Write more tests for signal handling

There's now a much stronger level of assurance that signaling on Windows
will be atomic, low-latency, low tail latency, and shall never deadlock.

libc/calls/clock_nanosleep-nt.c

@@ -40,7 +40,7 @@ static textwindows int sys_clock_nanosleep_nt_impl(int clock,
       return 0;
     msdelay = timespec_tomillis(timespec_sub(abs, now));
     msdelay = MIN(msdelay, -1u);
-    if (_park_norestart(msdelay, waitmask))
+    if (_park_norestart(msdelay, waitmask) == -1)
       return -1;
   }
 }

libc/calls/park.c

@@ -17,9 +17,17 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/internal.h"
+#include "libc/calls/sig.internal.h"
 #include "libc/calls/struct/sigset.h"
+#include "libc/calls/syscall_support-nt.internal.h"
 #include "libc/intrin/atomic.h"
+#include "libc/intrin/weaken.h"
+#include "libc/nt/enum/wait.h"
+#include "libc/nt/events.h"
+#include "libc/nt/runtime.h"
 #include "libc/nt/synchronization.h"
+#include "libc/sysv/consts/sicode.h"
+#include "libc/sysv/errfuns.h"
 #include "libc/thread/posixthread.internal.h"
 #ifdef __x86_64__
 
@@ -28,17 +36,39 @@
 // raises ECANCELED if this POSIX thread was canceled in masked mode
 textwindows static int _park_thread(uint32_t msdelay, sigset_t waitmask,
                                     bool restartable) {
-  if (__sigcheck(waitmask, restartable) == -1)
-    return -1;
-  int expect = 0;
-  atomic_int futex = 0;
   struct PosixThread *pt = _pthread_self();
+
+  // perform the wait operation
+  intptr_t sigev;
+  if (!(sigev = CreateEvent(0, 0, 0, 0)))
+    return __winerr();
+  pt->pt_event = sigev;
   pt->pt_blkmask = waitmask;
-  atomic_store_explicit(&pt->pt_blocker, &futex, memory_order_release);
-  bool32 ok = WaitOnAddress(&futex, &expect, sizeof(int), msdelay);
+  atomic_store_explicit(&pt->pt_blocker, PT_BLOCKER_EVENT,
+                        memory_order_release);
+  //!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!//
+  int sig = 0;
+  uint32_t ws = 0;
+  if (!_is_canceled() &&
+      !(_weaken(__sig_get) && (sig = _weaken(__sig_get)(waitmask))))
+    ws = WaitForSingleObject(sigev, msdelay);
+  //!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!/!//
   atomic_store_explicit(&pt->pt_blocker, 0, memory_order_release);
-  if (ok && __sigcheck(waitmask, restartable) == -1)
+  CloseHandle(sigev);
+
+  // recursion is now safe
+  if (ws == -1)
+    return __winerr();
+  int handler_was_called = 0;
+  if (sig)
+    handler_was_called = _weaken(__sig_relay)(sig, SI_KERNEL, waitmask);
+  if (_check_cancel())
     return -1;
+  if (handler_was_called & SIG_HANDLED_NO_RESTART)
+    return eintr();
+  if (handler_was_called & SIG_HANDLED_SA_RESTART)
+    if (!restartable)
+      return eintr();
   return 0;
 }
 

libc/calls/pause-nt.c

@@ -17,13 +17,21 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/internal.h"
+#include "libc/calls/struct/sigset.internal.h"
 #include "libc/calls/syscall_support-nt.internal.h"
 #ifdef __x86_64__
 
 textwindows int sys_pause_nt(void) {
   int rc;
+  // we don't strictly need to block signals, but it reduces signal
+  // delivery latency, by preventing other threads from delivering a
+  // signal asynchronously. it takes about ~5us to deliver a signal
+  // using SetEvent() whereas it takes ~30us to use SuspendThread(),
+  // GetThreadContext(), SetThreadContext(), and ResumeThread().
+  BLOCK_SIGNALS;
   while (!(rc = _park_norestart(-1u, 0)))
     donothing;
+  ALLOW_SIGNALS;
   return rc;
 }
 

libc/calls/sig.c

@@ -25,6 +25,7 @@
 #include "libc/calls/struct/siginfo.h"
 #include "libc/calls/struct/sigset.internal.h"
 #include "libc/calls/struct/ucontext.internal.h"
+#include "libc/calls/syscall_support-nt.internal.h"
 #include "libc/calls/ucontext.h"
 #include "libc/dce.h"
 #include "libc/errno.h"
@@ -135,7 +136,24 @@ static textwindows wontreturn void __sig_terminate(int sig) {
   TerminateThisProcess(sig);
 }
 
-static textwindows bool __sig_start(struct PosixThread *pt, int sig,
+textwindows static void __sig_wake(struct PosixThread *pt, int sig) {
+  atomic_int *blocker;
+  blocker = atomic_load_explicit(&pt->pt_blocker, memory_order_acquire);
+  if (!blocker)
+    return;
+  // threads can create semaphores on an as-needed basis
+  if (blocker == PT_BLOCKER_EVENT) {
+    STRACE("%G set %d's event object", sig, _pthread_tid(pt));
+    SetEvent(pt->pt_event);
+    return;
+  }
+  // all other blocking ops that aren't overlap should use futexes
+  // we force restartable futexes to churn by waking w/o releasing
+  STRACE("%G waking %d's futex", sig, _pthread_tid(pt));
+  WakeByAddressSingle(blocker);
+}
+
+textwindows static bool __sig_start(struct PosixThread *pt, int sig,
                                     unsigned *rva, unsigned *flags) {
   *rva = __sighandrvas[sig];
   *flags = __sighandflags[sig];
@@ -149,6 +167,7 @@ static textwindows bool __sig_start(struct PosixThread *pt, int sig,
     STRACE("enqueing %G on %d", sig, _pthread_tid(pt));
     atomic_fetch_or_explicit(&pt->tib->tib_sigpending, 1ull << (sig - 1),
                              memory_order_relaxed);
+    __sig_wake(pt, sig);
     return false;
   }
   if (*rva == (intptr_t)SIG_DFL) {
@@ -158,7 +177,7 @@ static textwindows bool __sig_start(struct PosixThread *pt, int sig,
   return true;
 }
 
-static textwindows sigaction_f __sig_handler(unsigned rva) {
+textwindows static sigaction_f __sig_handler(unsigned rva) {
   atomic_fetch_add_explicit(&__sig.count, 1, memory_order_relaxed);
   return (sigaction_f)(__executable_start + rva);
 }
@@ -228,34 +247,15 @@ textwindows int __sig_relay(int sig, int sic, sigset_t waitmask) {
   return handler_was_called;
 }
 
-// cancels blocking operations being performed by signaled thread
-textwindows void __sig_cancel(struct PosixThread *pt, int sig, unsigned flags) {
-  atomic_int *blocker;
-  blocker = atomic_load_explicit(&pt->pt_blocker, memory_order_acquire);
-  if (!blocker) {
-    STRACE("%G sent to %d asynchronously", sig, _pthread_tid(pt));
-    return;
-  }
-  // threads can create semaphores on an as-needed basis
-  if (blocker == PT_BLOCKER_EVENT) {
-    STRACE("%G set %d's event object", sig, _pthread_tid(pt));
-    SetEvent(pt->pt_event);
-    return;
-  }
-  // all other blocking ops that aren't overlap should use futexes
-  // we force restartable futexes to churn by waking w/o releasing
-  STRACE("%G waking %d's futex", sig, _pthread_tid(pt));
-  WakeByAddressSingle(blocker);
-}
-
 // the user's signal handler callback is wrapped with this trampoline
 static textwindows wontreturn void __sig_tramp(struct SignalFrame *sf) {
   int sig = sf->si.si_signo;
   struct CosmoTib *tib = __get_tls();
   struct PosixThread *pt = (struct PosixThread *)tib->tib_pthread;
+  atomic_store_explicit(&pt->pt_intoff, 0, memory_order_release);
   for (;;) {
 
-    // update the signal mask in preparation for signal handller
+    // update the signal mask in preparation for signal handler
     sigset_t blocksigs = __sighandmask[sig];
     if (!(sf->flags & SA_NODEFER))
       blocksigs |= 1ull << (sig - 1);
@@ -302,12 +302,16 @@ static textwindows int __sig_killer(struct PosixThread *pt, int sig, int sic) {
     return 0;
   }
 
-  // we can't preempt threads that masked sig or are blocked
-  if (atomic_load_explicit(&pt->tib->tib_sigmask, memory_order_acquire) &
-      (1ull << (sig - 1))) {
+  // we can't preempt threads that masked sig or are blocked. we aso
+  // need to ensure we don't the target thread's stack if many signals
+  // need to be delivered at once. we also need to make sure two threads
+  // can't deadlock by killing each other at the same time.
+  if ((atomic_load_explicit(&pt->tib->tib_sigmask, memory_order_acquire) &
+       (1ull << (sig - 1))) ||
+      atomic_exchange_explicit(&pt->pt_intoff, 1, memory_order_acquire)) {
     atomic_fetch_or_explicit(&pt->tib->tib_sigpending, 1ull << (sig - 1),
                              memory_order_relaxed);
-    __sig_cancel(pt, sig, flags);
+    __sig_wake(pt, sig);
     return 0;
   }
 
@@ -321,17 +325,16 @@ static textwindows int __sig_killer(struct PosixThread *pt, int sig, int sic) {
   uintptr_t th = _pthread_syshand(pt);
   if (atomic_load_explicit(&pt->tib->tib_sigpending, memory_order_acquire) &
       (1ull << (sig - 1))) {
+    atomic_store_explicit(&pt->pt_intoff, 0, memory_order_release);
     return 0;
   }
 
   // take control of thread
   // suspending the thread happens asynchronously
   // however getting the context blocks until it's frozen
-  static pthread_spinlock_t killer_lock;
-  pthread_spin_lock(&killer_lock);
   if (SuspendThread(th) == -1u) {
     STRACE("SuspendThread failed w/ %d", GetLastError());
-    pthread_spin_unlock(&killer_lock);
+    atomic_store_explicit(&pt->pt_intoff, 0, memory_order_release);
     return ESRCH;
   }
   struct NtContext nc;
@@ -339,10 +342,9 @@ static textwindows int __sig_killer(struct PosixThread *pt, int sig, int sic) {
   if (!GetThreadContext(th, &nc)) {
     STRACE("GetThreadContext failed w/ %d", GetLastError());
     ResumeThread(th);
-    pthread_spin_unlock(&killer_lock);
+    atomic_store_explicit(&pt->pt_intoff, 0, memory_order_release);
     return ESRCH;
   }
-  pthread_spin_unlock(&killer_lock);
 
   // we can't preempt threads that masked sig or are blocked
   // we can't preempt threads that are running in win32 code
@@ -354,7 +356,8 @@ static textwindows int __sig_killer(struct PosixThread *pt, int sig, int sic) {
     atomic_fetch_or_explicit(&pt->tib->tib_sigpending, 1ull << (sig - 1),
                              memory_order_relaxed);
     ResumeThread(th);
-    __sig_cancel(pt, sig, flags);
+    atomic_store_explicit(&pt->pt_intoff, 0, memory_order_release);
+    __sig_wake(pt, sig);
     return 0;
   }
 
@@ -387,10 +390,11 @@ static textwindows int __sig_killer(struct PosixThread *pt, int sig, int sic) {
   nc.Rsp = sp;
   if (!SetThreadContext(th, &nc)) {
     STRACE("SetThreadContext failed w/ %d", GetLastError());
+    atomic_store_explicit(&pt->pt_intoff, 0, memory_order_release);
     return ESRCH;
   }
   ResumeThread(th);
-  __sig_cancel(pt, sig, flags);
+  __sig_wake(pt, sig);
   return 0;
 }
 
@@ -404,6 +408,7 @@ textwindows int __sig_kill(struct PosixThread *pt, int sig, int sic) {
 }
 
 // sends signal to any other thread
+// this should only be called by non-posix threads
 textwindows void __sig_generate(int sig, int sic) {
   struct Dll *e;
   struct PosixThread *pt, *mark = 0;
@@ -450,6 +455,7 @@ textwindows void __sig_generate(int sig, int sic) {
   }
   _pthread_unlock();
   if (mark) {
+    // no lock needed since current thread is nameless and formless
     __sig_killer(mark, sig, sic);
     _pthread_unref(mark);
   } else {
@@ -610,13 +616,11 @@ __msabi textwindows dontinstrument bool32 __sig_console(uint32_t dwCtrlType) {
 // didn't have the SA_RESTART flag, and `2`, which means SA_RESTART
 // handlers were called (or `3` if both were the case).
 textwindows int __sig_check(void) {
-  int sig;
-  if ((sig = __sig_get(atomic_load_explicit(&__get_tls()->tib_sigmask,
-                                            memory_order_acquire)))) {
-    return __sig_raise(sig, SI_KERNEL);
-  } else {
-    return 0;
-  }
+  int sig, res = 0;
+  while ((sig = __sig_get(atomic_load_explicit(&__get_tls()->tib_sigmask,
+                                               memory_order_acquire))))
+    res |= __sig_raise(sig, SI_KERNEL);
+  return res;
 }
 
 // background thread for delivering inter-process signals asynchronously
@@ -642,7 +646,7 @@ textwindows dontinstrument static uint32_t __sig_worker(void *arg) {
       sigs &= ~(1ull << (sig - 1));
       __sig_generate(sig, SI_KERNEL);
     }
-    Sleep(1);
+    Sleep(POLL_INTERVAL_MS);
   }
   return 0;
 }

libc/calls/sigcheck.c

@@ -23,18 +23,18 @@
 #include "libc/sysv/errfuns.h"
 
 textwindows int __sigcheck(sigset_t waitmask, bool restartable) {
-  int sig, handler_was_called;
+  int sig, handler_was_called = 0;
   if (_check_cancel() == -1)
     return -1;
-  if (_weaken(__sig_get) && (sig = _weaken(__sig_get)(waitmask))) {
-    handler_was_called = _weaken(__sig_relay)(sig, SI_KERNEL, waitmask);
+  while (_weaken(__sig_get) && (sig = _weaken(__sig_get)(waitmask))) {
+    handler_was_called |= _weaken(__sig_relay)(sig, SI_KERNEL, waitmask);
     if (_check_cancel() == -1)
       return -1;
-    if (handler_was_called & SIG_HANDLED_NO_RESTART)
-      return eintr();
-    if (handler_was_called & SIG_HANDLED_SA_RESTART)
-      if (!restartable)
-        return eintr();
   }
+  if (handler_was_called & SIG_HANDLED_NO_RESTART)
+    return eintr();
+  if (handler_was_called & SIG_HANDLED_SA_RESTART)
+    if (!restartable)
+      return eintr();
   return 0;
 }

libc/calls/sigsuspend.c

@@ -53,8 +53,15 @@ int sigsuspend(const sigset_t *ignore) {
   } else {
     sigset_t waitmask = ignore ? *ignore : 0;
     if (IsWindows() || IsMetal()) {
+      // we don't strictly need to block signals, but it reduces signal
+      // delivery latency, by preventing other threads from delivering a
+      // signal asynchronously. it takes about ~5us to deliver a signal
+      // using SetEvent() whereas it takes ~30us to use SuspendThread(),
+      // GetThreadContext(), SetThreadContext(), and ResumeThread().
+      BLOCK_SIGNALS;
       while (!(rc = _park_norestart(-1u, waitmask)))
         donothing;
+      ALLOW_SIGNALS;
     } else {
       rc = sys_sigsuspend((uint64_t[2]){waitmask}, 8);
     }

libc/intrin/itoa16.c

@@ -18,7 +18,8 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/fmt/internal.h"
 
-__msabi textwindows char16_t *__itoa16(char16_t p[21], uint64_t x) {
+__msabi textwindows dontinstrument char16_t *__itoa16(char16_t p[21],
+                                                      uint64_t x) {
   char t;
   size_t a, b, i = 0;
   do {

libc/intrin/sig.c

@@ -44,9 +44,8 @@ void __sig_unblock(sigset_t m) {
   if (IsWindows() || IsMetal()) {
     if (__tls_enabled) {
       atomic_store_explicit(&__get_tls()->tib_sigmask, m, memory_order_release);
-      if (_weaken(__sig_check)) {
+      if (_weaken(__sig_check))
         _weaken(__sig_check)();
-      }
     }
   } else {
     sys_sigprocmask(SIG_SETMASK, &m, 0);

libc/intrin/sigproc.c

@@ -29,6 +29,7 @@
 #include "libc/nt/enum/pageflags.h"
 #include "libc/nt/files.h"
 #include "libc/nt/memory.h"
+#include "libc/nt/process.h"
 #include "libc/nt/runtime.h"
 #include "libc/nt/thunk/msabi.h"
 #ifdef __x86_64__
@@ -42,11 +43,16 @@ __msabi extern typeof(CreateFileMapping) *const __imp_CreateFileMappingW;
 __msabi extern typeof(MapViewOfFileEx) *const __imp_MapViewOfFileEx;
 __msabi extern typeof(SetEndOfFile) *const __imp_SetEndOfFile;
 __msabi extern typeof(SetFilePointer) *const __imp_SetFilePointer;
+__msabi extern typeof(GetEnvironmentVariable)
+    *const __imp_GetEnvironmentVariableW;
 
-__msabi textwindows char16_t *__sig_process_path(char16_t *path, uint32_t pid,
-                                                 int create_directories) {
+// Generates C:\ProgramData\cosmo\sig\x\y.pid like path
+__msabi textwindows dontinstrument char16_t *__sig_process_path(
+    char16_t *path, uint32_t pid, int create_directories) {
+  char16_t buf[3];
   char16_t *p = path;
-  *p++ = 'C';  // C:\ProgramData\cosmo\sig\x\y.pid
+  uint32_t vlen = __imp_GetEnvironmentVariableW(u"SYSTEMDRIVE", buf, 3);
+  *p++ = vlen == 2 ? buf[0] : 'C';
   *p++ = ':';
   *p++ = '\\';
   *p++ = 'P';

libc/intrin/sigprocmask-nt.c

@@ -47,6 +47,9 @@ textwindows int __sig_mask(int how, const sigset_t *neu, sigset_t *old) {
     } else {  // SIG_SETMASK
       oldmask = atomic_exchange_explicit(mask, *neu, memory_order_acq_rel);
     }
+    if (_weaken(__sig_check)) {
+      _weaken(__sig_check)();
+    }
   } else {
     oldmask = atomic_load_explicit(mask, memory_order_acquire);
   }
@@ -56,10 +59,6 @@ textwindows int __sig_mask(int how, const sigset_t *neu, sigset_t *old) {
     *old = oldmask;
   }
 
-  if (_weaken(__sig_check)) {
-    _weaken(__sig_check)();
-  }
-
   return 0;
 }
 

libc/log/oncrash_amd64.c

@@ -246,6 +246,7 @@ static relegated void ShowCrashReport(int err, int sig, siginfo_t *si,
   if (g_fds.n)
     kprintf("\n");
   __printfds(g_fds.p, g_fds.n);
+  kprintf("\n");
   if (__argv)
     for (i = 0; i < __argc; ++i)
       kprintf("%s ", __argv[i]);

libc/proc/proc.c

@@ -23,6 +23,7 @@
 #include "libc/calls/struct/rusage.h"
 #include "libc/calls/struct/siginfo.h"
 #include "libc/calls/struct/sigset.internal.h"
+#include "libc/calls/syscall_support-nt.internal.h"
 #include "libc/cosmo.h"
 #include "libc/errno.h"
 #include "libc/fmt/wintime.internal.h"
@@ -169,7 +170,7 @@ static textwindows dontinstrument uint32_t __proc_worker(void *arg) {
 
     // wait for something to happen
     if (n == 64) {
-      millis = 5;
+      millis = POLL_INTERVAL_MS;
     } else {
       millis = -1u;
       handles[n++] = __proc.onbirth;

libc/proc/wait4-nt.c

@@ -167,7 +167,7 @@ static textwindows int __proc_wait(int pid, int *wstatus, int options,
       }
       __proc_unlock();
       if (wi == 1) {
-        // __sig_cancel() woke our semaphore
+        // __sig_wake() woke our semaphore
         continue;
       } else {
         // neither posix or win32 define i/o error conditions for

libc/thread/posixthread.internal.h

@@ -89,6 +89,7 @@ struct PosixThread {
   locale_t pt_locale;
   jmp_buf pt_exiter;
   pthread_attr_t pt_attr;
+  atomic_bool pt_intoff;
 };
 
 typedef void (*atfork_f)(void);

libc/thread/pthread_kill.c

@@ -43,7 +43,9 @@ errno_t pthread_kill(pthread_t thread, int sig) {
   int err = 0;
   struct PosixThread *pt;
   pt = (struct PosixThread *)thread;
-  if (!(1 <= sig && sig <= 64)) {
+  if (!thread) {
+    err = EFAULT;
+  } else if (!(1 <= sig && sig <= 64)) {
     err = EINVAL;
   } else if (thread == __get_tls()->tib_pthread) {
     err = raise(sig);  // XNU will EDEADLK it otherwise
@@ -62,9 +64,8 @@ errno_t pthread_kill(pthread_t thread, int sig) {
         errno = e;
       }
     }
-    if (err == ESRCH) {
+    if (err == ESRCH)
       err = 0;  // we already reported this
-    }
   }
   STRACE("pthread_kill(%d, %G) → %s", _pthread_tid(pt), sig,
          DescribeErrno(err));

test/posix/signal_fight_test.c

@@ -0,0 +1,105 @@
+// Copyright 2024 Justine Alexandra Roberts Tunney
+//
+// Permission to use, copy, modify, and/or distribute this software for
+// any purpose with or without fee is hereby granted, provided that the
+// above copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
+// WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
+// WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
+// AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
+// DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
+// PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+// TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+#include <pthread.h>
+#include <signal.h>
+#include <stdlib.h>
+
+/**
+ * @fileoverview Tests two threads killing each other won't deadlock.
+ *
+ * Our Windows implementation of signals has surprisingly high
+ * throughput on this test. About 10x more signals get delivered than
+ * any other OS and in the same amount of time. The only exception was
+ * OpenBSD, which delivered a similar number of signals, but it took 10x
+ * longer for the process to execute.
+ */
+
+#define ITERATIONS 10000
+
+int gotsigs[2];
+pthread_t threads[2];
+pthread_t thread_ids[2];
+pthread_barrier_t barrier;
+pthread_barrier_t barrier2;
+
+void sig_handler(int signo) {
+  if (pthread_equal(pthread_self(), threads[0]))
+    ++gotsigs[0];
+  if (pthread_equal(pthread_self(), threads[1]))
+    ++gotsigs[1];
+}
+
+void *thread_func(void *arg) {
+  int idx = *(int *)arg;
+  int other_idx = 1 - idx;
+
+  thread_ids[idx] = pthread_self();
+
+  int s = pthread_barrier_wait(&barrier);
+  if (s != 0 && s != PTHREAD_BARRIER_SERIAL_THREAD)
+    exit(1);
+
+  pthread_t other_thread = thread_ids[other_idx];
+
+  for (int i = 0; i < ITERATIONS; ++i)
+    if (pthread_kill(other_thread, SIGUSR1))
+      exit(2);
+
+  s = pthread_barrier_wait(&barrier2);
+  if (s != 0 && s != PTHREAD_BARRIER_SERIAL_THREAD)
+    exit(1);
+
+  return 0;
+}
+
+int main() {
+  struct sigaction sa;
+  sa.sa_handler = sig_handler;
+  sa.sa_flags = 0;
+  sigemptyset(&sa.sa_mask);
+
+  if (sigaction(SIGUSR1, &sa, 0) == -1)
+    exit(3);
+
+  if (pthread_barrier_init(&barrier, 0, 2))
+    exit(4);
+  if (pthread_barrier_init(&barrier2, 0, 2))
+    exit(4);
+
+  int idx0 = 0, idx1 = 1;
+
+  if (pthread_create(&threads[0], 0, thread_func, &idx0))
+    exit(5);
+  if (pthread_create(&threads[1], 0, thread_func, &idx1))
+    exit(6);
+
+  if (pthread_join(threads[0], 0))
+    exit(7);
+  if (pthread_join(threads[1], 0))
+    exit(8);
+
+  if (pthread_barrier_destroy(&barrier2))
+    exit(9);
+  if (pthread_barrier_destroy(&barrier))
+    exit(9);
+
+  if (!gotsigs[0])
+    exit(10);
+  if (!gotsigs[1])
+    exit(11);
+
+  return 0;
+}

test/posix/signal_latency_test.c

@@ -0,0 +1,170 @@
+// Copyright 2024 Justine Alexandra Roberts Tunney
+//
+// Permission to use, copy, modify, and/or distribute this software for
+// any purpose with or without fee is hereby granted, provided that the
+// above copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
+// WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
+// WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
+// AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
+// DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
+// PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+// TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+#include <assert.h>
+#include <errno.h>
+#include <pthread.h>
+#include <signal.h>
+#include <stdatomic.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+#include <unistd.h>
+#include "libc/thread/posixthread.internal.h"
+
+#define ITERATIONS 10000
+
+pthread_t sender_thread;
+pthread_t receiver_thread;
+struct timespec send_time;
+double latencies[ITERATIONS];
+
+void sender_signal_handler(int signo) {
+  // Empty handler to unblock sigsuspend()
+}
+
+void receiver_signal_handler(int signo) {
+  struct timespec receive_time;
+  if (clock_gettime(CLOCK_MONOTONIC, &receive_time) == -1)
+    exit(1);
+
+  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;
+
+  static int iteration = 0;
+  if (iteration < ITERATIONS)
+    latencies[iteration++] = latency_ns;
+
+  // Send SIGUSR2 back to sender_thread
+  if (pthread_kill(sender_thread, SIGUSR2))
+    exit(2);
+
+  // Exit if we're done.
+  if (iteration >= ITERATIONS)
+    pthread_exit(0);
+}
+
+void *sender_func(void *arg) {
+  // Block SIGUSR2
+  sigset_t block_set;
+  sigemptyset(&block_set);
+  sigaddset(&block_set, SIGUSR2);
+  if (pthread_sigmask(SIG_BLOCK, &block_set, 0))
+    exit(3);
+
+  // Install signal handler for SIGUSR2
+  struct sigaction sa;
+  sa.sa_handler = sender_signal_handler;
+  sa.sa_flags = 0;
+  sigemptyset(&sa.sa_mask);
+  if (sigaction(SIGUSR2, &sa, 0))
+    exit(4);
+
+  for (int i = 0; i < ITERATIONS; i++) {
+    if (clock_gettime(CLOCK_MONOTONIC, &send_time))
+      exit(5);
+
+    // Send SIGUSR1 to receiver_thread
+    if (pthread_kill(receiver_thread, SIGUSR1))
+      exit(6);
+
+    // Unblock SIGUSR2 and wait for it
+    sigset_t wait_set;
+    sigemptyset(&wait_set);
+    if (sigsuspend(&wait_set) && errno != EINTR)
+      exit(7);
+  }
+
+  return 0;
+}
+
+void *receiver_func(void *arg) {
+  // Install signal handler for SIGUSR1
+  struct sigaction sa;
+  sa.sa_handler = receiver_signal_handler;
+  sa.sa_flags = 0;
+  sigemptyset(&sa.sa_mask);
+  if (sigaction(SIGUSR1, &sa, 0))
+    exit(8);
+
+  // Block all signals except SIGUSR1
+  sigset_t block_set;
+  sigfillset(&block_set);
+  sigdelset(&block_set, SIGUSR1);
+  if (pthread_sigmask(SIG_SETMASK, &block_set, 0))
+    exit(9);
+
+  // Wait indefinitely for signals
+  while (1)
+    pause();
+
+  return 0;
+}
+
+int compare(const void *a, const void *b) {
+  const double *x = a, *y = b;
+  if (*x < *y)
+    return -1;
+  else if (*x > *y)
+    return 1;
+  else
+    return 0;
+}
+
+int main() {
+
+  // Block SIGUSR1 and SIGUSR2 in main thread
+  sigset_t block_set;
+  sigemptyset(&block_set);
+  sigaddset(&block_set, SIGUSR1);
+  sigaddset(&block_set, SIGUSR2);
+  if (pthread_sigmask(SIG_BLOCK, &block_set, 0))
+    exit(10);
+
+  // Create receiver thread first
+  if (pthread_create(&receiver_thread, 0, receiver_func, 0))
+    exit(11);
+
+  // Create sender thread
+  if (pthread_create(&sender_thread, 0, sender_func, 0))
+    exit(12);
+
+  // Wait for threads to finish
+  if (pthread_join(sender_thread, 0))
+    exit(13);
+  if (pthread_join(receiver_thread, 0))
+    exit(14);
+
+  // 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
+  qsort(latencies, ITERATIONS, sizeof(double), compare);
+
+  double p50 = latencies[(int)(0.50 * ITERATIONS)];
+  double p90 = latencies[(int)(0.90 * ITERATIONS)];
+  double p95 = latencies[(int)(0.95 * ITERATIONS)];
+  double p99 = latencies[(int)(0.99 * ITERATIONS)];
+
+  printf("Mean latency: %.2f ns\n", mean_latency);
+  printf("50th percentile latency: %.2f ns\n", p50);
+  printf("90th percentile latency: %.2f ns\n", p90);
+  printf("95th percentile latency: %.2f ns\n", p95);
+  printf("99th percentile latency: %.2f ns\n", p99);
+}

third_party/nsync/common.c

@@ -182,7 +182,6 @@ static void free_waiters_populate (void) {
 	int n;
 	if (IsNetbsd ()) {
 		// netbsd needs a real file descriptor per semaphore
-		// tim cook wants us to use his lol central dispatch
 		n = 1;
 	} else {
 		n = __pagesize / sizeof(waiter);

third_party/nsync/futex.c

@@ -185,7 +185,7 @@ static int nsync_futex_wait_win32_ (atomic_int *w, int expect, char pshare,
 		}
 		ok = WaitOnAddress (w, &expect, sizeof(int), nsync_time_64to32u (timespec_tomillis (wait)));
 		if (pt) {
-			/* __sig_cancel wakes our futex without changing `w` after enqueing signals */
+			/* __sig_wake wakes our futex without changing `w` after enqueing signals */
 			atomic_store_explicit (&pt->pt_blocker, 0, memory_order_release);
 			if (ok && atomic_load_explicit (w, memory_order_acquire) == expect && (sig = __sig_get (waitmask))) {
 				__sig_relay (sig, SI_KERNEL, waitmask);