diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cpp
--- a/compiler-rt/lib/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cpp
@@ -513,6 +513,8 @@
 
 #elif SANITIZER_RISCV64
 typedef struct user_regs_struct regs_struct;
+// sys/ucontext.h already defines REG_SP as 2. Undefine it first.
+#undef REG_SP
 #define REG_SP sp
 static constexpr uptr kExtraRegs[] = {0};
 #define ARCH_IOVEC_FOR_GETREGSET