cosmopolitan/libc/intrin/pthread_mutex_unlock.c
Justine Tunney 2f48a02b44
Make recursive mutexes faster
Recursive mutexes now go as fast as normal mutexes. The tradeoff is they
are no longer safe to use in signal handlers. However you can still have
signal safe mutexes if you set your mutex to both recursive and pshared.
You can also make functions that use recursive mutexes signal safe using
sigprocmask to ensure recursion doesn't happen due to any signal handler

The impact of this change is that, on Windows, many functions which edit
the file descriptor table rely on recursive mutexes, e.g. open(). If you
develop your app so it uses pread() and pwrite() then your app should go
very fast when performing a heavily multithreaded and contended workload

For example, when scaling to 40+ cores, *NSYNC mutexes can go as much as
1000x faster (in CPU time) than the naive recursive lock implementation.
Now recursive will use *NSYNC under the hood when it's possible to do so
2024-09-10 00:08:59 -07:00

159 lines
6.2 KiB
C

/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
│ vi: set et ft=c ts=2 sts=2 sw=2 fenc=utf-8 :vi │
╞══════════════════════════════════════════════════════════════════════════════╡
│ Copyright 2022 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 "libc/calls/calls.h"
#include "libc/calls/state.internal.h"
#include "libc/dce.h"
#include "libc/errno.h"
#include "libc/intrin/atomic.h"
#include "libc/intrin/strace.h"
#include "libc/intrin/weaken.h"
#include "libc/runtime/internal.h"
#include "libc/thread/lock.h"
#include "libc/thread/thread.h"
#include "third_party/nsync/futex.internal.h"
#include "third_party/nsync/mu.h"
static void pthread_mutex_unlock_spin(atomic_int *word) {
atomic_store_explicit(word, 0, memory_order_release);
}
// see "take 3" algorithm in "futexes are tricky" by ulrich drepper
static void pthread_mutex_unlock_drepper(atomic_int *futex, char pshare) {
int word = atomic_fetch_sub_explicit(futex, 1, memory_order_release);
if (word == 2) {
atomic_store_explicit(futex, 0, memory_order_release);
_weaken(nsync_futex_wake_)(futex, 1, pshare);
}
}
static errno_t pthread_mutex_unlock_recursive(pthread_mutex_t *mutex,
uint64_t word) {
int me = gettid();
for (;;) {
// we allow unlocking an initialized lock that wasn't locked, but we
// don't allow unlocking a lock held by another thread, or unlocking
// recursive locks from a forked child, since it should be re-init'd
if (MUTEX_OWNER(word) && (MUTEX_OWNER(word) != me || mutex->_pid != __pid))
return EPERM;
// check if this is a nested lock with signal safety
if (MUTEX_DEPTH(word)) {
if (atomic_compare_exchange_weak_explicit(
&mutex->_word, &word, MUTEX_DEC_DEPTH(word), memory_order_relaxed,
memory_order_relaxed))
return 0;
continue;
}
// actually unlock the mutex
if (atomic_compare_exchange_weak_explicit(
&mutex->_word, &word, MUTEX_UNLOCK(word), memory_order_release,
memory_order_relaxed))
return 0;
}
}
#if PTHREAD_USE_NSYNC
static errno_t pthread_mutex_unlock_recursive_nsync(pthread_mutex_t *mutex,
uint64_t word) {
int me = gettid();
for (;;) {
// we allow unlocking an initialized lock that wasn't locked, but we
// don't allow unlocking a lock held by another thread, or unlocking
// recursive locks from a forked child, since it should be re-init'd
if (MUTEX_OWNER(word) && (MUTEX_OWNER(word) != me || mutex->_pid != __pid))
return EPERM;
// check if this is a nested lock with signal safety
if (MUTEX_DEPTH(word)) {
if (atomic_compare_exchange_strong_explicit(
&mutex->_word, &word, MUTEX_DEC_DEPTH(word), memory_order_relaxed,
memory_order_relaxed))
return 0;
continue;
}
// actually unlock the mutex
mutex->_word = MUTEX_UNLOCK(word);
_weaken(nsync_mu_unlock)((nsync_mu *)mutex->_nsyncx);
return 0;
}
}
#endif
/**
* Releases mutex.
*
* This function does nothing in vfork() children.
*
* @return 0 on success or error number on failure
* @raises EPERM if in error check mode and not owned by caller
* @vforksafe
*/
errno_t pthread_mutex_unlock(pthread_mutex_t *mutex) {
uint64_t word;
if (__vforked) {
LOCKTRACE("skipping pthread_mutex_lock(%t) due to vfork", mutex);
return 0;
}
LOCKTRACE("pthread_mutex_unlock(%t)", mutex);
// get current state of lock
word = atomic_load_explicit(&mutex->_word, memory_order_relaxed);
#if PTHREAD_USE_NSYNC
// use superior mutexes if possible
if (MUTEX_TYPE(word) == PTHREAD_MUTEX_NORMAL && //
MUTEX_PSHARED(word) == PTHREAD_PROCESS_PRIVATE && //
_weaken(nsync_mu_unlock)) {
// on apple silicon we should just put our faith in ulock
// otherwise *nsync gets struck down by the eye of sauron
if (!IsXnuSilicon()) {
_weaken(nsync_mu_unlock)((nsync_mu *)mutex);
return 0;
}
}
#endif
// implement barebones normal mutexes
if (MUTEX_TYPE(word) == PTHREAD_MUTEX_NORMAL) {
if (_weaken(nsync_futex_wake_)) {
pthread_mutex_unlock_drepper(&mutex->_futex, MUTEX_PSHARED(word));
} else {
pthread_mutex_unlock_spin(&mutex->_futex);
}
return 0;
}
// handle recursive and error checking mutexes
#if PTHREAD_USE_NSYNC
if (_weaken(nsync_mu_unlock) &&
MUTEX_PSHARED(word) == PTHREAD_PROCESS_PRIVATE) {
return pthread_mutex_unlock_recursive_nsync(mutex, word);
} else {
return pthread_mutex_unlock_recursive(mutex, word);
}
#else
return pthread_mutex_unlock_recursive(mutex, word);
#endif
}