This is a logical continuation of PR10757.

Test case:

/// --- cut ---

/* compile with "gcc -g -pthreads manythreads.c -o manythreads" */
#include <assert.h>
#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>

void *foo (void)
{
  // set breakpoint here
  usleep(1);
  return NULL;
}

void *fn (void *p)
{
  pthread_t tid = (pthread_t) p;
  int rc;

  if (p != NULL)
    assert (pthread_join (tid, NULL) == 0);

  while ((rc = pthread_create (&tid, NULL, fn, (void*) pthread_self ())) != 0)
    fprintf (stderr, "unexpected error from pthread_create: %d\n", rc);
  return foo();
}

int main (int argc, char *argv[])
{
  int i, n_threads = 100;

  if (argc > 1)
    n_threads = atoi (argv[1]);

  for (i = 0; i < n_threads; ++i)
    {
      pthread_t tid;

      while (pthread_create (&tid, NULL, fn, NULL) != 0)
        sleep (1);
    }
  sleep (300);
  exit (0);
}

/// --- cut ---

./manythreads & sleep 1 && ./gdb -nx ./manythreads $!
[1] 12880
GNU gdb (GDB) 7.0.50.20091031-cvs
Copyright (C) 2009 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i386-pc-solaris2.10".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /export/home/paul/tmp/manythreads...done.
Attaching to program `/export/home/paul/tmp/manythreads', process 12880
[New process 12880]
Retry #1:
Retry #2:
Retry #3:
Retry #4:
Reading symbols from /lib/libpthread.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/libpthread.so.1
Reading symbols from /lib/libc.so.1...(no debugging symbols found)...done.
[Thread debugging using libthread_db enabled]
[New LWP    7656        ]
[New LWP    7614        ]
...
[New Thread 7459        ]
[New Thread 7458        ]
[New Thread 7457        ]
Loaded symbols for /lib/libc.so.1
Reading symbols from /lib/ld.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/ld.so.1
[Switching to Thread 1 (LWP 1)]
0xfef19735 in ___nanosleep () from /lib/libc.so.1
(gdb) b foo
procfs: fetch_registers, get_gregs line 3800, /proc/12880/lwp/7457: No such file
or directory.

Another failure I've seen:

(gdb) b foo
Breakpoint 1 at 0x8050ab2: file manythreads.c, line 10.
(gdb) commands 1
Type commands for when breakpoint 1 is hit, one per line.
End with a line saying just "end".
>silent
>c
>end
(gdb) c
Continuing.
[LWP    2236         exited]
[New LWP    2247        ]
[LWP    2235         exited]
[LWP    2246         exited]
sol_thread_fetch_registers: td_ta_map_id2thr: no thread can be found to satisfy
query