From c9d15228ab7839b749d0ee3faf7cfd159dfc185e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ciro=20Santilli=20=E5=85=AD=E5=9B=9B=E4=BA=8B=E4=BB=B6=20?=
 =?UTF-8?q?=E6=B3=95=E8=BD=AE=E5=8A=9F?= <ciro.santilli@gmail.com>
Date: Thu, 25 Jun 2020 04:00:03 +0000
Subject: [PATCH] userland/arch/aarch64/inline_asm/futex_ldxr_stxr.c

---
 README.adoc                                   |  1 +
 .../arch/aarch64/inline_asm/futex_ldxr_stxr.c | 55 +++++++++++++++++++
 userland/linux/futex.c                        |  1 +
 3 files changed, 57 insertions(+)
 create mode 100644 userland/arch/aarch64/inline_asm/futex_ldxr_stxr.c

diff --git a/README.adoc b/README.adoc
index 4cfcbee..6f05f82 100644
--- a/README.adoc
+++ b/README.adoc
@@ -22968,6 +22968,7 @@ Examples:
 
 * link:userland/arch/aarch64/inline_asm/wfe_ldxr_stxr.cpp[]
 * link:userland/arch/aarch64/inline_asm/wfe_ldxr_str.cpp[]
+* link:userland/arch/aarch64/inline_asm/futex_ldxr_stxr.c[]: tests that ldxr and stxr do not interact with futexes. This was leading to problems in <<gem5-syscall-emulation-mode>> at one point: https://gem5.atlassian.net/browse/GEM5-537
 
 SEV is not the only thing that can wake up a WFE, it is only an explicit software way to do it.
 
diff --git a/userland/arch/aarch64/inline_asm/futex_ldxr_stxr.c b/userland/arch/aarch64/inline_asm/futex_ldxr_stxr.c
new file mode 100644
index 0000000..3fb4d66
--- /dev/null
+++ b/userland/arch/aarch64/inline_asm/futex_ldxr_stxr.c
@@ -0,0 +1,55 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#futex-system-call
+ * https://cirosantilli.com/linux-kernel-module-cheat#arm-wfe-global-monitor-events */
+
+#define _GNU_SOURCE
+#include <assert.h>
+#include <pthread.h>
+#include <stdatomic.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <inttypes.h>
+
+#include <lkmc/futex.h>
+
+static int futex1 = 1;
+static int futex2 = 1;
+atomic_int ldxr_done = 0;
+atomic_int stdr_wake_done = 0;
+static uint64_t ldxr_var = 0;
+
+void __attribute__ ((noinline)) busy_loop(
+    unsigned long long max,
+    unsigned long long max2
+) {
+    for (unsigned long long i = 0; i < max2; i++) {
+        for (unsigned long long j = 0; j < max; j++) {
+            __asm__ __volatile__ ("" : "+g" (i), "+g" (j) : :);
+        }
+    }
+}
+
+void* thread_main(void *arg) {
+    (void)arg;
+    __asm__ __volatile__ ("ldxr x0, [%0]" : : "r" (&ldxr_var) : "x0");
+    ldxr_done = 1;
+    lkmc_futex(&futex1, FUTEX_WAIT, futex1, NULL, NULL, 0);
+    lkmc_futex(&futex2, FUTEX_WAIT, futex2, NULL, NULL, 0);
+    return NULL;
+}
+
+int main(void) {
+    pthread_t thread;
+    pthread_create(&thread, NULL, thread_main, NULL);
+    while (!ldxr_done) {}
+    /* Wait for thread1 to sleep on futex1. */
+    busy_loop(1000, 1);
+    /* Wrongly wake up the thread with a SEV. */
+    __asm__ __volatile__ ("mov x0, 1;ldxr x0, [%0]; stxr w1, x0, [%0]" : : "r" (&ldxr_var) : "x0", "x1");
+    /* Wait for thread1 to sleep on futex2. */
+    busy_loop(1000, 1);
+    /* Wrongly wake thread from futex1 again. */
+    /* But it is now sleeping on futex2, so this is wrong. */
+    lkmc_futex(&futex1, FUTEX_WAKE, 1, NULL, NULL, 0);
+    assert(!pthread_join(thread, NULL));
+}
diff --git a/userland/linux/futex.c b/userland/linux/futex.c
index 2e25bec..7a8d44b 100644
--- a/userland/linux/futex.c
+++ b/userland/linux/futex.c
@@ -1,5 +1,6 @@
 /* https://cirosantilli.com/linux-kernel-module-cheat#futex-system-call */
 
+#define _GNU_SOURCE
 #include <assert.h>
 #include <pthread.h>
 #include <stdio.h>