userland/arch/aarch64/inline_asm/futex_ldxr_stxr.c

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.
 

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));
+}

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>