libuv/src/unix/async.c

/* Copyright Joyent, Inc. and other Node contributors. All rights reserved.
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to
 * deal in the Software without restriction, including without limitation the
 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
 * sell copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 */

#include "uv.h"
#include "internal.h"

#include <errno.h>
#include <assert.h>
#include <stdlib.h>
#include <unistd.h>

static int uv__async_init(uv_loop_t* loop);
static void uv__async_io(uv_loop_t* loop, uv__io_t* handle, int events);


static int uv__async_make_pending(volatile sig_atomic_t* ptr) {
  /* Do a cheap read first. */
  if (*ptr)
    return 1;

  /* Micro-optimization: use atomic memory operations to detect if we've been
   * preempted by another thread and don't have to make an expensive syscall.
   * This speeds up the heavily contended case by about 1-2% and has little
   * if any impact on the non-contended case.
   *
   * Use XCHG instead of the CMPXCHG that __sync_val_compare_and_swap() emits
   * on x86, it's about 4x faster. It probably makes zero difference in the
   * grand scheme of things but I'm OCD enough not to let this one pass.
   */
#if __i386__ || __x86_64__
  {
    unsigned int val = 1;
    __asm__ __volatile__("xchgl %0, %1" : "+r" (val) : "m" (*ptr));
    return val != 0;
  }
#elif __GNUC__ > 4 || __GNUC__ == 4 && __GNUC_MINOR__ >= 1 /* gcc >= 4.1 */
  return __sync_val_compare_and_swap(ptr, 0, 1) != 0;
#else
  *ptr = 1;
  return 0;
#endif
}


int uv_async_init(uv_loop_t* loop, uv_async_t* handle, uv_async_cb async_cb) {
  if (uv__async_init(loop))
    return uv__set_sys_error(loop, errno);

  uv__handle_init(loop, (uv_handle_t*)handle, UV_ASYNC);
  handle->async_cb = async_cb;
  handle->pending = 0;

  ngx_queue_insert_tail(&loop->async_handles, &handle->queue);
  uv__handle_start(handle);

  return 0;
}


int uv_async_send(uv_async_t* handle) {
  int r;

  if (uv__async_make_pending(&handle->pending))
    return 0; /* already pending */

  do
    r = write(handle->loop->async_pipefd[1], "x", 1);
  while (r == -1 && errno == EINTR);

  if (r == -1 && errno != EAGAIN && errno != EWOULDBLOCK)
    return uv__set_sys_error(handle->loop, errno);

  return 0;
}


void uv__async_close(uv_async_t* handle) {
  ngx_queue_remove(&handle->queue);
  uv__handle_stop(handle);
}


static int uv__async_init(uv_loop_t* loop) {
  if (loop->async_pipefd[0] != -1)
    return 0;

  if (uv__make_pipe(loop->async_pipefd, UV__F_NONBLOCK))
    return -1;

  uv__io_init(&loop->async_watcher,
              uv__async_io,
              loop->async_pipefd[0],
              UV__IO_READ);
  uv__io_start(loop, &loop->async_watcher);

  return 0;
}


static void uv__async_io(uv_loop_t* loop, uv__io_t* handle, int events) {
  char buf[1024];
  ngx_queue_t* q;
  uv_async_t* h;
  ssize_t r;

  while (1) {
    r = read(loop->async_pipefd[0], buf, sizeof(buf));

    if (r == sizeof(buf))
      continue;

    if (r != -1)
      break;

    if (errno == EAGAIN || errno == EWOULDBLOCK)
      break;

    if (errno == EINTR)
      continue;

    abort();
  }

  ngx_queue_foreach(q, &loop->async_handles) {
    h = ngx_queue_data(q, uv_async_t, queue);
    if (!h->pending) continue;
    h->pending = 0;
    h->async_cb(h, 0);
  }
}