINFO_PL_TDNAME)
+/* Fetch the external variant of the kernel's internal process
+   structure for the process PID into KP.  */
+
+static void
+fbsd_fetch_kinfo_proc (pid_t pid, struct kinfo_proc *kp)
+{
+  size_t len;
+  int mib[4];
+
+  len = sizeof *kp;
+  mib[0] = CTL_KERN;
+  mib[1] = KERN_PROC;
+  mib[2] = KERN_PROC_PID;
+  mib[3] = pid;
+  if (sysctl (mib, 4, kp, &len, NULL, 0) == -1)
+    perror_with_name (("sysctl"));
+}
+#endif
+
+/*
+  FreeBSD's first thread support was via a "reentrant" version of libc
+  (libc_r) that first shipped in 2.2.7.  This library multiplexed all
+  of the threads in a process onto a single kernel thread.  This
+  library is supported via the bsd-uthread target.
+
+  FreeBSD 5.1 introduced two new threading libraries that made use of
+  multiple kernel threads.  The first (libkse) scheduled M user
+  threads onto N (<= M) kernel threads (LWPs).  The second (libthr)
+  bound each user thread to a dedicated kernel thread.  libkse shipped
+  as the default threading library (libpthread).
+
+  FreeBSD 5.3 added a libthread_db to abstract the interface across
+  the various thread libraries (libc_r, libkse, and libthr).
+
+  FreeBSD 7.0 switched the default threading library from from libkse
+  to libpthread and removed libc_r.
+
+  FreeBSD 8.0 removed libkse and the in-kernel support for it.  The
+  only threading library supported by 8.0 and later is libthr which
+  ties each user thread directly to an LWP.  To simplify the
+  implementation, this target only supports LWP-backed threads using
+  ptrace directly rather than libthread_db.
+
+  FreeBSD 11.0 introduced LWP event reporting via PT_LWP_EVENTS.
+*/
+
+/* Return true if PTID is still active in the inferior.  */
+
+static int
+fbsd_thread_alive (struct target_ops *ops, ptid_t ptid)
+{
+  if (ptid_lwp_p (ptid))
+    {
+      struct ptrace_lwpinfo pl;
+
+      if (ptrace (PT_LWPINFO, ptid_get_lwp (ptid), (caddr_t) &pl, sizeof pl)
+	  == -1)
+	return 0;
+#ifdef PL_FLAG_EXITED
+      if (pl.pl_flags & PL_FLAG_EXITED)
+	return 0;
+#endif
+    }
+
+  return 1;
+}
+
+/* Convert PTID to a string.  Returns the string in a static
+   buffer.  */
+
+static char *
+fbsd_pid_to_str (struct target_ops *ops, ptid_t ptid)
+{
+  lwpid_t lwp;
+
+  lwp = ptid_get_lwp (ptid);
+  if (lwp != 0)
+    {
+      static char buf[64];
+      int pid = ptid_get_pid (ptid);
+
+      xsnprintf (buf, sizeof buf, "process %d, LWP %d", pid, lwp);
+      return buf;
+    }
+
+  return normal_pid_to_str (ptid);
+}
+
+#ifdef HAVE_STRUCT_PTRACE_LWPINFO_PL_TDNAME
+/* Return the name assigned to a thread by an application.  Returns
+   the string in a static buffer.  */
+
+static const char *
+fbsd_thread_name (struct target_ops *self, struct thread_info *thr)
+{
+  struct ptrace_lwpinfo pl;
+  struct kinfo_proc kp;
+  int pid = ptid_get_pid (thr->ptid);
+  long lwp = ptid_get_lwp (thr->ptid);
+  static char buf[sizeof pl.pl_tdname + 1];
+
+  /* Note that ptrace_lwpinfo returns the process command in pl_tdname
+     if a name has not been set explicitly.  Return a NULL name in
+     that case.  */
+  fbsd_fetch_kinfo_proc (pid, &kp);
+  if (ptrace (PT_LWPINFO, lwp, (caddr_t) &pl, sizeof pl) == -1)
+    perror_with_name (("ptrace"));
+  if (strcmp (kp.ki_comm, pl.pl_tdname) == 0)
+    return NULL;
+  xsnprintf (buf, sizeof buf, "%s", pl.pl_tdname);
+  return buf;
+}
+#endif
+
+#ifdef PT_LWP_EVENTS
+/* Enable LWP events for a specific process.
+
+   To catch LWP events, PT_LWP_EVENTS is set on every traced process.
+   This enables stops on the birth for new LWPs (excluding the "main" LWP)
+   and the death of LWPs (excluding the last LWP in a process).  Note
+   that unlike fork events, the LWP that creates a new LWP does not
+   report an event.  */
+
+static void
+fbsd_enable_lwp_events (pid_t pid)
+{
+  if (ptrace (PT_LWP_EVENTS, pid, (PTRACE_TYPE_ARG3)0, 1) == -1)
+    perror_with_name (("ptrace"));
+}
+#endif
+
+/* Add threads for any new LWPs in a process.
+
+   When LWP events are used, this function is only used to detect existing
+   threads when attaching to a process.  On older systems, this function is
+   called to discover new threads each time the thread list is updated.  */
+
+static void
+fbsd_add_threads (pid_t pid)
+{
+  struct cleanup *cleanup;
+  lwpid_t *lwps;
+  int i, nlwps;
+
+  gdb_assert (!in_thread_list (pid_to_ptid (pid)));
+  nlwps = ptrace (PT_GETNUMLWPS, pid, NULL, 0);
+  if (nlwps == -1)
+    perror_with_name (("ptrace"));
+
+  lwps = XCNEWVEC (lwpid_t, nlwps);
+  cleanup = make_cleanup (xfree, lwps);
+
+  nlwps = ptrace (PT_GETLWPLIST, pid, (caddr_t) lwps, nlwps);
+  if (nlwps == -1)
+    perror_with_name (("ptrace"));
+
+  for (i = 0; i < nlwps; i++)
+    {
+      ptid_t ptid = ptid_build (pid, lwps[i], 0);
+
+      if (!in_thread_list (ptid))
+	{
+#ifdef PT_LWP_EVENTS
+	  struct ptrace_lwpinfo pl;
+
+	  /* Don't add exited threads.  Note that this is only called
+	     when attaching to a multi-threaded process.  */
+	  if (ptrace (PT_LWPINFO, lwps[i], (caddr_t) &pl, sizeof pl) == -1)
+	    perror_with_name (("ptrace"));
+	  if (pl.pl_flags & PL_FLAG_EXITED)
+	    continue;
+#endif
+	  if (debug_fbsd_lwp)
+	    fprintf_unfiltered (gdb_stdlog,
+				"FLWP: adding thread for LWP %u\n",
+				lwps[i]);
+	  add_thread (ptid);
+	}
+    }
+  do_cleanups (cleanup);
+}
+
+/* Implement the "to_update_thread_list" target_ops method.  */
+
+static void
+fbsd_update_thread_list (struct target_ops *ops)
+{
+#ifdef PT_LWP_EVENTS
+  /* With support for thread events, threads are added/deleted from the
+     list as events are reported, so just try deleting exited threads.  */
+  delete_exited_threads ();
+#else
+  prune_threads ();
+
+  fbsd_add_threads (ptid_get_pid (inferior_ptid));
+#endif
+}
+
+static void (*super_resume) (struct target_ops *,
+			     ptid_t,
+			     int,
+			     enum gdb_signal);
+
+static int
+resume_one_thread_cb (struct thread_info *tp, void *data)
+{
+  ptid_t *ptid = data;
+  int request;
+
+  if (ptid_get_pid (tp->ptid) != ptid_get_pid (*ptid))
+    return 0;
+
+  if (ptid_get_lwp (tp->ptid) == ptid_get_lwp (*ptid))
+    request = PT_RESUME;
+  else
+    request = PT_SUSPEND;
+
+  if (ptrace (request, ptid_get_lwp (tp->ptid), NULL, 0) == -1)
+    perror_with_name (("ptrace"));
+  return 0;
+}
+
+static int
+resume_all_threads_cb (struct thread_info *tp, void *data)
+{
+  ptid_t *filter = data;
+
+  if (!ptid_match (tp->ptid, *filter))
+    return 0;
+
+  if (ptrace (PT_RESUME, ptid_get_lwp (tp->ptid), NULL, 0) == -1)
+    perror_with_name (("ptrace"));
+  return 0;
+}
+
+/* Implement the "to_resume" target_ops method.  */
+
+static void
+fbsd_resume (struct target_ops *ops,
+	     ptid_t ptid, int step, enum gdb_signal signo)
+{
+
+  if (debug_fbsd_lwp)
+    fprintf_unfiltered (gdb_stdlog,
+			"FLWP: fbsd_resume for ptid (%d, %ld, %ld)\n",
+			ptid_get_pid (ptid), ptid_get_lwp (ptid),
+			ptid_get_tid (ptid));
+  if (ptid_lwp_p (ptid))
+    {
+      /* If ptid is a specific LWP, suspend all other LWPs in the process.  */
+      iterate_over_threads (resume_one_thread_cb, &ptid);
+    }
+  else
+    {
+      /* If ptid is a wildcard, resume all matching threads (they won't run
+	 until the process is continued however).  */
+      iterate_over_threads (resume_all_threads_cb, &ptid);
+      ptid = inferior_ptid;
+    }
+  super_resume (ops, ptid, step, signo);
+}
+
 #ifdef TDP_RFPPWAIT
 /*
   To catch fork events, PT_FOLLOW_FORK is set on every traced process
@@ -246,7 +519,7 @@ static ptid_t (*super_wait) (struct target_ops *,
 struct fbsd_fork_child_info
 {
   struct fbsd_fork_child_info *next;
-  pid_t child;			/* Pid of new child.  */
+  ptid_t child;			/* Pid of new child.  */
 };
 
 static struct fbsd_fork_child_info *fbsd_pending_children;
@@ -255,7 +528,7 @@ static struct fbsd_fork_child_info *fbsd_pending_children;
    corresponding fork event in the parent.  */
 
 static void
-fbsd_remember_child (pid_t pid)
+fbsd_remember_child (ptid_t pid)
 {
   struct fbsd_fork_child_info *info = XCNEW (struct fbsd_fork_child_info);
 
@@ -265,45 +538,29 @@ fbsd_remember_child (pid_t pid)
 }
 
 /* Check for a previously-recorded new child process event for PID.
-   If one is found, remove it from the list.  */
+   If one is found, remove it from the list and return the PTID.  */
 
-static int
+static ptid_t
 fbsd_is_child_pending (pid_t pid)
 {
   struct fbsd_fork_child_info *info, *prev;
+  ptid_t ptid;
 
   prev = NULL;
   for (info = fbsd_pending_children; info; prev = info, info = info->next)
     {
-      if (info->child == pid)
+      if (ptid_get_pid (info->child) == pid)
 	{
 	  if (prev == NULL)
 	    fbsd_pending_children = info->next;
 	  else
 	    prev->next = info->next;
+	  ptid = info->child;
 	  xfree (info);
-	  return 1;
+	  return ptid;
 	}
     }
-  return 0;
-}
-
-/* Fetch the external variant of the kernel's internal process
-   structure for the process PID into KP.  */
-
-static void
-fbsd_fetch_kinfo_proc (pid_t pid, struct kinfo_proc *kp)
-{
-  size_t len;
-  int mib[4];
-
-  len = sizeof *kp;
-  mib[0] = CTL_KERN;
-  mib[1] = KERN_PROC;
-  mib[2] = KERN_PROC_PID;
-  mib[3] = pid;
-  if (sysctl (mib, 4, kp, &len, NULL, 0) == -1)
-    perror_with_name (("sysctl"));
+  return null_ptid;
 }
 #endif
 
@@ -328,21 +585,83 @@ fbsd_wait (struct target_ops *ops,
 	  int status;
 
 	  pid = ptid_get_pid (wptid);
-	  if (ptrace (PT_LWPINFO, pid, (caddr_t)&pl, sizeof pl) == -1)
+	  if (ptrace (PT_LWPINFO, pid, (caddr_t) &pl, sizeof pl) == -1)
 	    perror_with_name (("ptrace"));
 
+	  wptid = ptid_build (pid, pl.pl_lwpid, 0);
+
+#ifdef PT_LWP_EVENTS
+	  if (pl.pl_flags & PL_FLAG_EXITED)
+	    {
+	      /* If GDB attaches to a multi-threaded process, exiting
+		 threads might be skipped during fbsd_post_attach that
+		 have not yet reported their PL_FLAG_EXITED event.
+		 Ignore EXITED events for an unknown LWP.  */
+	      if (in_thread_list (wptid))
+		{
+		  if (debug_fbsd_lwp)
+		    fprintf_unfiltered (gdb_stdlog,
+					"FLWP: deleting thread for LWP %u\n",
+					pl.pl_lwpid);
+		  if (print_thread_events)
+		    printf_unfiltered (_("[%s exited]\n"), target_pid_to_str
+				       (wptid));
+		  delete_thread (wptid);
+		}
+	      if (ptrace (PT_CONTINUE, pid, (caddr_t) 1, 0) == -1)
+		perror_with_name (("ptrace"));
+	      continue;
+	    }
+#endif
+
+	  /* Switch to an LWP PTID on the first stop in a new process.
+	     This is done after handling PL_FLAG_EXITED to avoid
+	     switching to an exited LWP.  It is done before checking
+	     PL_FLAG_BORN in case the first stop reported after
+	     attaching to an existing process is a PL_FLAG_BORN
+	     event.  */
+	  if (in_thread_list (pid_to_ptid (pid)))
+	    {
+	      if (debug_fbsd_lwp)
+		fprintf_unfiltered (gdb_stdlog,
+				    "FLWP: using LWP %u for first thread\n",
+				    pl.pl_lwpid);
+	      thread_change_ptid (pid_to_ptid (pid), wptid);
+	    }
+
+#ifdef PT_LWP_EVENTS
+	  if (pl.pl_flags & PL_FLAG_BORN)
+	    {
+	      /* If GDB attaches to a multi-threaded process, newborn
+		 threads might be added by fbsd_add_threads that have
+		 not yet reported their PL_FLAG_BORN event.  Ignore
+		 BORN events for an already-known LWP.  */
+	      if (!in_thread_list (wptid))
+		{
+		  if (debug_fbsd_lwp)
+		    fprintf_unfiltered (gdb_stdlog,
+					"FLWP: adding thread for LWP %u\n",
+					pl.pl_lwpid);
+		  add_thread (wptid);
+		}
+	      ourstatus->kind = TARGET_WAITKIND_SPURIOUS;
+	      return wptid;
+	    }
+#endif
+
 #ifdef TDP_RFPPWAIT
 	  if (pl.pl_flags & PL_FLAG_FORKED)
 	    {
 	      struct kinfo_proc kp;
+	      ptid_t child_ptid;
 	      pid_t child;
 
 	      child = pl.pl_child_pid;
 	      ourstatus->kind = TARGET_WAITKIND_FORKED;
-	      ourstatus->value.related_pid = pid_to_ptid (child);
 
 	      /* Make sure the other end of the fork is stopped too.  */
-	      if (!fbsd_is_child_pending (child))
+	      child_ptid = fbsd_is_child_pending (child);
+	      if (ptid_equal (child_ptid, null_ptid))
 		{
 		  pid = waitpid (child, &status, 0);
 		  if (pid == -1)
@@ -354,6 +673,7 @@ fbsd_wait (struct target_ops *ops,
 		    perror_with_name (("ptrace"));
 
 		  gdb_assert (pl.pl_flags & PL_FLAG_CHILD);
+		  child_ptid = ptid_build (child, pl.pl_lwpid, 0);
 		}
 
 	      /* For vfork, the child process will have the P_PPWAIT
@@ -361,6 +681,7 @@ fbsd_wait (struct target_ops *ops,
 	      fbsd_fetch_kinfo_proc (child, &kp);
 	      if (kp.ki_flag & P_PPWAIT)
 		ourstatus->kind = TARGET_WAITKIND_VFORKED;
+	      ourstatus->value.related_pid = child_ptid;
 
 	      return wptid;
 	    }
@@ -370,7 +691,7 @@ fbsd_wait (struct target_ops *ops,
 	      /* Remember that this child forked, but do not report it
 		 until the parent reports its corresponding fork
 		 event.  */
-	      fbsd_remember_child (ptid_get_pid (wptid));
+	      fbsd_remember_child (wptid);
 	      continue;
 	    }
 #endif
@@ -449,13 +770,19 @@ fbsd_enable_follow_fork (pid_t pid)
   if (ptrace (PT_FOLLOW_FORK, pid, (PTRACE_TYPE_ARG3)0, 1) == -1)
     perror_with_name (("ptrace"));
 }
+#endif
 
 /* Implement the "to_post_startup_inferior" target_ops method.  */
 
 static void
 fbsd_post_startup_inferior (struct target_ops *self, ptid_t pid)
 {
+#ifdef TDP_RFPPWAIT
   fbsd_enable_follow_fork (ptid_get_pid (pid));
+#endif
+#ifdef PT_LWP_EVENTS
+  fbsd_enable_lwp_events (ptid_get_pid (pid));
+#endif
 }
 
 /* Implement the "to_post_attach" target_ops method.  */
@@ -463,9 +790,14 @@ fbsd_post_startup_inferior (struct target_ops *self, ptid_t pid)
 static void
 fbsd_post_attach (struct target_ops *self, int pid)
 {
+#ifdef TDP_RFPPWAIT
   fbsd_enable_follow_fork (pid);
-}
 #endif
+#ifdef PT_LWP_EVENTS
+  fbsd_enable_lwp_events (pid);
+#endif
+  fbsd_add_threads (pid);
+}
 
 #ifdef PL_FLAG_EXEC
 /* If the FreeBSD kernel supports PL_FLAG_EXEC, then traced processes
@@ -491,16 +823,25 @@ fbsd_nat_add_target (struct target_ops *t)
   t->to_pid_to_exec_file = fbsd_pid_to_exec_file;
   t->to_find_memory_regions = fbsd_find_memory_regions;
 #ifdef PT_LWPINFO
+  t->to_thread_alive = fbsd_thread_alive;
+  t->to_pid_to_str = fbsd_pid_to_str;
+#ifdef HAVE_STRUCT_PTRACE_LWPINFO_PL_TDNAME
+  t->to_thread_name = fbsd_thread_name;
+#endif
+  t->to_update_thread_list = fbsd_update_thread_list;
+  t->to_has_thread_control = tc_schedlock;
+  super_resume = t->to_resume;
+  t->to_resume = fbsd_resume;
   super_wait = t->to_wait;
   t->to_wait = fbsd_wait;
+  t->to_post_startup_inferior = fbsd_post_startup_inferior;
+  t->to_post_attach = fbsd_post_attach;
 #ifdef TDP_RFPPWAIT
   t->to_follow_fork = fbsd_follow_fork;
   t->to_insert_fork_catchpoint = fbsd_insert_fork_catchpoint;
   t->to_remove_fork_catchpoint = fbsd_remove_fork_catchpoint;
   t->to_insert_vfork_catchpoint = fbsd_insert_vfork_catchpoint;
   t->to_remove_vfork_catchpoint = fbsd_remove_vfork_catchpoint;
-  t->to_post_startup_inferior = fbsd_post_startup_inferior;
-  t->to_post_attach = fbsd_post_attach;
 #endif
 #ifdef PL_FLAG_EXEC
   t->to_insert_exec_catchpoint = fbsd_insert_exec_catchpoint;
@@ -509,3 +850,21 @@ fbsd_nat_add_target (struct target_ops *t)
 #endif
   add_target (t);
 }
+
+/* Provide a prototype to silence -Wmissing-prototypes.  */
+extern initialize_file_ftype _initialize_fbsd_nat;
+
+void
+_initialize_fbsd_nat (void)
+{
+#ifdef PT_LWPINFO
+  add_setshow_boolean_cmd ("fbsd-lwp", class_maintenance,
+			   &debug_fbsd_lwp, _("\
+Set debugging of FreeBSD lwp module."), _("\
+Show debugging of FreeBSD lwp module."), _("\
+Enables printf debugging output."),
+			   NULL,
+			   &show_fbsd_lwp_debug,
+			   &setdebuglist, &showdebuglist);
+#endif
+}