Implement critical section (Lamport's Baker s algorithm)...

thanks @fincs for sample code

exosphere/src/lock.h

@@ -1,26 +0,0 @@
-#ifndef EXOSPHERE_LOCK_H
-#define EXOSPHERE_LOCK_H
-
-#include <stdatomic.h>
-#include <stdbool.h>
-
-/* Simple atomics driver for Exosphere. */
-
-/* Acquire a lock. */
-static inline void lock_acquire(atomic_flag *flag) {
-    while (atomic_flag_test_and_set_explicit(flag, memory_order_acquire)) {
-        /* Wait to acquire lock. */
-    }
-}
-
-/* Release a lock. */
-static inline void lock_release(atomic_flag *flag) {
-    atomic_flag_clear_explicit(flag, memory_order_release);
-}
-
-/* Try to acquire a lock. */
-static inline bool lock_try_acquire(atomic_flag *flag) {
-    return atomic_flag_test_and_set_explicit(flag, memory_order_acquire);
-}
-
-#endif

exosphere/src/smc_api.c

@@ -6,7 +6,7 @@
 
 #include "configitem.h"
 #include "cpu_context.h"
-#include "lock.h"
+#include "synchronization.h"
 #include "masterkey.h"
 #include "mc.h"
 #include "memory_map.h"

exosphere/src/synchronization.h

@@ -0,0 +1,115 @@
+#ifndef EXOSPHERE_SYNCHRONIZATION_H
+#define EXOSPHERE_SYNCHRONIZATION_H
+
+#include <stdatomic.h>
+#include "utils.h"
+
+/* Simple atomics driver for Exosphere. */
+
+typedef struct {
+    struct {
+        uint8_t ticket_number   : 7;
+        uint8_t is_entering     : 1;
+    } customers[4];
+} critical_section_t;
+
+static inline void __dsb_sy(void) {
+    __asm__ __volatile__ ("dsb sy" ::: "memory");
+}
+
+static inline void __dmb_sy(void) {
+    __asm__ __volatile__ ("dmb sy" ::: "memory");
+}
+
+static inline void __sev(void) {
+    __asm__ __volatile__ ("sev");
+}
+
+static inline void __sevl(void) {
+    __asm__ __volatile__ ("sevl");
+}
+
+static inline void __wfe(void) {
+    __asm__ __volatile__ ("wfe");
+}
+
+/* Acquire a lock. */
+static inline void lock_acquire(atomic_flag *flag) {
+    while (atomic_flag_test_and_set_explicit(flag, memory_order_acquire)) {
+        /* Wait to acquire lock. */
+    }
+}
+
+/* Release a lock. */
+static inline void lock_release(atomic_flag *flag) {
+    atomic_flag_clear_explicit(flag, memory_order_release);
+}
+
+/* Try to acquire a lock. */
+static inline bool lock_try_acquire(atomic_flag *flag) {
+    return atomic_flag_test_and_set_explicit(flag, memory_order_acquire);
+}
+
+/*
+    Enter a critical section, using the Lamport's bakery algorithm.
+    https://en.wikipedia.org/wiki/Lamport%27s_bakery_algorithm
+
+    This is invoked on warmboot before the MMU is turned on, therefore
+    exclusive load/store instructions can't be used.
+
+    Note: Nintendo has tried to implement that algorithm, but it seems that
+    they didn't understand how it works, and their implementation
+    is therefore a complete failure: in particular, the "ticket number" is
+    always the same (core0 will always enter the critical section before all the cores,
+    and so on...), and thus there can be starvation, etc.
+
+    Nintendo, wtf.
+*/
+ALINLINE static inline unsigned int critical_section_enter(volatile critical_section_t *critical_section) {
+    unsigned int id = get_core_id();
+    uint8_t my_ticket_number = 0;
+    uint8_t tmp;
+
+    critical_section->customers[id].is_entering = 1;
+    for (unsigned int i = 0; i < 4; i++) {
+        tmp = critical_section->customers[id].ticket_number;
+        my_ticket_number = tmp > my_ticket_number ? tmp : my_ticket_number;
+    }
+
+    critical_section->customers[id].ticket_number = ++my_ticket_number;
+    critical_section->customers[id].is_entering = 0;
+    __dsb_sy();
+    __sev();
+
+    for (unsigned int i = 0; i < 4; i++) {
+        __sevl();
+        do {
+            __wfe();
+        } while (critical_section->customers[i].is_entering);
+ 
+        __sevl();
+        do {
+            __wfe();
+            tmp = critical_section->customers[i].ticket_number;
+        } while (tmp && (tmp < my_ticket_number || (tmp == my_ticket_number && i < id)));
+    }
+
+    __dmb_sy();
+    return id;
+}
+
+/*
+    Leaves a critical section, using the Lamport's bakery algorithm (see above).
+
+    Note: Nintendo failed even that: they're clearing the entire critical section state
+    instead of just the counter associated to the current core.
+
+    Nintendo, wtf.
+*/
+ALINLINE static inline void critical_section_leave(volatile critical_section_t *critical_section) {
+    critical_section->customers[get_core_id()].ticket_number = 0;
+    __dsb_sy();
+    __sev();
+}
+
+#endif

exosphere/src/utils.h

@@ -16,7 +16,6 @@
 #define PACKED          __attribute__((packed))
 
 #define ALINLINE        __attribute__((always_inline))
-#define FAR_REACHING    __attribute__((target("cmodel=large"), noinline))
 
 /* Custom stuff below */