From c9f8973de78cfda9e909abe6993975b820396bf9 Mon Sep 17 00:00:00 2001
From: Lemaitre <lemaitre@users.noreply.github.com>
Date: Sun, 10 Oct 2021 12:06:18 +0200
Subject: [PATCH] Exponential back-off

---
 examples/thread.c       |  4 +-
 libc/thread/nativesem.c | 84 +++++++++++++++++++++--------------------
 libc/thread/nativesem.h |  2 +-
 3 files changed, 47 insertions(+), 43 deletions(-)

diff --git a/examples/thread.c b/examples/thread.c
index 06ae57914..d156f89b6 100644
--- a/examples/thread.c
+++ b/examples/thread.c
@@ -35,7 +35,7 @@ int main() {
   cthread_t thread;
   int rc = cthread_create(&thread, NULL, &worker, NULL);
   if (rc == 0) {
-    cthread_native_sem_wait(&semaphore, 0, 0, NULL);
+    cthread_native_sem_wait(&semaphore, 0, NULL);
     //printf("thread created: %p\n", thread);
     sleep(1);
 #if 1
@@ -45,7 +45,7 @@ int main() {
     sleep(2);
 #endif
     cthread_native_sem_signal(&semaphore);
-    cthread_native_sem_wait(&semaphore, 0, 0, NULL);
+    cthread_native_sem_wait(&semaphore, 0, NULL);
     //printf("thread joined: %p -> %d\n", thread, rc);
   } else {
     printf("ERROR: thread could not be started: %d\n", rc);
diff --git a/libc/thread/nativesem.c b/libc/thread/nativesem.c
index 2e7eccafb..f260e177e 100644
--- a/libc/thread/nativesem.c
+++ b/libc/thread/nativesem.c
@@ -24,6 +24,16 @@
 
 #define CTHREAD_THREAD_VAL_BITS 32
 
+static void pause(int attempt) {
+  if (attempt < 16) {
+    for (int i = 0; i < (1 << attempt); ++i) {
+      asm("pause");
+    }
+  } else {
+    cthread_yield();
+  }
+}
+
 int cthread_native_sem_init(cthread_native_sem_t* sem, int count) {
   sem->linux.count = count;
   return 0;
@@ -54,8 +64,8 @@ int cthread_native_sem_signal(cthread_native_sem_t* sem) {
   return 0;
 }
 
-int cthread_native_sem_wait_slow(cthread_native_sem_t* sem,
-                                 const struct timespec* timeout) {
+int cthread_native_sem_wait_futex(cthread_native_sem_t* sem,
+                                  const struct timespec* timeout) {
   uint64_t count;
 
   // record current thread as waiter
@@ -66,12 +76,15 @@ int cthread_native_sem_wait_slow(cthread_native_sem_t* sem,
 
   for (;;) {
     // try to acquire the semaphore, as well as remove itself from waiters
-    if ((uint32_t)count > 0 &&
-        atomic_compare_exchange_weak(
-            &sem->linux.count, count,
-            count - 1 - ((uint64_t)1 << CTHREAD_THREAD_VAL_BITS)))
-      break;
-
+    while ((uint32_t)count > 0) {
+      // without spin, we could miss a futex wake
+      if (atomic_compare_exchange_weak(
+              &sem->linux.count, count,
+              count - 1 - ((uint64_t)1 << CTHREAD_THREAD_VAL_BITS))) {
+        return 0;
+      }
+    }
+    
     int flags = FUTEX_WAIT;
     register struct timespec* timeout_ asm("r10") = timeout;
 
@@ -88,45 +101,36 @@ int cthread_native_sem_wait_slow(cthread_native_sem_t* sem,
   return 0;
 }
 
-int cthread_native_sem_wait_spin_yield(cthread_native_sem_t* sem,
-                                       uint64_t count, int yield,
-                                       const struct timespec* timeout) {
-  // spin on yield
-  while (yield-- > 0) {
-    if ((count >> CTHREAD_THREAD_VAL_BITS) != 0)
-      break;  // a thread is already waiting in queue
-    if ((uint32_t)count > 0 &&
-        atomic_compare_exchange_weak(&sem->linux.count, count, count - 1))
-      return 0;
-    cthread_yield();
-  }
-
-  return cthread_native_sem_wait_slow(sem, timeout);
-}
-
 int cthread_native_sem_wait_spin(cthread_native_sem_t* sem, uint64_t count,
-                                 int spin, int yield,
-                                 const struct timespec* timeout) {
+                                 int spin, const struct timespec* timeout) {
   // spin on pause
-  while (spin-- > 0) {
-    if ((count >> CTHREAD_THREAD_VAL_BITS) != 0) break;
-    if ((uint32_t)count > 0 &&
-        atomic_compare_exchange_weak(&sem->linux.count, count, count - 1))
-      return 0;
-    asm volatile("pause");
+  for (int attempt = 0; attempt < spin; ++attempt) {
+    //if ((count >> CTHREAD_THREAD_VAL_BITS) != 0) break;
+    while ((uint32_t)count > 0) {
+      // spin is useful if multiple waiters can acquire the semaphore at the same time
+      if (atomic_compare_exchange_weak(
+              &sem->linux.count, count, count - 1)) {
+        return 0;
+      }
+    }
+    pause(attempt);
   }
-
-  return cthread_native_sem_wait_spin_yield(sem, count, yield, timeout);
+  
+  return cthread_native_sem_wait_futex(sem, timeout);
 }
 
-int cthread_native_sem_wait(cthread_native_sem_t* sem, int spin, int yield,
+int cthread_native_sem_wait(cthread_native_sem_t* sem, int spin,
                             const struct timespec* timeout) {
   uint64_t count = atomic_load(&sem->linux.count);
 
   // uncontended
-  if ((count >> 32) == 0 && (uint32_t)count > 0 &&
-      atomic_compare_exchange_weak(&sem->linux.count, count, count - 1))
-    return 0;
-
-  return cthread_native_sem_wait_spin(sem, count, spin, yield, timeout);
+  while ((uint32_t)count > 0) {
+    // spin is useful if multiple waiters can acquire the semaphore at the same time
+    if (atomic_compare_exchange_weak(
+            &sem->linux.count, count, count - 1)) {
+      return 0;
+    }
+  }
+  
+  return cthread_native_sem_wait_spin(sem, count, spin, timeout);
 }
diff --git a/libc/thread/nativesem.h b/libc/thread/nativesem.h
index df1b8583c..aab938b2a 100644
--- a/libc/thread/nativesem.h
+++ b/libc/thread/nativesem.h
@@ -18,7 +18,7 @@ struct timespec;
 int cthread_native_sem_init(cthread_native_sem_t*, int);
 int cthread_native_sem_destroy(cthread_native_sem_t*);
 
-int cthread_native_sem_wait(cthread_native_sem_t*, int, int, const struct timespec*);
+int cthread_native_sem_wait(cthread_native_sem_t*, int, const struct timespec*);
 int cthread_native_sem_signal(cthread_native_sem_t*);