Comment on gem5's broken GDB on secondary core

Try to assert on all C programs if thread creation failed. C++ already
throws by default.

README.adoc

@@ -2391,6 +2391,8 @@ to switch between two simultaneous live threads with different affinities, it ju
 b main_thread_0
 ....
 
+Note that secondary cores in gem5 are kind of broken however: <<gem5-gdb-step-debug-secondary-cores>>.
+
 Bibliography:
 
 * https://stackoverflow.com/questions/10490756/how-to-use-sched-getaffinity-and-sched-setaffinity-in-linux-from-c/50117787#50117787
@@ -11509,6 +11511,20 @@ TODO:
 
 breaks when `m5` is run on guest, but does not show the source code.
 
+==== gem5 GDB step debug secondary cores
+
+gem5's secondary core GDB setup is a hack and spawns one gdbserver for each core in separate ports, e.g. 7000, 7001, etc.
+
+Partly because of this, it is basically unusable/very hard to use, because you can't attach to a core that is stopped either because it hasn't been initialized, or if you are already currently debugging another core.
+
+This affects both full system and <<gdb-step-debug-multicore-userland,userland>>, and is described in more detail at: https://gem5.atlassian.net/browse/GEM5-626
+
+In LKMC 0a3ce2f41f12024930bcdc74ff646b66dfc46999, we can easily test attaching to another core by passing `--run-id`, e.g. to connect to the second core we can use `--run-id 1`:
+
+....
+./run-gdb --arch aarch64 --emulator gem5 --userland userland/gcc/busy_loop.c --run-id 1
+....
+
 === gem5 checkpoint
 
 Analogous to QEMU's <<snapshot>>, but better since it can be started from inside the guest, so we can easily checkpoint after a specific guest event, e.g. just before `init` is done.

userland/c/atomic.c

@@ -1,6 +1,7 @@
 /* https://cirosantilli.com/linux-kernel-module-cheat#atomic-c */
 
 #if __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_THREADS__)
+#include <assert.h>
 #include <stdatomic.h>
 #include <stdio.h>
 #include <threads.h>
@@ -36,9 +37,9 @@ int main(int argc, char **argv) {
     }
     threads = malloc(sizeof(thrd_t) * nthreads);
     for(size_t i = 0; i < nthreads; ++i)
-        thrd_create(threads + i, my_thread_main, &niters);
+        assert(thrd_create(threads + i, my_thread_main, &niters) == thrd_success);
     for(size_t i = 0; i < nthreads; ++i)
-        thrd_join(threads[i], NULL);
+        assert(thrd_join(threads[i], NULL) == thrd_success);
     free(threads);
     printf("atomic %u\n", acnt);
     printf("non-atomic %u\n", cnt);

userland/linux/sched_getaffinity_threads.c

@@ -43,8 +43,8 @@ int main(void) {
     enum NUM_THREADS {NUM_THREADS = 2};
     pthread_t threads[NUM_THREADS];
     int thread_args[NUM_THREADS];
-    pthread_create(&threads[0], NULL, main_thread_0, (void*)&thread_args[0]);
-    pthread_create(&threads[1], NULL, main_thread_1, (void*)&thread_args[1]);
+    assert(!pthread_create(&threads[0], NULL, main_thread_0, (void*)&thread_args[0]));
+    assert(!pthread_create(&threads[1], NULL, main_thread_1, (void*)&thread_args[1]));
     pthread_join(threads[0], NULL);
     pthread_join(threads[1], NULL);
     return EXIT_SUCCESS;

userland/posix/pthread_mutex.c

@@ -40,9 +40,9 @@ int main(int argc, char **argv) {
 
     /* Action */
     for (i = 0; i < nthreads; ++i)
-        pthread_create(&threads[i], NULL, main_thread, &niters);
+        assert(!pthread_create(&threads[i], NULL, main_thread, &niters));
     for (i = 0; i < nthreads; ++i)
-        pthread_join(threads[i], NULL);
+        assert(!pthread_join(threads[i], NULL));
     assert(global == nthreads * niters);
 
     /* Cleanup. */