[asan] fix one more async-signal-safety issue with use-after-return

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@191004 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/asan/asan_thread.h b/lib/asan/asan_thread.h
index 00a1233..bd92cfe 100644
--- a/lib/asan/asan_thread.h
+++ b/lib/asan/asan_thread.h
@@ -62,7 +62,7 @@
 
   uptr stack_top() { return stack_top_; }
   uptr stack_bottom() { return stack_bottom_; }
-  uptr stack_size() { return stack_top_ - stack_bottom_; }
+  uptr stack_size() { return stack_size_; }
   uptr tls_begin() { return tls_begin_; }
   uptr tls_end() { return tls_end_; }
   u32 tid() { return context_->tid; }
@@ -116,6 +116,9 @@
   void *arg_;
   uptr  stack_top_;
   uptr  stack_bottom_;
+  // stack_size_ == stack_top_ - stack_bottom_;
+  // It needs to be set in a async-signal-safe manner.
+  uptr  stack_size_;
   uptr tls_begin_;
   uptr tls_end_;