cosmopolitan/third_party/nsync/futex.c

/*-*- mode:c;indent-tabs-mode:t;c-basic-offset:8;tab-width:8;coding:utf-8   -*-│
│vi: set et ft=c ts=8 tw=8 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/struct/timespec.internal.h"
#include "libc/calls/syscall_support-nt.internal.h"
#include "libc/dce.h"
#include "libc/errno.h"
#include "libc/intrin/describeflags.internal.h"
#include "libc/intrin/strace.internal.h"
#include "libc/limits.h"
#include "libc/nt/runtime.h"
#include "libc/nt/synchronization.h"
#include "libc/sysv/consts/futex.h"
#include "libc/thread/thread.h"
#include "third_party/nsync/common.internal.h"
#include "third_party/nsync/futex.internal.h"
#include "third_party/nsync/mu_semaphore.internal.h"
// clang-format off

/* futex() polyfill w/ sched_yield() fallback */

#define FUTEX_WAIT_BITS_ FUTEX_BITSET_MATCH_ANY

int _futex (int *, int, int, const struct timespec *, int *, int);

static int FUTEX_WAIT_;
static int FUTEX_PRIVATE_FLAG_;
static bool FUTEX_IS_SUPPORTED;
bool FUTEX_TIMEOUT_IS_ABSOLUTE;

__attribute__((__constructor__)) static void nsync_futex_init_ (void) {
	int x = 0;

	if (NSYNC_FUTEX_WIN32 && IsWindows ()) {
		FUTEX_IS_SUPPORTED = true;
		FUTEX_WAIT_ = FUTEX_WAIT;
		return;
	}

        if (!(FUTEX_IS_SUPPORTED = IsLinux () || IsOpenbsd ())) {
		// we're using sched_yield() so let's
		// avoid needless clock_gettime calls
		FUTEX_TIMEOUT_IS_ABSOLUTE = true;
		return;
	}

	// In our testing, we found that the monotonic clock on various
	// popular systems (such as Linux, and some BSD variants) was no
	// better behaved than the realtime clock, and routinely took
	// large steps backwards, especially on multiprocessors. Given
	// that "monotonic" doesn't seem to mean what it says,
	// implementers of nsync_time might consider retaining the
	// simplicity of a single epoch within an address space, by
	// configuring any time synchronization mechanism (like ntp) to
	// adjust for leap seconds by adjusting the rate, rather than
	// with a backwards step.
	if (IsLinux () &&
	    _futex (&x, FUTEX_WAIT_BITSET | FUTEX_CLOCK_REALTIME,
		    1, 0, 0, FUTEX_BITSET_MATCH_ANY) == -EAGAIN) {
		FUTEX_WAIT_ = FUTEX_WAIT_BITSET | FUTEX_CLOCK_REALTIME;
		FUTEX_PRIVATE_FLAG_ = FUTEX_PRIVATE_FLAG;
		FUTEX_TIMEOUT_IS_ABSOLUTE = true;
	} else if (IsLinux () && 
		   _futex (&x, FUTEX_WAIT_BITSET, 1, 0, 0,
			   FUTEX_BITSET_MATCH_ANY) == -EAGAIN) {
		FUTEX_WAIT_ = FUTEX_WAIT_BITSET;
		FUTEX_PRIVATE_FLAG_ = FUTEX_PRIVATE_FLAG;
		FUTEX_TIMEOUT_IS_ABSOLUTE = true;
	} else if (IsOpenbsd () ||
		   (IsLinux () && 
		    !_futex (&x, FUTEX_WAKE_PRIVATE, 1, 0, 0, 0))) {
		FUTEX_WAIT_ = FUTEX_WAIT;
		FUTEX_PRIVATE_FLAG_ = FUTEX_PRIVATE_FLAG;
	} else {
		FUTEX_WAIT_ = FUTEX_WAIT;
	}
}

int nsync_futex_wait_ (int *p, int expect, char pshare, struct timespec *timeout) {
	int rc, op;
	uint32_t ms;

	if (!FUTEX_IS_SUPPORTED) {
		nsync_yield_ ();
		if (timeout) {
			return -EINTR;
		} else {
			return 0;
		}
	}

	op = FUTEX_WAIT_;
	if (NSYNC_FUTEX_WIN32 && IsWindows ()) {
		if (timeout) {
			ms = _timespec_tomillis (*timeout);
		} else {
			ms = -1;
		}
		if (WaitOnAddress (p, &expect, sizeof(int), ms)) {
			rc = 0;
		} else {
			rc = -GetLastError ();
		}
	} else {
		if (pshare == PTHREAD_PROCESS_PRIVATE) {
			op |= FUTEX_PRIVATE_FLAG_;
		}
		rc = _futex (p, op, expect, timeout, 0, FUTEX_WAIT_BITS_);
		if (IsOpenbsd() && rc > 0) {
			// [jart] openbsd does this without setting carry flag
			rc = -rc;
		}
	}

	STRACE("futex(%t, %s, %d, %s) → %s",
	       p, DescribeFutexOp (op), expect,
	       DescribeTimespec (0, timeout),
	       DescribeFutexResult (rc));

	return rc;
}

int nsync_futex_wake_ (int *p, int count, char pshare) {
	int rc, op;
	int wake (void *, int, int) asm ("_futex");

	ASSERT (count == 1 || count == INT_MAX);

	if (!FUTEX_IS_SUPPORTED) {
		nsync_yield_ ();
		return 0;
	}

	op = FUTEX_WAKE;
	if (NSYNC_FUTEX_WIN32 && IsWindows ()) {
		if (count == 1) {
			WakeByAddressSingle (p);
		} else {
			WakeByAddressAll (p);
		}
		rc = 0;
	} else {
		if (pshare == PTHREAD_PROCESS_PRIVATE) {
			op |= FUTEX_PRIVATE_FLAG_;
		}
		rc = wake (p, op, count);
	}

	STRACE("futex(%t, %s, %d) → %s", p,
	       DescribeFutexOp(op),
	       count, DescribeFutexResult(rc));

	return rc;
}