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libthread_db: Skip attaching to terminated and joined threads

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I wrote a test that attaches to a program that constantly spawns
short-lived threads, which exposed several issues.  This is one of
them.

On GNU/Linux, attaching to a multi-threaded program sometimes prints
out warnings like:

 ...
 [New LWP 20700]
 warning: unable to open /proc file '/proc/-1/status'
 [New LWP 20850]
 [New LWP 21019]
 ...

That happens because when a thread exits, and is joined, glibc does:

nptl/pthread_join.c:
pthread_join ()
{
...
  if (__glibc_likely (result == 0))
    {
      /* We mark the thread as terminated and as joined.  */
      pd->tid = -1;
...
     /* Free the TCB.  */
      __free_tcb (pd);
    }

So if we attach or interrupt the program (which does an implicit "info
threads") at just the right (or rather, wrong) time, we can find and
return threads in the libthread_db/pthreads thread list with kernel
thread ID -1.  I've filed glibc PR nptl/17707 for this.  You'll find
more info there.

This patch handles this as a special case in GDB.

This is actually more than just a cosmetic issue.  lin_lwp_attach_lwp
will think that this -1 is an LWP we're not attached to yet, and after
failing to attach will try to check we were already attached to the
process, using a waitpid call, which in this case ends up being
"waitpid (-1, ...", which obviously results in GDB potentially
discarding an event when it shouldn't...

Tested on x86_64 Fedora 20, native and gdbserver.

gdb/gdbserver/
2015-01-09  Pedro Alves  <palves@redhat.com>

	* thread-db.c (find_new_threads_callback): Ignore thread if the
	kernel thread ID is -1.

gdb/
2015-01-09  Pedro Alves  <palves@redhat.com>

	* linux-nat.c (lin_lwp_attach_lwp): Assert that the lwp id we're
	about to wait for is > 0.
	* linux-thread-db.c (find_new_threads_callback): Ignore thread if
	the kernel thread ID is -1.
This commit is contained in:
Pedro Alves 2014-12-16 16:12:24 +00:00
parent 8784d56326
commit a33e39599c
5 changed files with 35 additions and 0 deletions
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11
gdb/linux-thread-db.c
View file

@ -1610,6 +1610,17 @@ find_new_threads_callback (const td_thrhandle_t *th_p, void *data)
error (_("find_new_threads_callback: cannot get thread info: %s"),
thread_db_err_str (err));
if (ti.ti_lid == -1)
{
/* A thread with kernel thread ID -1 is either a thread that
exited and was joined, or a thread that is being created but
hasn't started yet, and that is reusing the tcb/stack of a
thread that previously exited and was joined. (glibc marks
terminated and joined threads with kernel thread ID -1. See
glibc PR17707. */
return 0;
}
if (ti.ti_tid == 0)
{
/* A thread ID of zero means that this is the main thread, but