diff --git a/compiler-rt/lib/tsan/rtl/tsan_rtl.cpp b/compiler-rt/lib/tsan/rtl/tsan_rtl.cpp
--- a/compiler-rt/lib/tsan/rtl/tsan_rtl.cpp
+++ b/compiler-rt/lib/tsan/rtl/tsan_rtl.cpp
@@ -132,6 +132,10 @@
   ctx->trace_part_finished_excess = 0;
 }
 
+uptr mapped_shadow_begin;
+uptr mapped_shadow_end;
+bool mapped_shadow_captured;
+
 static void DoResetImpl(uptr epoch) {
   ThreadRegistryLock lock0(&ctx->thread_registry);
   Lock lock1(&ctx->slot_mtx);
@@ -197,8 +201,13 @@
   }
 
   DPrintf("Resetting shadow...\n");
-  if (!MmapFixedSuperNoReserve(ShadowBeg(), ShadowEnd() - ShadowBeg(),
-                               "shadow")) {
+  auto shadow_begin = ShadowBeg();
+  auto shadow_end = ShadowEnd();
+  if (mapped_shadow_captured) {
+    shadow_begin = mapped_shadow_begin;
+    shadow_end = mapped_shadow_end;
+  }
+  if (!MmapFixedSuperNoReserve(shadow_begin, shadow_end - shadow_begin, "shadow")) {
     Printf("failed to reset shadow memory\n");
     Die();
   }
@@ -559,15 +568,42 @@
 #endif
 
 void MapShadow(uptr addr, uptr size) {
-  // Global data is not 64K aligned, but there are no adjacent mappings,
-  // so we can get away with unaligned mapping.
-  // CHECK_EQ(addr, addr & ~((64 << 10) - 1));  // windows wants 64K alignment
-  const uptr kPageSize = GetPageSizeCached();
-  uptr shadow_begin = RoundDownTo((uptr)MemToShadow(addr), kPageSize);
-  uptr shadow_end = RoundUpTo((uptr)MemToShadow(addr + size), kPageSize);
-  if (!MmapFixedSuperNoReserve(shadow_begin, shadow_end - shadow_begin,
-                               "shadow"))
-    Die();
+  // Ensure thead registry lock held, so as to synchronize
+  // with DoReset, which also access the mapped_shadow_* vars.
+  ThreadRegistryLock lock0(&ctx->thread_registry);
+
+  /* no longer relevant
+          |
+          v
+   // Global data is not 64K aligned, but there are no adjacent mappings,
+   // so we can get away with unaligned mapping.
+   // CHECK_EQ(addr, addr & ~((64 << 10) - 1));  // windows wants 64K alignment
+          ^
+          |
+   no longer relevant */
+
+  static bool first_complete = false;
+  uptr shadow_begin = RoundDownTo((uptr)MemToShadow(addr), (64 << 10));
+  uptr shadow_end = RoundUpTo((uptr)MemToShadow(addr + size), (64 << 10));
+  VPrintf(2, "=-= mapping shadow for (0x%zx-0x%zx) at (0x%zx-0x%zx)\n",
+          addr, addr + size, shadow_begin, shadow_end);
+  if (!first_complete) {
+    first_complete = true;
+    // First call maps data+bss.
+    if (!MmapFixedSuperNoReserve(shadow_begin, shadow_end - shadow_begin, "shadow"))
+      Die();
+  } else {
+    if (shadow_end <= mapped_shadow_end)
+      return;
+    if (mapped_shadow_begin < shadow_begin)
+      mapped_shadow_begin = shadow_begin;
+    if (shadow_begin < mapped_shadow_end)
+      shadow_begin = mapped_shadow_end;
+    if (!MmapFixedSuperNoReserve(shadow_begin, shadow_end - shadow_begin,
+                                 "shadow"))
+      Die();
+    mapped_shadow_end = shadow_end;
+  }
 
   // Meta shadow is 2:1, so tread carefully.
   static bool data_mapped = false;
@@ -587,8 +623,10 @@
     // Windows wants 64K alignment.
     meta_begin = RoundDownTo(meta_begin, 64 << 10);
     meta_end = RoundUpTo(meta_end, 64 << 10);
-    if (meta_end <= mapped_meta_end)
-      return;
+    if (meta_end <= mapped_meta_end) {
+      Printf("should never happen");
+      Die();
+    }
     if (meta_begin < mapped_meta_end)
       meta_begin = mapped_meta_end;
     if (!MmapFixedSuperNoReserve(meta_begin, meta_end - meta_begin,