/proc visibility patch breaks GDB, etc.

[Peter Chubb]

In fs/proc/base.c:proc_pid_lookup(), the patch

        read_unlock(&tasklist_lock); 
        if (!task) 
                goto out; 
+       if (!thread_group_leader(task)) 
+               goto out_drop_task; 
  
        inode = proc_pid_make_inode(dir->i_sb, task, PROC_TGID_INO); 

means that threads other than the thread group leader don't appear in
the /proc top-level directory.  Programs that are informed via pid of
events can no longer find the appropriate process -- for example,
using gdb on a multi-threaded process, or profiling using perfmon.

The immediate symptom is GDB saying:
    Could not open /proc/757/status
when 757 is a TID not a PID.

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