GNU/Linux: Stop using libthread_db/td_ta_thr_iter

TL;DR - GDB can hang if something refreshes the thread list out of the
target while the target is running.  GDB hangs inside td_ta_thr_iter.
The fix is to not use that libthread_db function anymore.

Long version:

Running the testsuite against my all-stop-on-top-of-non-stop series is
still exposing latent non-stop bugs.

I was originally seeing this with the multi-create.exp test, back when
we were still using libthread_db thread event breakpoints.  The
all-stop-on-top-of-non-stop series forces a thread list refresh each
time GDB needs to start stepping over a breakpoint (to pause all
threads).  That test hits the thread event breakpoint often, resulting
in a bunch of step-over operations, thus a bunch of thread list
refreshes while some threads in the target are running.

The commit adds a real non-stop mode test that triggers the issue,
based on multi-create.exp, that does an explicit "info threads" when a
breakpoint is hit.  IOW, it does the same things the as-ns series was
doing when testing multi-create.exp.

The bug is a race, so it unfortunately takes several runs for the test
to trigger it.  In fact, even when setting the test running in a loop,
it sometimes takes several minutes for it to trigger for me.

The race is related to libthread_db's td_ta_thr_iter.  This is
libthread_db's entry point for walking the thread list of the
inferior.

Sometimes, when GDB refreshes the thread list from the target,
libthread_db's td_ta_thr_iter can somehow see glibc's thread list as a
cycle, and get stuck in an infinite loop.

The issue is that when a thread exits, its thread control structure in
glibc is moved from a "used" list to a "cache" list.  These lists are
simply circular linked lists where the "next/prev" pointers are
embedded in the thread control structure itself.  The "next" pointer
of the last element of the list points back to the list's sentinel
"head".  There's only one set of "next/prev" pointers for both lists;
thus a thread can only be in one of the lists at a time, not in both
simultaneously.

So when thread C exits, simplifying, the following happens.  A-C are
threads.  stack_used and stack_cache are the list's heads.

Before:

  stack_used -> A -> B -> C -> (&stack_used)
  stack_cache -> (&stack_cache)

After:

  stack_used -> A -> B -> (&stack_used)
  stack_cache -> C -> (&stack_cache)

td_ta_thr_iter starts by iterating at the list's head's next, and
iterates until it sees a thread whose next pointer points to the
list's head again.  Thus in the before case above, C's next points to
stack_used, indicating end of list.  In the same case, the stack_cache
list is empty.

For each thread being iterated, td_ta_thr_iter reads the whole thread
object out of the inferior.  This includes the thread's "next"
pointer.

In the scenario above, it may happen that td_ta_thr_iter is iterating
thread B and has already read B's thread structure just before thread
C exits and its control structure moves to the cached list.

Now, recall that td_ta_thr_iter is running in the context of GDB, and
there's no locking between GDB and the inferior.  From it's local copy
of B, td_ta_thr_iter believes that the next thread after B is thread
C, so it happilly continues iterating to C, a thread that has already
exited, and is now in the stack cache list.

After iterating C, td_ta_thr_iter finds the stack_cache head, which
because it is not stack_used, td_ta_thr_iter assumes it's just another
thread.  After this, unless the reverse race triggers, GDB gets stuck
in td_ta_thr_iter forever walking the stack_cache list, as no thread
in thatlist has a next pointer that points back to stack_used (the
terminating condition).

Before fully understanding the issue, I tried adding cycle detection
to GDB's td_ta_thr_iter callback.  However, td_ta_thr_iter skips
calling the callback in some cases, which means that it's possible
that the callback isn't called at all, making it impossible for GDB to
break the loop.  I did manage to get GDB stuck in that state more than
once.

Fortunately, we can avoid the issue altogether.  We don't really need
td_ta_thr_iter for live debugging nowadays, given PTRACE_EVENT_CLONE.
We already know how to map and lwp id to a thread id without iterating
(thread_from_lwp), so use that more.

gdb/ChangeLog:
2015-02-20  Pedro Alves  <palves@redhat.com>

	* linux-nat.c (linux_handle_extended_wait): Call
	thread_db_notice_clone whenever a new clone LWP is detected.
	(linux_stop_and_wait_all_lwps, linux_unstop_all_lwps): New
	functions.
	* linux-nat.h (thread_db_attach_lwp): Delete declaration.
	(thread_db_notice_clone, linux_stop_and_wait_all_lwps)
	(linux_unstop_all_lwps): Declare.
	* linux-thread-db.c (struct thread_get_info_inout): Delete.
	(thread_get_info_callback): Delete.
	(thread_from_lwp): Use td_thr_get_info and record_thread.
	(thread_db_attach_lwp): Delete.
	(thread_db_notice_clone): New function.
	(try_thread_db_load_1): If /proc is mounted and shows the
	process'es task list, walk over all LWPs and call thread_from_lwp
	instead of relying on td_ta_thr_iter.
	(attach_thread): Don't call check_thread_signals here.  Split the
	tail part of the function (which adds the thread to the core GDB
	thread list) to ...
	(record_thread): ... this function.  Call check_thread_signals
	here.
	(thread_db_wait): Don't call thread_db_find_new_threads_1.  Always
	call thread_from_lwp.
	(thread_db_update_thread_list): Rename to ...
	(thread_db_update_thread_list_org): ... this.
	(thread_db_update_thread_list): New function.
	(thread_db_find_thread_from_tid): Delete.
	(thread_db_get_ada_task_ptid): Simplify.
	* nat/linux-procfs.c: Include <sys/stat.h>.
	(linux_proc_task_list_dir_exists): New function.
	* nat/linux-procfs.h (linux_proc_task_list_dir_exists): Declare.

gdb/gdbserver/ChangeLog:
2015-02-20  Pedro Alves  <palves@redhat.com>

	* thread-db.c: Include "nat/linux-procfs.h".
	(thread_db_init): Skip listing new threads if the kernel supports
	PTRACE_EVENT_CLONE and /proc/PID/task/ is accessible.

gdb/testsuite/ChangeLog:
2015-02-20  Pedro Alves  <palves@redhat.com>

	* gdb.threads/multi-create-ns-info-thr.exp: New file.

10 files changed, 291 insertions, 172 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 52a641b..b7a72d9 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,38 @@
 2015-02-20  Pedro Alves  <palves@redhat.com>
 
+	* linux-nat.c (linux_handle_extended_wait): Call
+	thread_db_notice_clone whenever a new clone LWP is detected.
+	(linux_stop_and_wait_all_lwps, linux_unstop_all_lwps): New
+	functions.
+	* linux-nat.h (thread_db_attach_lwp): Delete declaration.
+	(thread_db_notice_clone, linux_stop_and_wait_all_lwps)
+	(linux_unstop_all_lwps): Declare.
+	* linux-thread-db.c (struct thread_get_info_inout): Delete.
+	(thread_get_info_callback): Delete.
+	(thread_from_lwp): Use td_thr_get_info and record_thread.
+	(thread_db_attach_lwp): Delete.
+	(thread_db_notice_clone): New function.
+	(try_thread_db_load_1): If /proc is mounted and shows the
+	process'es task list, walk over all LWPs and call thread_from_lwp
+	instead of relying on td_ta_thr_iter.
+	(attach_thread): Don't call check_thread_signals here.  Split the
+	tail part of the function (which adds the thread to the core GDB
+	thread list) to ...
+	(record_thread): ... this function.  Call check_thread_signals
+	here.
+	(thread_db_wait): Don't call thread_db_find_new_threads_1.  Always
+	call thread_from_lwp.
+	(thread_db_update_thread_list): Rename to ...
+	(thread_db_update_thread_list_org): ... this.
+	(thread_db_update_thread_list): New function.
+	(thread_db_find_thread_from_tid): Delete.
+	(thread_db_get_ada_task_ptid): Simplify.
+	* nat/linux-procfs.c: Include <sys/stat.h>.
+	(linux_proc_task_list_dir_exists): New function.
+	* nat/linux-procfs.h (linux_proc_task_list_dir_exists): Declare.
+
+2015-02-20  Pedro Alves  <palves@redhat.com>
+
 	* linux-nat.c (lin_lwp_attach_lwp): No longer special case the
 	main LWP.  Handle the case of waitpid returning 0 if we're already
 	attached to the LWP.  Don't set the LWP's last_resume_kind to
diff --git a/gdb/gdbserver/ChangeLog b/gdb/gdbserver/ChangeLog
index cd79d75..cfdb3e7 100644
--- a/gdb/gdbserver/ChangeLog
+++ b/gdb/gdbserver/ChangeLog
@@ -1,5 +1,11 @@
 2015-02-20  Pedro Alves  <palves@redhat.com>
 
+	* thread-db.c: Include "nat/linux-procfs.h".
+	(thread_db_init): Skip listing new threads if the kernel supports
+	PTRACE_EVENT_CLONE and /proc/PID/task/ is accessible.
+
+2015-02-20  Pedro Alves  <palves@redhat.com>
+
 	* linux-low.c (status_pending_p_callback): Use ptid_match.
 
 2015-02-19  Antoine Tremblay  <antoine.tremblay@ericsson.com>
diff --git a/gdb/gdbserver/thread-db.c b/gdb/gdbserver/thread-db.c
index 2185245..a8b6f42 100644
--- a/gdb/gdbserver/thread-db.c
+++ b/gdb/gdbserver/thread-db.c
@@ -29,6 +29,7 @@ static int thread_db_use_events;
 #include "gdb_proc_service.h"
 #include "nat/gdb_thread_db.h"
 #include "gdb_vecs.h"
+#include "nat/linux-procfs.h"
 
 #ifndef USE_LIBTHREAD_DB_DIRECTLY
 #include <dlfcn.h>
@@ -859,7 +860,22 @@ thread_db_init (int use_events)
 	  thread_db_mourn (proc);
 	  return 0;
 	}
-      thread_db_find_new_threads ();
+
+      /* It's best to avoid td_ta_thr_iter if possible.  That walks
+	 data structures in the inferior's address space that may be
+	 corrupted, or, if the target is running, the list may change
+	 while we walk it.  In the latter case, it's possible that a
+	 thread exits just at the exact time that causes GDBserver to
+	 get stuck in an infinite loop.  If the kernel supports clone
+	 events, and /proc/PID/task/ exits, then we already know about
+	 all threads in the process.  When we need info out of
+	 thread_db on a given thread (e.g., for TLS), we'll use
+	 find_one_thread then.  That uses thread_db entry points that
+	 do not walk libpthread's thread list, so should be safe, as
+	 well as more efficient.  */
+      if (use_events
+	  || !linux_proc_task_list_dir_exists (pid_of (proc)))
+	thread_db_find_new_threads ();
       thread_db_look_up_symbols ();
       return 1;
     }
diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index cfde1fd..2e1133d 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -264,6 +264,7 @@ async_file_mark (void)
 static int kill_lwp (int lwpid, int signo);
 
 static int stop_callback (struct lwp_info *lp, void *data);
+static int resume_stopped_resumed_lwps (struct lwp_info *lp, void *data);
 
 static void block_child_signals (sigset_t *prev_mask);
 static void restore_child_signals_mask (sigset_t *prev_mask);
@@ -2003,34 +2004,24 @@ linux_handle_extended_wait (struct lwp_info *lp, int status,
 		status = 0;
 	    }
 
-	  if (non_stop)
+	  /* If the thread_db layer is active, let it record the user
+	     level thread id and status, and add the thread to GDB's
+	     list.  */
+	  if (!thread_db_notice_clone (lp->ptid, new_lp->ptid))
 	    {
-	      /* Add the new thread to GDB's lists as soon as possible
-		 so that:
-
-		 1) the frontend doesn't have to wait for a stop to
-		 display them, and,
-
-		 2) we tag it with the correct running state.  */
-
-	      /* If the thread_db layer is active, let it know about
-		 this new thread, and add it to GDB's list.  */
-	      if (!thread_db_attach_lwp (new_lp->ptid))
-		{
-		  /* We're not using thread_db.  Add it to GDB's
-		     list.  */
-		  target_post_attach (ptid_get_lwp (new_lp->ptid));
-		  add_thread (new_lp->ptid);
-		}
+	      /* The process is not using thread_db.  Add the LWP to
+		 GDB's list.  */
+	      target_post_attach (ptid_get_lwp (new_lp->ptid));
+	      add_thread (new_lp->ptid);
+	    }
 
-	      if (!stopping)
-		{
-		  set_running (new_lp->ptid, 1);
-		  set_executing (new_lp->ptid, 1);
-		  /* thread_db_attach_lwp -> lin_lwp_attach_lwp forced
-		     resume_stop.  */
-		  new_lp->last_resume_kind = resume_continue;
-		}
+	  if (!stopping)
+	    {
+	      set_running (new_lp->ptid, 1);
+	      set_executing (new_lp->ptid, 1);
+	      /* thread_db_attach_lwp -> lin_lwp_attach_lwp forced
+		 resume_stop.  */
+	      new_lp->last_resume_kind = resume_continue;
 	    }
 
 	  if (status != 0)
@@ -2285,6 +2276,28 @@ linux_stop_lwp (struct lwp_info *lwp)
   stop_callback (lwp, NULL);
 }
 
+/* See linux-nat.h  */
+
+void
+linux_stop_and_wait_all_lwps (void)
+{
+  /* Stop all LWP's ...  */
+  iterate_over_lwps (minus_one_ptid, stop_callback, NULL);
+
+  /* ... and wait until all of them have reported back that
+     they're no longer running.  */
+  iterate_over_lwps (minus_one_ptid, stop_wait_callback, NULL);
+}
+
+/* See linux-nat.h  */
+
+void
+linux_unstop_all_lwps (void)
+{
+  iterate_over_lwps (minus_one_ptid,
+		     resume_stopped_resumed_lwps, &minus_one_ptid);
+}
+
 /* Return non-zero if LWP PID has a pending SIGINT.  */
 
 static int
diff --git a/gdb/linux-nat.h b/gdb/linux-nat.h
index 669450d..8c2ceb5 100644
--- a/gdb/linux-nat.h
+++ b/gdb/linux-nat.h
@@ -142,7 +142,12 @@ extern struct lwp_info *lwp_list;
 /* Attempt to initialize libthread_db.  */
 void check_for_thread_db (void);
 
-int thread_db_attach_lwp (ptid_t ptid);
+/* Called from the LWP layer to inform the thread_db layer that PARENT
+   spawned CHILD.  Both LWPs are currently stopped.  This function
+   does whatever is required to have the child LWP under the
+   thread_db's control --- e.g., enabling event reporting.  Returns
+   true on success, false if the process isn't using libpthread.  */
+extern int thread_db_notice_clone (ptid_t parent, ptid_t child);
 
 /* Return the set of signals used by the threads library.  */
 extern void lin_thread_get_thread_signals (sigset_t *mask);
@@ -155,6 +160,15 @@ extern int lin_lwp_attach_lwp (ptid_t ptid);
 
 extern void linux_stop_lwp (struct lwp_info *lwp);
 
+/* Stop all LWPs, synchronously.  (Any events that trigger while LWPs
+   are being stopped are left pending.)  */
+extern void linux_stop_and_wait_all_lwps (void);
+
+/* Set resumed LWPs running again, as they were before being stopped
+   with linux_stop_and_wait_all_lwps.  (LWPS with pending events are
+   left stopped.)  */
+extern void linux_unstop_all_lwps (void);
+
 /* Iterator function for lin-lwp's lwp list.  */
 struct lwp_info *iterate_over_lwps (ptid_t filter,
 				    int (*callback) (struct lwp_info *,
diff --git a/gdb/linux-thread-db.c b/gdb/linux-thread-db.c
index 60d8742..f09685e 100644
--- a/gdb/linux-thread-db.c
+++ b/gdb/linux-thread-db.c
@@ -217,6 +217,13 @@ struct thread_db_info *thread_db_list;
 static void thread_db_find_new_threads_1 (ptid_t ptid);
 static void thread_db_find_new_threads_2 (ptid_t ptid, int until_no_new);
 
+static void check_thread_signals (void);
+
+static void record_thread (struct thread_db_info *info,
+			   struct thread_info *tp,
+			   ptid_t ptid, const td_thrhandle_t *th_p,
+			   const td_thrinfo_t *ti_p);
+
 /* Add the current inferior to the list of processes using libpthread.
    Return a pointer to the newly allocated object that was added to
    THREAD_DB_LIST.  HANDLE is the handle returned by dlopen'ing
@@ -398,64 +405,6 @@ have_threads (ptid_t ptid)
   return iterate_over_threads (have_threads_callback, &pid) != NULL;
 }
 
-struct thread_get_info_inout
-{
-  struct thread_info *thread_info;
-  struct thread_db_info *thread_db_info;
-};
-
-/* A callback function for td_ta_thr_iter, which we use to map all
-   threads to LWPs.
-
-   THP is a handle to the current thread; if INFOP is not NULL, the
-   struct thread_info associated with this thread is returned in
-   *INFOP.
-
-   If the thread is a zombie, TD_THR_ZOMBIE is returned.  Otherwise,
-   zero is returned to indicate success.  */
-
-static int
-thread_get_info_callback (const td_thrhandle_t *thp, void *argp)
-{
-  td_thrinfo_t ti;
-  td_err_e err;
-  ptid_t thread_ptid;
-  struct thread_get_info_inout *inout;
-  struct thread_db_info *info;
-
-  inout = argp;
-  info = inout->thread_db_info;
-
-  err = info->td_thr_get_info_p (thp, &ti);
-  if (err != TD_OK)
-    error (_("thread_get_info_callback: cannot get thread info: %s"),
-	   thread_db_err_str (err));
-
-  if (ti.ti_lid == -1)
-    {
-      /* We'll get this if a threaded program has a thread call clone
-	 with CLONE_VM.  `clone' sets the pthread LID of the new LWP
-	 to -1, which ends up clearing the parent thread's LID.  */
-      return 0;
-    }
-
-  /* Fill the cache.  */
-  thread_ptid = ptid_build (info->pid, ti.ti_lid, 0);
-  inout->thread_info = find_thread_ptid (thread_ptid);
-
-  if (inout->thread_info == NULL)
-    {
-      /* New thread.  Attach to it now (why wait?).  */
-      if (!have_threads (thread_ptid))
- 	thread_db_find_new_threads_1 (thread_ptid);
-      else
-	attach_thread (thread_ptid, thp, &ti);
-      inout->thread_info = find_thread_ptid (thread_ptid);
-      gdb_assert (inout->thread_info != NULL);
-    }
-
-  return 0;
-}
 
 /* Fetch the user-level thread id of PTID.  */
 
@@ -463,9 +412,10 @@ static void
 thread_from_lwp (ptid_t ptid)
 {
   td_thrhandle_t th;
+  td_thrinfo_t ti;
   td_err_e err;
   struct thread_db_info *info;
-  struct thread_get_info_inout io = {0};
+  struct thread_info *tp;
 
   /* Just in case td_ta_map_lwp2thr doesn't initialize it completely.  */
   th.th_unique = 0;
@@ -484,56 +434,37 @@ thread_from_lwp (ptid_t ptid)
     error (_("Cannot find user-level thread for LWP %ld: %s"),
 	   ptid_get_lwp (ptid), thread_db_err_str (err));
 
-  /* Long-winded way of fetching the thread info.  */
-  io.thread_db_info = info;
-  io.thread_info = NULL;
-  thread_get_info_callback (&th, &io);
+  err = info->td_thr_get_info_p (&th, &ti);
+  if (err != TD_OK)
+    error (_("thread_get_info_callback: cannot get thread info: %s"),
+	   thread_db_err_str (err));
+
+  /* Fill the cache.  */
+  tp = find_thread_ptid (ptid);
+  record_thread (info, tp, ptid, &th, &ti);
 }
 
 
-/* Attach to lwp PTID, doing whatever else is required to have this
-   LWP under the debugger's control --- e.g., enabling event
-   reporting.  Returns true on success.  */
+/* See linux-nat.h.  */
+
 int
-thread_db_attach_lwp (ptid_t ptid)
+thread_db_notice_clone (ptid_t parent, ptid_t child)
 {
   td_thrhandle_t th;
   td_thrinfo_t ti;
   td_err_e err;
   struct thread_db_info *info;
 
-  info = get_thread_db_info (ptid_get_pid (ptid));
+  info = get_thread_db_info (ptid_get_pid (child));
 
   if (info == NULL)
     return 0;
 
-  /* This ptid comes from linux-nat.c, which should always fill in the
-     LWP.  */
-  gdb_assert (ptid_get_lwp (ptid) != 0);
+  thread_from_lwp (child);
 
-  /* Access an lwp we know is stopped.  */
-  info->proc_handle.ptid = ptid;
-
-  /* If we have only looked at the first thread before libpthread was
-     initialized, we may not know its thread ID yet.  Make sure we do
-     before we add another thread to the list.  */
-  if (!have_threads (ptid))
-    thread_db_find_new_threads_1 (ptid);
-
-  err = info->td_ta_map_lwp2thr_p (info->thread_agent, ptid_get_lwp (ptid),
-				   &th);
-  if (err != TD_OK)
-    /* Cannot find user-level thread.  */
-    return 0;
-
-  err = info->td_thr_get_info_p (&th, &ti);
-  if (err != TD_OK)
-    {
-      warning (_("Cannot get thread info: %s"), thread_db_err_str (err));
-      return 0;
-    }
-
-  attach_thread (ptid, &th, &ti);
+  /* If we do not know about the main thread yet, this would be a good
+     time to find it.  */
+  thread_from_lwp (parent);
   return 1;
 }
 
@@ -821,7 +752,30 @@ try_thread_db_load_1 (struct thread_db_info *info)
   info->td_thr_tls_get_addr_p = dlsym (info->handle, "td_thr_tls_get_addr");
   info->td_thr_tlsbase_p = dlsym (info->handle, "td_thr_tlsbase");
 
-  if (thread_db_find_new_threads_silently (inferior_ptid) != 0)
+  /* It's best to avoid td_ta_thr_iter if possible.  That walks data
+     structures in the inferior's address space that may be corrupted,
+     or, if the target is running, may change while we walk them.  If
+     there's execution (and /proc is mounted), then we're already
+     attached to all LWPs.  Use thread_from_lwp, which uses
+     td_ta_map_lwp2thr instead, which does not walk the thread list.
+
+     td_ta_map_lwp2thr uses ps_get_thread_area, but we can't use that
+     currently on core targets, as it uses ptrace directly.  */
+  if (target_has_execution
+      && linux_proc_task_list_dir_exists (ptid_get_pid (inferior_ptid)))
+    {
+      struct lwp_info *lp;
+      int pid = ptid_get_pid (inferior_ptid);
+
+      linux_stop_and_wait_all_lwps ();
+
+      ALL_LWPS (lp)
+	if (ptid_get_pid (lp->ptid) == pid)
+	  thread_from_lwp (lp->ptid);
+
+      linux_unstop_all_lwps ();
+    }
+  else if (thread_db_find_new_threads_silently (inferior_ptid) != 0)
     {
       /* Even if libthread_db initializes, if the thread list is
          corrupted, we'd not manage to list any threads.  Better reject this
@@ -1302,9 +1256,7 @@ static int
 attach_thread (ptid_t ptid, const td_thrhandle_t *th_p,
 	       const td_thrinfo_t *ti_p)
 {
-  struct private_thread_info *private;
   struct thread_info *tp;
-  td_err_e err;
   struct thread_db_info *info;
 
   /* If we're being called after a TD_CREATE event, we may already
@@ -1337,9 +1289,6 @@ attach_thread (ptid_t ptid, const td_thrhandle_t *th_p,
 	}
     }
 
-  if (target_has_execution)
-    check_thread_signals ();
-
   /* Under GNU/Linux, we have to attach to each and every thread.  */
   if (target_has_execution
       && tp == NULL)
@@ -1363,31 +1312,48 @@ attach_thread (ptid_t ptid, const td_thrhandle_t *th_p,
       /* Otherwise, we sucessfully attached to the thread.  */
     }
 
+  info = get_thread_db_info (ptid_get_pid (ptid));
+  record_thread (info, tp, ptid, th_p, ti_p);
+  return 1;
+}
+
+/* Record a new thread in GDB's thread list.  Creates the thread's
+   private info.  If TP is NULL, creates a new thread.  Otherwise,
+   uses TP.  */
+
+static void
+record_thread (struct thread_db_info *info,
+	       struct thread_info *tp,
+	       ptid_t ptid, const td_thrhandle_t *th_p,
+	       const td_thrinfo_t *ti_p)
+{
+  td_err_e err;
+  struct private_thread_info *private;
+  int new_thread = (tp == NULL);
+
+  /* A thread ID of zero may mean the thread library has not
+     initialized yet.  Leave private == NULL until the thread library
+     has initialized.  */
+  if (ti_p->ti_tid == 0)
+    return;
+
   /* Construct the thread's private data.  */
   private = xmalloc (sizeof (struct private_thread_info));
   memset (private, 0, sizeof (struct private_thread_info));
 
-  /* A thread ID of zero may mean the thread library has not initialized
-     yet.  But we shouldn't even get here if that's the case.  FIXME:
-     if we change GDB to always have at least one thread in the thread
-     list this will have to go somewhere else; maybe private == NULL
-     until the thread_db target claims it.  */
-  gdb_assert (ti_p->ti_tid != 0);
   private->th = *th_p;
   private->tid = ti_p->ti_tid;
   update_thread_state (private, ti_p);
 
   /* Add the thread to GDB's thread list.  */
   if (tp == NULL)
-    add_thread_with_info (ptid, private);
+    tp = add_thread_with_info (ptid, private);
   else
     tp->private = private;
 
-  info = get_thread_db_info (ptid_get_pid (ptid));
-
   /* Enable thread event reporting for this thread, except when
      debugging a core file.  */
-  if (target_has_execution && thread_db_use_events ())
+  if (target_has_execution && thread_db_use_events () && new_thread)
     {
       err = info->td_thr_event_enable_p (th_p, 1);
       if (err != TD_OK)
@@ -1395,7 +1361,8 @@ attach_thread (ptid_t ptid, const td_thrhandle_t *th_p,
 	       target_pid_to_str (ptid), thread_db_err_str (err));
     }
 
-  return 1;
+  if (target_has_execution)
+    check_thread_signals ();
 }
 
 static void
@@ -1581,21 +1548,13 @@ thread_db_wait (struct target_ops *ops,
       return ptid;
     }
 
-  /* If we do not know about the main thread yet, this would be a good time to
-     find it.  */
-  if (ourstatus->kind == TARGET_WAITKIND_STOPPED && !have_threads (ptid))
-    thread_db_find_new_threads_1 (ptid);
-
   if (ourstatus->kind == TARGET_WAITKIND_STOPPED
       && ourstatus->value.sig == GDB_SIGNAL_TRAP)
     /* Check for a thread event.  */
     check_event (ptid);
 
-  if (have_threads (ptid))
-    {
-      /* Fill in the thread's user-level thread id.  */
-      thread_from_lwp (ptid);
-    }
+  /* Fill in the thread's user-level thread id and status.  */
+  thread_from_lwp (ptid);
 
   return ptid;
 }
@@ -1727,6 +1686,9 @@ find_new_threads_once (struct thread_db_info *info, int iteration,
   data.info = info;
   data.new_threads = 0;
 
+  /* See comment in thread_db_update_thread_list.  */
+  gdb_assert (!target_has_execution || thread_db_use_events ());
+
   TRY_CATCH (except, RETURN_MASK_ERROR)
     {
       /* Iterate over all user-space threads to discover new threads.  */
@@ -1805,8 +1767,10 @@ update_thread_core (struct lwp_info *info, void *closure)
   return 0;
 }
 
+/* Update the thread list using td_ta_thr_iter.  */
+
 static void
-thread_db_update_thread_list (struct target_ops *ops)
+thread_db_update_thread_list_td_ta_thr_iter (struct target_ops *ops)
 {
   struct thread_db_info *info;
   struct inferior *inf;
@@ -1830,6 +1794,29 @@ thread_db_update_thread_list (struct target_ops *ops)
 
       thread_db_find_new_threads_1 (thread->ptid);
     }
+}
+
+/* Implement the to_update_thread_list target method for this
+   target.  */
+
+static void
+thread_db_update_thread_list (struct target_ops *ops)
+{
+  /* It's best to avoid td_ta_thr_iter if possible.  That walks data
+     structures in the inferior's address space that may be corrupted,
+     or, if the target is running, the list may change while we walk
+     it.  In the latter case, it's possible that a thread exits just
+     at the exact time that causes GDB to get stuck in an infinite
+     loop.  To avoid pausing all threads whenever the core wants to
+     refresh the thread list, if the kernel supports clone events
+     (meaning we're always already attached to all LWPs), we use
+     thread_from_lwp immediately when we see an LWP stop.  That uses
+     thread_db entry points that do not walk libpthread's thread list,
+     so should be safe, as well as more efficient.  */
+  if (target_has_execution && !thread_db_use_events ())
+    ops->beneath->to_update_thread_list (ops->beneath);
+  else
+    thread_db_update_thread_list_td_ta_thr_iter (ops);
 
   if (target_has_execution)
     iterate_over_lwps (minus_one_ptid /* iterate over all */,
@@ -1962,33 +1949,13 @@ thread_db_get_thread_local_address (struct target_ops *ops,
   return beneath->to_get_thread_local_address (beneath, ptid, lm, offset);
 }
 
-/* Callback routine used to find a thread based on the TID part of
-   its PTID.  */
-
-static int
-thread_db_find_thread_from_tid (struct thread_info *thread, void *data)
-{
-  long *tid = (long *) data;
-
-  if (thread->private->tid == *tid)
-    return 1;
-
-  return 0;
-}
-
 /* Implement the to_get_ada_task_ptid target method for this target.  */
 
 static ptid_t
 thread_db_get_ada_task_ptid (struct target_ops *self, long lwp, long thread)
 {
-  struct thread_info *thread_info;
-
-  thread_db_find_new_threads_1 (inferior_ptid);
-  thread_info = iterate_over_threads (thread_db_find_thread_from_tid, &thread);
-
-  gdb_assert (thread_info != NULL);
-
-  return (thread_info->ptid);
+  /* NPTL uses a 1:1 model, so the LWP id suffices.  */
+  return ptid_build (ptid_get_pid (inferior_ptid), lwp, 0);
 }
 
 static void
diff --git a/gdb/nat/linux-procfs.c b/gdb/nat/linux-procfs.c
index 5dd2b92..1e922b9 100644
--- a/gdb/nat/linux-procfs.c
+++ b/gdb/nat/linux-procfs.c
@@ -20,6 +20,7 @@
 #include "linux-procfs.h"
 #include "filestuff.h"
 #include <dirent.h>
+#include <sys/stat.h>
 
 /* Return the TGID of LWPID from /proc/pid/status.  Returns -1 if not
    found.  */
@@ -251,3 +252,15 @@ linux_proc_attach_tgid_threads (pid_t pid,
 
   closedir (dir);
 }
+
+/* See linux-procfs.h.  */
+
+int
+linux_proc_task_list_dir_exists (pid_t pid)
+{
+  char pathname[128];
+  struct stat buf;
+
+  xsnprintf (pathname, sizeof (pathname), "/proc/%ld/task", (long) pid);
+  return (stat (pathname, &buf) == 0);
+}
diff --git a/gdb/nat/linux-procfs.h b/gdb/nat/linux-procfs.h
index 6f2a404..979ae0d 100644
--- a/gdb/nat/linux-procfs.h
+++ b/gdb/nat/linux-procfs.h
@@ -68,4 +68,7 @@ typedef int (*linux_proc_attach_lwp_func) (ptid_t ptid);
 extern void linux_proc_attach_tgid_threads (pid_t pid,
 					    linux_proc_attach_lwp_func func);
 
+/* Return true if the /proc/PID/task/ directory exists.  */
+extern int linux_proc_task_list_dir_exists (pid_t pid);
+
 #endif /* COMMON_LINUX_PROCFS_H */
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 050f98c..2ea6940 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,9 @@
 2015-02-20  Pedro Alves  <palves@redhat.com>
 
+	* gdb.threads/multi-create-ns-info-thr.exp: New file.
+
+2015-02-20  Pedro Alves  <palves@redhat.com>
+
 	* gdb.trace/no-attach-trace.exp: Don't run to main.  Do
 	clean_restart before gdb_target_supports_trace.
 
diff --git a/gdb/testsuite/gdb.threads/multi-create-ns-info-thr.exp b/gdb/testsuite/gdb.threads/multi-create-ns-info-thr.exp
new file mode 100644
index 0000000..906532e
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/multi-create-ns-info-thr.exp
@@ -0,0 +1,50 @@
+# Copyright (C) 2007-2015 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 3 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, see <http://www.gnu.org/licenses/>.
+
+# Test running "info threads" while threads are being created and
+# exiting, in non-stop mode.  Originally based on multi-create.exp.
+
+standard_testfile multi-create.c
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug pthreads}] == -1} {
+    return -1
+}
+
+gdb_test_no_output "set pagination off"
+gdb_test_no_output "set non-stop on"
+
+runto_main
+
+# Create a breakpoint that does "info threads" when hit, which will be
+# just while other threads are being created or exiting.
+set bp_location1 [gdb_get_line_number "set breakpoint 1 here"]
+gdb_breakpoint $srcfile:$bp_location1
+gdb_test "commands\ninfo threads\ncontinue&\nend" ".*" "set breakpoint commands"
+
+set test "continue -a&"
+gdb_test_multiple $test $test {
+    -re "$gdb_prompt " {
+	pass $test
+    }
+}
+
+for {set i 0} {$i < 32} {incr i} {
+    set test "continue to breakpoint $i"
+    gdb_test_multiple "" $test {
+	-re "Breakpoint $decimal,.*$srcfile:$bp_location1" {
+	    pass $test
+	}
+    }
+}