Fix ordering of pthread_create(pthread_t *thread)

This change fixes a bug where signal_latency_async_test would flake less
than 1/1000 of the time. What was happening was pthread_kill(sender_thr)
would return EFAULT. This was because pthread_create() was not returning
the thread object pointer until after clone() had been called. So it was
actually possible for the main thread to stall after calling clone() and
during that time the receiver would launch and receive a signal from the
sender thread, and then fail when it tried to send a pong. I thought I'd
use a barrier at first, in the test, to synchronize thread creation, but
I firmly believe that pthread_create() was to blame and now that's fixed

Author: jart

libc/intrin/stack.c

@@ -492,7 +492,7 @@ relegated bool TellOpenbsdThisIsStackMemory(void *addr, size_t size) {
 
 // OpenBSD only permits RSP to occupy memory that's been explicitly
 // defined as stack memory, i.e. `lo <= %rsp < hi` must be the case
-relegated errno_t FixupCustomStackOnOpenbsd(pthread_attr_t *attr) {
+relegated bool FixupCustomStackOnOpenbsd(pthread_attr_t *attr) {
 
   // get interval
   uintptr_t lo = (uintptr_t)attr->__stackaddr;
@@ -503,15 +503,11 @@ relegated errno_t FixupCustomStackOnOpenbsd(pthread_attr_t *attr) {
   hi = hi & -__pagesize;
 
   // tell os it's stack memory
-  errno_t olderr = errno;
-  if (!TellOpenbsdThisIsStackMemory((void *)lo, hi - lo)) {
-    errno_t err = errno;
-    errno = olderr;
-    return err;
-  }
+  if (!TellOpenbsdThisIsStackMemory((void *)lo, hi - lo))
+    return false;
 
   // update attributes with usable stack address
   attr->__stackaddr = (void *)lo;
   attr->__stacksize = hi - lo;
-  return 0;
+  return true;
 }

libc/intrin/stack.h

@@ -8,7 +8,7 @@ void cosmo_stack_unlock(void);
 void cosmo_stack_wipe(void);
 
 bool TellOpenbsdThisIsStackMemory(void *, size_t);
-errno_t FixupCustomStackOnOpenbsd(pthread_attr_t *);
+bool FixupCustomStackOnOpenbsd(pthread_attr_t *);
 
 COSMOPOLITAN_C_END_
 #endif /* COSMOPOLITAN_LIBC_STACK_H_ */

libc/thread/posixthread.internal.h

@@ -128,7 +128,7 @@ forceinline pureconst struct PosixThread *_pthread_self(void) {
 }
 
 forceinline void _pthread_ref(struct PosixThread *pt) {
-  atomic_fetch_add_explicit(&pt->pt_refs, 1, memory_order_acq_rel);
+  atomic_fetch_add_explicit(&pt->pt_refs, 1, memory_order_relaxed);
 }
 
 forceinline void _pthread_unref(struct PosixThread *pt) {

libc/thread/pthread_create.c

@@ -199,14 +199,12 @@ static errno_t pthread_create_impl(pthread_t *thread,
                                    const pthread_attr_t *attr,
                                    void *(*start_routine)(void *), void *arg,
                                    sigset_t oldsigs) {
-  int rc, e = errno;
+  errno_t err;
   struct PosixThread *pt;
 
   // create posix thread object
-  if (!(pt = calloc(1, sizeof(struct PosixThread)))) {
-    errno = e;
+  if (!(pt = calloc(1, sizeof(struct PosixThread))))
     return EAGAIN;
-  }
   dll_init(&pt->list);
   pt->pt_locale = &__global_locale;
   pt->pt_start = start_routine;
@@ -215,7 +213,6 @@ static errno_t pthread_create_impl(pthread_t *thread,
   // create thread local storage memory
   if (!(pt->pt_tls = _mktls(&pt->tib))) {
     free(pt);
-    errno = e;
     return EAGAIN;
   }
 
@@ -232,9 +229,9 @@ static errno_t pthread_create_impl(pthread_t *thread,
     // caller supplied their own stack
     // assume they know what they're doing as much as possible
     if (IsOpenbsd()) {
-      if ((rc = FixupCustomStackOnOpenbsd(&pt->pt_attr))) {
+      if (!FixupCustomStackOnOpenbsd(&pt->pt_attr)) {
         _pthread_free(pt);
-        return rc;
+        return EPERM;
       }
     }
   } else {
@@ -259,7 +256,7 @@ static errno_t pthread_create_impl(pthread_t *thread,
       if (!(pt->pt_attr.__sigaltstackaddr =
                 malloc(pt->pt_attr.__sigaltstacksize))) {
         _pthread_free(pt);
-        return errno;
+        return EAGAIN;
       }
       pt->pt_flags |= PT_OWNSIGALTSTACK;
     }
@@ -282,35 +279,41 @@ static errno_t pthread_create_impl(pthread_t *thread,
                             memory_order_relaxed);
       break;
     default:
-      _pthread_free(pt);
-      return EINVAL;
+      // pthread_attr_setdetachstate() makes this impossible
+      __builtin_unreachable();
   }
 
+  // if pthread_attr_setdetachstate() was used then it's possible for
+  // the `pt` object to be freed before this clone call has returned!
+  atomic_store_explicit(&pt->pt_refs, 1, memory_order_relaxed);
+
   // add thread to global list
   // we add it to the beginning since zombies go at the end
   _pthread_lock();
   dll_make_first(&_pthread_list, &pt->list);
   _pthread_unlock();
 
-  // if pthread_attr_setdetachstate() was used then it's possible for
-  // the `pt` object to be freed before this clone call has returned!
-  _pthread_ref(pt);
+  // we don't normally do this, but it's important to write the result
+  // memory before spawning the thread, so it's visible to the threads
+  *thread = (pthread_t)pt;
 
   // launch PosixThread(pt) in new thread
-  if ((rc = clone(
+  if ((err = clone(
            PosixThread, pt->pt_attr.__stackaddr, pt->pt_attr.__stacksize,
            CLONE_VM | CLONE_THREAD | CLONE_FS | CLONE_FILES | CLONE_SIGHAND |
                CLONE_SYSVSEM | CLONE_SETTLS | CLONE_PARENT_SETTID |
                CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID,
            pt, &pt->tib->tib_ptid, __adj_tls(pt->tib), &pt->tib->tib_ctid))) {
+    *thread = 0;  // posix doesn't require we do this
     _pthread_lock();
     dll_remove(&_pthread_list, &pt->list);
     _pthread_unlock();
     _pthread_free(pt);
-    return rc;
+    if (err == ENOMEM)
+      err = EAGAIN;
+    return err;
   }
 
-  *thread = (pthread_t)pt;
   return 0;
 }
 
@@ -359,7 +362,7 @@ static const char *DescribeHandle(char buf[12], errno_t err, pthread_t *th) {
  *                 └──────────────┘
  *
  * @param thread is used to output the thread id upon success, which
- *     must be non-null
+ *     must be non-null; upon failure, its value is undefined
  * @param attr points to launch configuration, or may be null
  *     to use sensible defaults; it must be initialized using
  *     pthread_attr_init()
@@ -375,14 +378,18 @@ static const char *DescribeHandle(char buf[12], errno_t err, pthread_t *th) {
 errno_t pthread_create(pthread_t *thread, const pthread_attr_t *attr,
                        void *(*start_routine)(void *), void *arg) {
   errno_t err;
+  errno_t olderr = errno;
   _pthread_decimate(kPosixThreadZombie);
   BLOCK_SIGNALS;
   err = pthread_create_impl(thread, attr, start_routine, arg, _SigMask);
   ALLOW_SIGNALS;
   STRACE("pthread_create([%s], %p, %t, %p) → %s",
          DescribeHandle(alloca(12), err, thread), attr, start_routine, arg,
          DescribeErrno(err));
-  if (!err)
+  if (!err) {
     _pthread_unref(*(struct PosixThread **)thread);
+  } else {
+    errno = olderr;
+  }
   return err;
 }

test/posix/signal_latency_async_test.c

@@ -40,11 +40,10 @@ void receiver_signal_handler(int signo) {
 }
 
 void *sender_func(void *arg) {
-
   for (int i = 0; i < ITERATIONS; i++) {
 
     // Wait a bit sometimes
-    if (rand() % 2 == 1) {
+    if (rand() % 2) {
       volatile unsigned v = 0;
       for (;;)
         if (++v == 4000)
@@ -67,32 +66,25 @@ void *sender_func(void *arg) {
 }
 
 void *receiver_func(void *arg) {
-
-  // Wait for asynchronous signals
-  for (;;) {
+  static int iteration = 0;
+  do {
+    // wait for signal handler to be called
     if (atomic_exchange_explicit(&receiver_got_signal, 0,
                                  memory_order_acq_rel)) {
+
+      // record received time
       struct timespec receive_time;
       clock_gettime(CLOCK_MONOTONIC, &receive_time);
-
       long sec_diff = receive_time.tv_sec - send_time.tv_sec;
       long nsec_diff = receive_time.tv_nsec - send_time.tv_nsec;
       double latency_ns = sec_diff * 1e9 + nsec_diff;
+      latencies[iteration++] = latency_ns;
 
-      static int iteration = 0;
-      if (iteration < ITERATIONS)
-        latencies[iteration++] = latency_ns;
-
-      // Pong sender
+      // pong sender
       if (pthread_kill(sender_thread, SIGUSR2))
         exit(2);
-
-      // Exit if done
-      if (iteration >= ITERATIONS)
-        pthread_exit(0);
     }
-  }
-
+  } while (iteration < ITERATIONS);
   return 0;
 }
 
@@ -108,11 +100,7 @@ int compare(const void *a, const void *b) {
 
 int main() {
 
-  // TODO(jart): fix flakes
-  if (1)
-    return 0;
-
-  // Install signal handlers
+  // install handlers
   struct sigaction sa;
   sa.sa_handler = receiver_signal_handler;
   sa.sa_flags = 0;
@@ -121,27 +109,27 @@ int main() {
   sa.sa_handler = sender_signal_handler;
   sigaction(SIGUSR2, &sa, 0);
 
-  // Create receiver thread first
+  // create receiver thread first
   if (pthread_create(&receiver_thread, 0, receiver_func, 0))
     exit(11);
 
-  // Create sender thread
+  // create sender thread
   if (pthread_create(&sender_thread, 0, sender_func, 0))
     exit(12);
 
-  // Wait for threads to finish
+  // wait for threads to finish
   if (pthread_join(sender_thread, 0))
     exit(13);
   if (pthread_join(receiver_thread, 0))
     exit(14);
 
-  // Compute mean latency
+  // compute mean latency
   double total_latency = 0;
   for (int i = 0; i < ITERATIONS; i++)
     total_latency += latencies[i];
   double mean_latency = total_latency / ITERATIONS;
 
-  // Sort latencies to compute percentiles
+  // sort latencies to compute percentiles
   qsort(latencies, ITERATIONS, sizeof(double), compare);
 
   double p50 = latencies[(int)(0.50 * ITERATIONS)];

tool/scripts/flakes

@@ -6,17 +6,20 @@ import concurrent.futures
 from collections import Counter
 from typing import List, Dict, Tuple
 
-NUM_PARALLEL = int(os.cpu_count() * 1.5)
+NUM_PARALLEL = int(os.cpu_count() * 20)
 
-def find_test_files(root_dir: str) -> List[str]:
+def find_test_files(root: str) -> List[str]:
     """Find all executable files ending with _test recursively."""
     test_files = []
-    for root, _, files in os.walk(root_dir):
-        for file in files:
-            if file.endswith('_test'):
-                file_path = os.path.join(root, file)
-                if os.access(file_path, os.X_OK):
-                    test_files.append(file_path)
+    if os.path.isdir(root):
+        for root, _, files in os.walk(root):
+            for file in files:
+                if file.endswith('_test'):
+                    file_path = os.path.join(root, file)
+                    if os.access(file_path, os.X_OK):
+                        test_files.append(file_path)
+    elif root.endswith('_test'):
+        test_files.append(root)
     return test_files
 
 def run_single_test(test_path: str) -> int: