[PATCH 5/6] Add support for LWP-based threads on FreeBSD.

[John Baldwin <jhb at freebsd dot org>]

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_switch_to_threaded): New function.
	[PT_LWP_EVENTS] (fbsd_enable_lwp_events): New function.
	[!PT_LWP_EVENTS] (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_wait): Include lwp in returned ptid.
	[PT_LWP_EVENTS] Handle LWP events.
	(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/ChangeLog    |  35 +++++
 gdb/config.in    |   3 +
 gdb/configure    |  16 ++
 gdb/configure.ac |   7 +
 gdb/fbsd-nat.c   | 452 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
 5 files changed, 492 insertions(+), 21 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 0accac9..4e911f8 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,4 +1,39 @@
 2016-01-10  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_switch_to_threaded): New function.
+	[PT_LWP_EVENTS] (fbsd_enable_lwp_events): New function.
+	[!PT_LWP_EVENTS] (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_wait): Include lwp in returned ptid.
+	[PT_LWP_EVENTS] Handle LWP events.
+	(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.
+
+2016-01-10  John Baldwin  <jhb@FreeBSD.org>
+
 	* amd64bsd-nat.c (get_ptrace_pid): New function.
 	(amd64bsd_fetch_inferior_registers): Use new function.
 	(amd64bsd_store_inferior_registers): Use new function.
diff --git a/gdb/config.in b/gdb/config.in
index 6196fa0..dc9da0a 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 15383d6..2dc1c9d 100755
--- a/gdb/configure
+++ b/gdb/configure
@@ -12964,6 +12964,22 @@ $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 d102640..ad62310 100644
--- a/gdb/configure.ac
+++ b/gdb/configure.ac
@@ -1510,6 +1510,13 @@ 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/fbsd-nat.c b/gdb/fbsd-nat.c
index 702e51d..8eca7b9 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 +205,338 @@ 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);
 
+#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.
+*/
+
+/* 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
+static 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[64];
+
+  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
+static void
+fbsd_switch_to_threaded (pid_t pid)
+{
+  struct cleanup *cleanup;
+  lwpid_t *lwps;
+  struct ptrace_lwpinfo *pl;
+  int i, nlwps;
+
+  nlwps = ptrace (PT_GETNUMLWPS, pid, NULL, 0);
+  if (nlwps == -1)
+    perror_with_name (("ptrace"));
+
+  lwps = XCNEWVEC (lwpid_t, nwlps);
+  cleanup = make_cleanup (xfree, lwps);
+
+  nlwps = ptrace (PT_GETLWPLIST, pid, (caddr_t)lwps, nlwps);
+  if (nlwps == -1)
+    perror_with_name (("ptrace"));
+  pl = XCNEWVEC (struct ptrace_lwpinfo, nlwps);
+  make_cleanup (xfree, pl);
+  for (i = 0; i < nlwps; i++)
+    {
+      if (ptrace (PT_LWPINFO, lwps[i], (caddr_t)&pl[i], sizeof pl[i]) == -1)
+	perror_with_name (("ptrace"));
+    }
+
+  /* Choose a candidate thread for the main thread.  Prefer the first
+     non-BORN and non-EXITED thread.  If all threads are newborns, use
+     the first non-EXITED thread.  */
+  for (i = 0; i < nlwps; i++)
+    {
+      ptid_t ptid = ptid_build (pid, lwps[i], 0);
+
+      if ((pl[i].pl_flags & (PL_FLAG_BORN | PL_FLAG_EXITED)) != 0)
+	continue;
+
+      if (debug_fbsd_lwp)
+	fprintf_unfiltered (gdb_stdlog,
+			    "FLWP: using LWP %u as main thread\n",
+			    lwps[i]);
+      gdb_assert (!in_thread_list (ptid));
+      thread_change_ptid (inferior_ptid, ptid);
+      break;
+    }
+  for (i = 0; i < nlwps; i++)
+    {
+      ptid_t ptid = ptid_build (pid, lwps[i], 0);
+
+      if (pl[i].pl_flags & PL_FLAG_EXITED)
+	continue;
+
+      if (!ptid_lwp_p (inferior_ptid))
+	{
+	  if (debug_fbsd_lwp)
+	    fprintf_unfiltered (gdb_stdlog,
+				"FLWP: using LWP %u as main thread\n",
+				lwps[i]);
+	  gdb_assert (!in_thread_list (ptid));
+	  thread_change_ptid (inferior_ptid, ptid);
+	}
+      else if (!in_thread_list (ptid))
+	{
+	  if (debug_fbsd_lwp)
+	    fprintf_unfiltered (gdb_stdlog,
+				"FLWP: adding thread for LWP %u\n",
+				lwps[i]);
+	  add_thread (ptid);
+	}
+    }
+
+  do_cleanups (cleanup);
+}
+
+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"));
+}
+#else
+static void
+fbsd_add_threads (pid_t pid, int nlwps)
+{
+  struct cleanup *cleanup;
+  lwpid_t *lwps;
+  int i;
+
+  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 this inferior is not using LWP ptids, use the first LWP as
+	 the main thread.  */
+      if (!ptid_lwp_p (inferior_ptid))
+	{
+	  if (debug_fbsd_lwp)
+	    fprintf_unfiltered (gdb_stdlog,
+				"FLWP: using LWP %u as main thread\n",
+				lwps[i]);
+	  gdb_assert (!in_thread_list (ptid));
+	  thread_change_ptid (inferior_ptid, ptid);
+	}
+      else if (!in_thread_list (ptid))
+	{
+	  if (debug_fbsd_lwp)
+	    fprintf_unfiltered (gdb_stdlog,
+				"FLWP: adding thread for LWP %u\n",
+				lwps[i]);
+	  gdb_assert (ptid_lwp_p (inferior_ptid));
+	  add_thread (ptid);
+	}
+    }
+  do_cleanups (cleanup);
+}
+#endif
+
+/* Implement the to_update_thread_list target method for this
+   target.  */
+
+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
+  int nlwps;
+  int pid = ptid_get_pid (inferior_ptid);
+
+  prune_threads();
+
+  nlwps = ptrace (PT_GETNUMLWPS, pid, NULL, 0);
+  if (nlwps == -1)
+    perror_with_name (("ptrace"));
+
+  /* Leave single-threaded processes with a non-threaded ptid alone.  */
+  if (nlwps == 1 && !ptid_lwp_p (inferior_ptid))
+    return;
+
+  fbsd_add_threads (pid, nlwps);
+#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), (caddr_t)0, 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;
+
+  /* Ignore single-threaded processes.  */
+  if (!ptid_lwp_p (tp->ptid))
+    return 0;
+
+  if (ptrace (PT_RESUME, ptid_get_lwp (tp->ptid), (caddr_t)0, 0) == -1)
+    perror_with_name (("ptrace"));
+  return 0;
+}
+
+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
@@ -287,24 +615,6 @@ fbsd_is_child_pending (pid_t pid)
     }
   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"));
-}
 #endif
 
 /* Wait for the child specified by PTID to do something.  Return the
@@ -330,6 +640,8 @@ fbsd_wait (struct target_ops *ops,
 	  pid = ptid_get_pid (wptid);
 	  if (ptrace (PT_LWPINFO, pid, (caddr_t)&pl, sizeof pl) == -1)
 	    perror_with_name (("ptrace"));
+	  if (!in_thread_list (wptid))
+	    wptid = ptid_build (pid, pl.pl_lwpid, 0);
 
 #ifdef TDP_RFPPWAIT
 	  if (pl.pl_flags & PL_FLAG_FORKED)
@@ -384,6 +696,52 @@ fbsd_wait (struct target_ops *ops,
 	      return wptid;
 	    }
 #endif
+
+#ifdef PT_LWP_EVENTS
+	  if (pl.pl_flags & PL_FLAG_BORN)
+	    {
+	      if (ptid_lwp_p (inferior_ptid))
+		{
+		  ptid_t ptid = ptid_build (pid, pl.pl_lwpid, 0);
+
+		  /* If a process stops with multiple newborn threads,
+		     the first thread birth event will add all of the
+		     pending threads in fbsd_switch_to_threaded.
+		     However, each thread will still report a newborn
+		     stop.  */
+		  if (!in_thread_list (ptid))
+		    {
+		      if (debug_fbsd_lwp)
+			fprintf_unfiltered (gdb_stdlog,
+					    "FLWP: adding thread for LWP %u\n",
+					    pl.pl_lwpid);
+		      gdb_assert(!in_thread_list (ptid));
+		      add_thread (ptid);
+		    }
+		}
+	      else
+		fbsd_switch_to_threaded (pid);
+	      ourstatus->kind = TARGET_WAITKIND_SPURIOUS;
+	      return ptid_build (pid, pl.pl_lwpid, 0);
+	    }
+	  if (pl.pl_flags & PL_FLAG_EXITED)
+	    {
+	      ptid_t ptid = ptid_build (pid, pl.pl_lwpid, 0);
+
+	      gdb_assert (in_thread_list (ptid));
+	      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
+				   (ptid));
+	      delete_thread (ptid);
+	      if (ptrace (PT_CONTINUE, pid, (PTRACE_TYPE_ARG3)1, 0) == -1)
+		perror_with_name (("ptrace"));
+	      continue;
+	    }
+#endif
 	}
       return wptid;
     }
@@ -449,13 +807,19 @@ 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 +827,28 @@ fbsd_post_startup_inferior (struct target_ops *self, ptid_t pid)
 static void
 fbsd_post_attach (struct target_ops *self, int pid)
 {
+  int nlwps;
+
+#ifdef TDP_RFPPWAIT
   fbsd_enable_follow_fork (pid);
-}
 #endif
+#ifdef PT_LWP_EVENTS
+  fbsd_enable_lwp_events (pid);
+#endif
+
+  /* Add threads for other LWPs when attaching to a threaded process.  */
+  nlwps = ptrace (PT_GETNUMLWPS, pid, NULL, 0);
+  if (nlwps == -1)
+    perror_with_name (("ptrace"));
+  if (nlwps > 1)
+    {
+#ifdef PT_LWP_EVENTS
+      fbsd_switch_to_threaded (pid);
+#else
+      fbsd_add_threads (pid, nlwps);
+#endif
+    }
+}
 
 #ifdef PL_FLAG_EXEC
 /* If the FreeBSD kernel supports PL_FLAG_EXEC, then traced processes
@@ -491,16 +874,25 @@ 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 +901,21 @@ 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,
+			   NULL,
+			   &setdebuglist, &showdebuglist);
+#endif
+}
-- 
2.7.0