[Linux] Optimize PID -> struct lwp_info lookup

[Linux] Optimize PID -> struct lwp_info lookup

Hacking the gdb.threads/attach-many-short-lived-threads.exp test to
spawn thousands of threads instead of dozens, and running gdb under
perf, I saw that GDB was spending most of the time in find_lwp_pid:

   - captured_main
      - 93.61% catch_command_errors
         - 87.41% attach_command
            - 87.40% linux_nat_attach
               - 87.40% linux_proc_attach_tgid_threads
                  - 82.38% attach_proc_task_lwp_callback
                     - 81.01% find_lwp_pid
                          5.30% ptid_get_lwp
                        + 0.10% ptid_lwp_p
                     + 0.64% add_thread
                     + 0.26% set_running
                     + 0.24% set_executing
                       0.12% ptid_get_lwp
                     + 0.01% ptrace
                     + 0.01% add_lwp

attach_proc_task_lwp_callback is called once for each LWP that we
attach to, found by listing the /proc/PID/task/ directory.  In turn,
attach_proc_task_lwp_callback calls find_lwp_pid to check whether the
LWP we're about to try to attach to is already known.  Since
find_lwp_pid does a linear walk over the whole LWP list, this becomes
quadratic.  We do the /proc/PID/task/ listing until we get two
iterations in a row where we found no new threads.  So the second and
following times we walk the /proc/PID/task/ dir, we're going to take
an even worse find_lwp_pid hit.

Fix this by adding a hash table keyed by LWP PID, for fast lookup.

The linked list embedded in the LWP structure itself is kept, and made
a double-linked list, so that removals from that list are O(1).  An
earlier version of this patch got rid of this list altogether, but
that revealed hidden dependencies / assumptions on how the list is
sorted.  For example, killing a process and then waiting for all the
LWPs status using iterate_over_lwps only works as is because the
leader LWP is always last in the list.  So I thought it better to take
an incremental approach and make this patch concern itself _only_ with
the PID lookup optimization.

gdb/ChangeLog:
2016-05-24  Pedro Alves  <palves@redhat.com>

	PR gdb/19828
	* linux-nat.c (lwp_lwpid_htab): New htab.
	(lwp_info_hash, lwp_lwpid_htab_eq, lwp_lwpid_htab_create)
	(lwp_lwpid_htab_add_lwp): New functions.
	(lwp_list): Tweak comment.
	(lwp_list_add, lwp_list_remove, lwp_lwpid_htab_remove_pid): New
	functions.
	(purge_lwp_list): Rewrite, using htab_traverse_noresize.
	(add_initial_lwp): Add lwp to htab too.  Use lwp_list_add.
	(delete_lwp): Use lwp_list_remove.  Remove htab too.
	(find_lwp_pid): Search in htab.
	(_initialize_linux_nat): Call lwp_lwpid_htab_create.
	* linux-nat.h (struct lwp_info) <prev>: New field.