AArch64: Racy: Don't set empty set of hardware BPs/WPs on new thread

On some heavily loaded AArch64 boxes, GDB will sometimes hang forever when
the inferior creates a thread.  This hang happens inside the kernel during
the ptrace call to set hardware watchpoints or hardware breakpoints.
Currently, GDB will always set hw wp/bp at the start of each thread even if
there are none set in the process.

This patch works around the issue by avoiding setting hw wp/bp if there
are none set for the process.

On an effected machine, this fix drastically reduces the racy nature of the
gdb.threads test set.  I ran the entire gdb test suite across all processors
for 100 iterations, then ran the results through the racy tests script.
Without the patch, 58 .exp files in gdb.threads were marked as racy.  After
the patch this reduced to the same ~14 tests as the non effected boxes.

Clearly GDB will still be subject to hangs on an effect box if hw wp/bp's are
used prior to creating inferior threads on a heavily loaded system.

To enable this in gdbserver, the sequence in gdbserver add_lwp() is switched
to the same as gdb order as gdb, to ensure the thread is registered before
calling new_thread().  This allows aarch64_linux_new_thread() to read the
ptid.

gdb/ChangeLog:

	* nat/aarch64-linux-hw-point.c
	(aarch64_linux_any_set_debug_regs_state): New function.
	* nat/aarch64-linux-hw-point.h
	(aarch64_linux_any_set_debug_regs_state): New declaration.
	* nat/aarch64-linux.c (aarch64_linux_new_thread): Check if any
	BPs or WPs are set.

gdb/gdbserver/ChangeLog:

	* linux-low.c (add_lwp): Switch ordering.

1 files changed, 2 insertions, 2 deletions

diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index 4d84927..9e1faf3 100644
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -951,11 +951,11 @@ add_lwp (ptid_t ptid)
 
   lwp->waitstatus.kind = TARGET_WAITKIND_IGNORE;
 
+  lwp->thread = add_thread (ptid, lwp);
+
   if (the_low_target.new_thread != NULL)
     the_low_target.new_thread (lwp);
 
-  lwp->thread = add_thread (ptid, lwp);
-
   return lwp;
 }