[libcpu/arm64] add C11 atomic ticket spinlock (#8882)

* [libcpu/arm64] add C11 atomic ticket spinlock

Replace the former implementation of flag-based spinlock which is unfair

Besides, C11 atomic implementation is more readable (it's C anyway),
and maintainable. Cause toolchain can use their builtin optimization and
tune for different micro-architectures. For example armv8.5 introduces a
better instruction. The compiler can help with that when it knows your
target platform in support of it.

Signed-off-by: Shell <smokewood@qq.com>

* fixup: RT_CPUS_NR

---------

Signed-off-by: Shell <smokewood@qq.com>

examples/utest/testcases/kernel/sched_sem_tc.c

@@ -29,7 +29,7 @@
 #error the thread priority should at least be greater than idle
 #endif
 
-static rt_atomic_t _star_counter = 1;
+static rt_atomic_t _star_counter;
 static struct rt_semaphore _thr_exit_sem;
 static struct rt_semaphore _level_waiting[TEST_LEVEL_COUNTS];
 static rt_thread_t _thread_matrix[TEST_LEVEL_COUNTS][KERN_TEST_CONCURRENT_THREADS];
@@ -157,6 +157,8 @@ static void scheduler_tc(void)
 static rt_err_t utest_tc_init(void)
 {
     LOG_I("Setup environment...");
+    _star_counter = 1;
+    rt_memset(_load_average, 0, sizeof(_load_average));
     rt_sem_init(&_thr_exit_sem, "test", 0, RT_IPC_FLAG_PRIO);
 
     for (size_t i = 0; i < TEST_LEVEL_COUNTS; i++)

libcpu/Kconfig

@@ -12,6 +12,9 @@ if ARCH_ARMV8 && ARCH_CPU_64BIT
     config ARCH_HAVE_EFFICIENT_UNALIGNED_ACCESS
         bool
         default y
+    config ARCH_USING_GENERIC_CPUID
+        bool "Using generic cpuid implemenation"
+        default n
     endmenu
 endif
 

libcpu/aarch64/common/context_gcc.S

@@ -44,7 +44,11 @@ int rt_hw_cpu_id(void)
 .weak rt_hw_cpu_id
 .type rt_hw_cpu_id, @function
 rt_hw_cpu_id:
+#if RT_CPUS_NR > 1
     mrs x0, tpidr_el1
+#else
+    mov x0, xzr
+#endif
     ret
 
 /*

libcpu/aarch64/common/cpu.c

@@ -8,6 +8,7 @@
  * 2011-09-15     Bernard      first version
  * 2019-07-28     zdzn         add smp support
  * 2023-02-21     GuEe-GUI     mov cpu ofw init to setup
+ * 2024-04-29     Shell        Add generic ticket spinlock using C11 atomic
  */
 
 #include <rthw.h>
@@ -55,65 +56,101 @@ rt_weak rt_uint64_t rt_cpu_mpidr_early[] =
 };
 #endif /* RT_USING_SMART */
 
-static inline void arch_spin_lock(arch_spinlock_t *lock)
+/* in support of C11 atomic */
-{
+#if __STDC_VERSION__ >= 201112L
-    unsigned int tmp;
+#include <stdatomic.h>
 
-    asm volatile(
+union _spinlock
-    "   sevl\n"
+{
-    "1: wfe\n"
+    _Atomic(rt_uint32_t) _value;
-    "2: ldaxr   %w0, %1\n"
+    struct
-    "   cbnz    %w0, 1b\n"
+    {
-    "   stxr    %w0, %w2, %1\n"
+        _Atomic(rt_uint16_t) owner;
-    "   cbnz    %w0, 2b\n"
+        _Atomic(rt_uint16_t) next;
-    : "=&r" (tmp), "+Q" (lock->lock)
+    } ticket;
-    : "r" (1)
+};
-    : "cc", "memory");
+
+void rt_hw_spin_lock_init(rt_hw_spinlock_t *_lock)
+{
+    union _spinlock *lock = (void *)_lock;
+
+    /**
+     * just a dummy note that this is an atomic operation, though it alway is
+     * even without usage of atomic API in arm64
+     */
+    atomic_store_explicit(&lock->_value, 0, memory_order_relaxed);
 }
 
-static inline int arch_spin_trylock(arch_spinlock_t *lock)
+rt_bool_t rt_hw_spin_trylock(rt_hw_spinlock_t *_lock)
 {
-    unsigned int tmp;
+    rt_bool_t rc;
+    rt_uint32_t readonce;
+    union _spinlock temp;
+    union _spinlock *lock = (void *)_lock;
 
-    asm volatile(
+    readonce = atomic_load_explicit(&lock->_value, memory_order_acquire);
-    "  ldaxr   %w0, %1\n"
+    temp._value = readonce;
-    "  cbnz    %w0, 1f\n"
-    "  stxr    %w0, %w2, %1\n"
-    "1:\n"
-    : "=&r" (tmp), "+Q" (lock->lock)
-    : "r" (1)
-    : "cc", "memory");
 
-    return !tmp;
+    if (temp.ticket.owner != temp.ticket.next)
+    {
+        rc = RT_FALSE;
+    }
+    else
+    {
+        temp.ticket.next += 1;
+        rc = atomic_compare_exchange_strong_explicit(
+            &lock->_value, &readonce, temp._value,
+            memory_order_acquire, memory_order_relaxed);
+    }
+    return rc;
 }
 
-static inline void arch_spin_unlock(arch_spinlock_t *lock)
+rt_inline rt_base_t _load_acq_exclusive(_Atomic(rt_uint16_t) *halfword)
 {
-    asm volatile(
+    rt_uint32_t old;
-    " stlr    %w1, %0\n"
+    __asm__ volatile("ldaxrh %w0, [%1]"
-    : "=Q" (lock->lock) : "r" (0) : "memory");
+                     : "=&r"(old)
+                     : "r"(halfword)
+                     :  "memory");
+    return old;
 }
 
-void rt_hw_spin_lock_init(arch_spinlock_t *lock)
+rt_inline void _send_event_local(void)
 {
-    lock->lock = 0;
+    __asm__ volatile("sevl");
 }
 
-void rt_hw_spin_lock(rt_hw_spinlock_t *lock)
+rt_inline void _wait_for_event(void)
 {
-    arch_spin_lock(lock);
+    __asm__ volatile("wfe" ::: "memory");
 }
 
-void rt_hw_spin_unlock(rt_hw_spinlock_t *lock)
+void rt_hw_spin_lock(rt_hw_spinlock_t *_lock)
 {
-    arch_spin_unlock(lock);
+    union _spinlock *lock = (void *)_lock;
+    rt_uint16_t ticket =
+        atomic_fetch_add_explicit(&lock->ticket.next, 1, memory_order_relaxed);
+
+    if (atomic_load_explicit(&lock->ticket.owner, memory_order_acquire) !=
+        ticket)
+    {
+        _send_event_local();
+        do
+        {
+            _wait_for_event();
+        }
+        while (_load_acq_exclusive(&lock->ticket.owner) != ticket);
+    }
 }
 
-rt_bool_t rt_hw_spin_trylock(rt_hw_spinlock_t *lock)
+void rt_hw_spin_unlock(rt_hw_spinlock_t *_lock)
 {
-    return arch_spin_trylock(lock);
+    union _spinlock *lock = (void *)_lock;
+    atomic_fetch_add_explicit(&lock->ticket.owner, 1, memory_order_release);
 }
 
+#endif
+
 static int _cpus_init_data_hardcoded(int num_cpus, rt_uint64_t *cpu_hw_ids, struct cpu_ops_t *cpu_ops[])
 {
     // load in cpu_hw_ids in cpuid_to_hwid,

libcpu/aarch64/common/cpu_gcc.S

@@ -1,10 +1,11 @@
 /*
- * Copyright (c) 2006-2020, RT-Thread Development Team
+ * Copyright (c) 2006-2024, RT-Thread Development Team
  *
  * SPDX-License-Identifier: Apache-2.0
  *
  * Date           Author       Notes
  * 2018-10-06     ZhaoXiaowei    the first version
+ * 2024-04-28     Shell        add generic spinlock implementation
  */
 
 .text
@@ -80,7 +81,7 @@ rt_hw_set_elx_env:
 0:
     RET
 
-.global rt_cpu_vector_set_base
+.globl rt_cpu_vector_set_base
 rt_cpu_vector_set_base:
     MSR        VBAR_EL1,X0
     RET
@@ -89,7 +90,7 @@ rt_cpu_vector_set_base:
 /**
  * unsigned long rt_hw_ffz(unsigned long x)
  */
-.global rt_hw_ffz
+.globl rt_hw_ffz
 rt_hw_ffz:
     mvn     x1, x0
     clz     x0, x1
@@ -97,7 +98,80 @@ rt_hw_ffz:
     sub     x0, x1, x0
     ret
 
-.global rt_hw_clz
+.globl rt_hw_clz
 rt_hw_clz:
     clz     x0, x0
     ret
+
+/**
+ * Spinlock (fallback implementation)
+ */
+
+rt_hw_spin_lock_init:
+    .weak   rt_hw_spin_lock_init
+    stlr    wzr, [x0]
+    ret
+
+rt_hw_spin_trylock:
+    .weak   rt_hw_spin_trylock
+    sub     sp, sp, #16
+    ldar    w2, [x0]
+    add     x1, sp, 8
+    stlr    w2, [x1]
+    ldarh   w1, [x1]
+    and     w1, w1, 65535
+    add     x3, sp, 10
+    ldarh   w3, [x3]
+    cmp     w1, w3, uxth
+    beq     1f
+    mov     w0, 0
+    add     sp, sp, 16
+    ret
+1:
+    add     x1, sp, 10
+2:
+    ldaxrh  w3, [x1]
+    add     w3, w3, 1
+    stlxrh  w4, w3, [x1]
+    cbnz    w4, 2b
+    add     x1, sp, 8
+    ldar    w1, [x1]
+3:
+    ldaxr   w3, [x0]
+    cmp     w3, w2
+    bne     4f
+    stxr    w4, w1, [x0]
+    cbnz    w4, 3b
+4:
+    cset    w0, eq
+    add     sp, sp, 16
+    ret
+
+rt_hw_spin_lock:
+    .weak   rt_hw_spin_lock
+    add     x1, x0, 2
+1:
+    ldxrh   w2, [x1]
+    add     w3, w2, 1
+    stxrh   w4, w3, [x1]
+    cbnz    w4, 1b
+    and     w2, w2, 65535
+    ldarh   w1, [x0]
+    cmp     w2, w1, uxth
+    beq     3f
+    sevl
+2:
+    wfe
+    ldaxrh  w1, [x0]
+    cmp     w2, w1
+    bne     2b
+3:
+    ret
+
+rt_hw_spin_unlock:
+    .weak   rt_hw_spin_unlock
+    ldxrh   w1, [x0]
+    add     w1, w1, 1
+    stlxrh  w2, w1, [x0]
+    cbnz    w2, rt_hw_spin_unlock
+    ret

libcpu/aarch64/common/cpuport.h

@@ -17,10 +17,42 @@
 #include <rtdef.h>
 
 #ifdef RT_USING_SMP
-typedef struct {
+
-    volatile unsigned int lock;
+/**
+ * Spinlock
+ */
+
+typedef struct
+{
+    rt_uint32_t value;
 } rt_hw_spinlock_t;
-#endif
+
+/**
+ * Generic hw-cpu-id
+ */
+#ifdef ARCH_USING_GENERIC_CPUID
+
+#if RT_CPUS_NR > 1
+
+rt_inline int rt_hw_cpu_id(void)
+{
+    long cpuid;
+    __asm__ volatile("mrs %0, tpidr_el1":"=r"(cpuid));
+    return cpuid;
+}
+
+#else
+
+rt_inline int rt_hw_cpu_id(void)
+{
+    return 0;
+}
+
+#endif /* RT_CPUS_NR > 1 */
+
+#endif /* ARCH_USING_GENERIC_CPUID */
+
+#endif /* RT_USING_SMP */
 
 #define rt_hw_barrier(cmd, ...) \
     __asm__ volatile (RT_STRINGIFY(cmd) " "RT_STRINGIFY(__VA_ARGS__):::"memory")

src/cpu_mp.c

@@ -146,7 +146,7 @@ rt_base_t rt_cpus_lock(void)
     pcpu = rt_cpu_self();
     if (pcpu->current_thread != RT_NULL)
     {
-        register rt_ubase_t lock_nest = rt_atomic_load(&(pcpu->current_thread->cpus_lock_nest));
+        rt_ubase_t lock_nest = rt_atomic_load(&(pcpu->current_thread->cpus_lock_nest));
 
         rt_atomic_add(&(pcpu->current_thread->cpus_lock_nest), 1);
         if (lock_nest == 0)

src/scheduler_mp.c

@@ -1089,6 +1089,7 @@ void rt_exit_critical_safe(rt_base_t critical_level)
 
 void rt_exit_critical_safe(rt_base_t critical_level)
 {
+    RT_UNUSED(critical_level);
     return rt_exit_critical();
 }
 

src/thread.c

@@ -363,7 +363,8 @@ rt_thread_t rt_thread_self(void)
     self = rt_cpu_self()->current_thread;
     rt_hw_local_irq_enable(lock);
     return self;
-#else
+
+#else /* !RT_USING_SMP */
     extern rt_thread_t rt_current_thread;
 
     return rt_current_thread;