diff --git a/lldb/test/API/commands/register/register/aarch64_sve_registers/rw_access_dynamic_resize/TestSVEThreadedDynamic.py b/lldb/test/API/commands/register/register/aarch64_sve_registers/rw_access_dynamic_resize/TestSVEThreadedDynamic.py
--- a/lldb/test/API/commands/register/register/aarch64_sve_registers/rw_access_dynamic_resize/TestSVEThreadedDynamic.py
+++ b/lldb/test/API/commands/register/register/aarch64_sve_registers/rw_access_dynamic_resize/TestSVEThreadedDynamic.py
@@ -148,10 +148,6 @@
 
         self.runCmd("process continue", RUN_SUCCEEDED)
 
-        # If we start the checks too quickly, thread 3 may not have started.
-        while process.GetNumThreads() < 3:
-            pass
-
         for idx in range(1, process.GetNumThreads()):
             thread = process.GetThreadAtIndex(idx)
             if thread.GetStopReason() != lldb.eStopReasonBreakpoint:
diff --git a/lldb/test/API/commands/register/register/aarch64_sve_registers/rw_access_dynamic_resize/main.c b/lldb/test/API/commands/register/register/aarch64_sve_registers/rw_access_dynamic_resize/main.c
--- a/lldb/test/API/commands/register/register/aarch64_sve_registers/rw_access_dynamic_resize/main.c
+++ b/lldb/test/API/commands/register/register/aarch64_sve_registers/rw_access_dynamic_resize/main.c
@@ -1,4 +1,5 @@
 #include <pthread.h>
+#include <stdbool.h>
 #include <sys/prctl.h>
 
 #ifndef PR_SME_SET_VL
@@ -62,7 +63,18 @@
 
 int SET_VL_OPT = PR_SVE_SET_VL;
 
+// These ensure that when lldb stops in one of threadX / threadY, the other has
+// at least been created. That means we can continue the other onto the expected
+// breakpoint. Otherwise we could get to the breakpoint in one thread before the
+// other has started.
+volatile bool threadX_ready = false;
+volatile bool threadY_ready = false;
+
 void *threadX_func(void *x_arg) {
+  threadX_ready = true;
+  while (!threadY_ready) {
+  }
+
   prctl(SET_VL_OPT, 8 * 4);
 #ifdef USE_SSVE
   SMSTART();
@@ -73,6 +85,10 @@
 }
 
 void *threadY_func(void *y_arg) {
+  threadY_ready = true;
+  while (!threadX_ready) {
+  }
+
   prctl(SET_VL_OPT, 8 * 2);
 #ifdef USE_SSVE
   SMSTART();