src/core/lib/iomgr/ev_epoll_linux.c

/*
 *
 * Copyright 2016, Google Inc.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *     * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above
 * copyright notice, this list of conditions and the following disclaimer
 * in the documentation and/or other materials provided with the
 * distribution.
 *     * Neither the name of Google Inc. nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 */

#include <grpc/support/port_platform.h>

#ifdef GPR_POSIX_SOCKET

#include "src/core/lib/iomgr/ev_epoll_posix.h"

#include <assert.h>
#include <errno.h>
#include <poll.h>
#include <signal.h>
#include <string.h>
#include <sys/epoll.h>
#include <sys/socket.h>
#include <unistd.h>

#include <grpc/support/alloc.h>
#include <grpc/support/log.h>
#include <grpc/support/string_util.h>
#include <grpc/support/tls.h>
#include <grpc/support/useful.h>

#include "src/core/lib/iomgr/ev_posix.h"
#include "src/core/lib/iomgr/iomgr_internal.h"
#include "src/core/lib/iomgr/wakeup_fd_posix.h"
#include "src/core/lib/profiling/timers.h"
#include "src/core/lib/support/block_annotate.h"

struct polling_island;

/*******************************************************************************
 * Fd Declarations
 */
struct grpc_fd {
  int fd;
  /* refst format:
       bit 0    : 1=Active / 0=Orphaned
       bits 1-n : refcount
     Ref/Unref by two to avoid altering the orphaned bit */
  gpr_atm refst;

  gpr_mu mu;
  bool shutdown;
  int closed;
  bool released;

  grpc_closure *read_closure;
  grpc_closure *write_closure;

  /* The polling island to which this fd belongs to and the mutex protecting the
     the field */
  gpr_mu pi_mu;
  struct polling_island *polling_island;

  struct grpc_fd *freelist_next;
  grpc_closure *on_done_closure;

  grpc_iomgr_object iomgr_object;
};

/* Reference counting for fds */
#ifdef GRPC_FD_REF_COUNT_DEBUG
static void fd_ref(grpc_fd *fd, const char *reason, const char *file, int line);
static void fd_unref(grpc_fd *fd, const char *reason, const char *file,
                     int line);
#define GRPC_FD_REF(fd, reason) fd_ref(fd, reason, __FILE__, __LINE__)
#define GRPC_FD_UNREF(fd, reason) fd_unref(fd, reason, __FILE__, __LINE__)
#else
static void fd_ref(grpc_fd *fd);
static void fd_unref(grpc_fd *fd);
#define GRPC_FD_REF(fd, reason) fd_ref(fd)
#define GRPC_FD_UNREF(fd, reason) fd_unref(fd)
#endif

static void fd_global_init(void);
static void fd_global_shutdown(void);

#define CLOSURE_NOT_READY ((grpc_closure *)0)
#define CLOSURE_READY ((grpc_closure *)1)

/*******************************************************************************
 * Polling-island Declarations
 */
typedef struct polling_island {
  gpr_mu mu;
  int ref_cnt;

  /* Points to the polling_island this merged into.
   * If merged_to is not NULL, all the remaining fields (except mu and ref_cnt)
   * are invalid and must be ignored */
  struct polling_island *merged_to;

  /* The fd of the underlying epoll set */
  int epoll_fd;

  /* The file descriptors in the epoll set */
  size_t fd_cnt;
  size_t fd_capacity;
  grpc_fd **fds;

  /* Polling islands that are no longer needed are kept in a freelist so that
     they can be reused. This field points to the next polling island in the
     free list */
  struct polling_island *next_free;
} polling_island;

/*******************************************************************************
 * Pollset Declarations
 */

struct grpc_pollset_worker {
  int kicked_specifically;
  pthread_t pt_id; /* TODO (sreek) - Add an abstraction here */
  struct grpc_pollset_worker *next;
  struct grpc_pollset_worker *prev;
};

struct grpc_pollset {
  gpr_mu mu;
  grpc_pollset_worker root_worker;
  bool kicked_without_pollers;

  bool shutting_down;          /* Is the pollset shutting down ? */
  bool finish_shutdown_called; /* Is the 'finish_shutdown_locked()' called ? */
  grpc_closure *shutdown_done; /* Called after after shutdown is complete */

  /* The polling island to which this pollset belongs to and the mutex
     protecting the field */
  gpr_mu pi_mu;
  struct polling_island *polling_island;
};

/*******************************************************************************
 * Pollset-set Declarations
 */
struct grpc_pollset_set {
  gpr_mu mu;

  size_t pollset_count;
  size_t pollset_capacity;
  grpc_pollset **pollsets;

  size_t pollset_set_count;
  size_t pollset_set_capacity;
  struct grpc_pollset_set **pollset_sets;

  size_t fd_count;
  size_t fd_capacity;
  grpc_fd **fds;
};

/*******************************************************************************
 * Polling-island Definitions
 */

/* Polling island freelist */
static gpr_mu g_pi_freelist_mu;
static polling_island *g_pi_freelist = NULL;

/* The caller is expected to hold pi->mu lock before calling this function */
static void polling_island_add_fds_locked(polling_island *pi, grpc_fd **fds,
                                          size_t fd_count, bool add_fd_refs) {
  int err;
  size_t i;
  struct epoll_event ev;

  for (i = 0; i < fd_count; i++) {
    ev.events = (uint32_t)(EPOLLIN | EPOLLOUT | EPOLLET);
    ev.data.ptr = fds[i];
    err = epoll_ctl(pi->epoll_fd, EPOLL_CTL_ADD, fds[i]->fd, &ev);

    if (err < 0) {
      if (errno != EEXIST) {
        /* TODO: sreek - We need a better way to bubble up this error instead of
           just logging a message */
        gpr_log(GPR_ERROR, "epoll_ctl add for fd: %d failed with error: %s",
                fds[i]->fd, strerror(errno));
      }

      continue;
    }

    if (pi->fd_cnt == pi->fd_capacity) {
      pi->fd_capacity = GPR_MAX(pi->fd_capacity + 8, pi->fd_cnt * 3 / 2);
      pi->fds = gpr_realloc(pi->fds, sizeof(grpc_fd *) * pi->fd_capacity);
    }

    pi->fds[pi->fd_cnt++] = fds[i];
    if (add_fd_refs) {
      GRPC_FD_REF(fds[i], "polling_island");
    }
  }
}

/* The caller is expected to hold pi->mu lock before calling this function */
static void polling_island_remove_all_fds_locked(polling_island *pi,
                                                 bool remove_fd_refs) {
  int err;
  size_t i;

  for (i = 0; i < pi->fd_cnt; i++) {
    if (remove_fd_refs) {
      GRPC_FD_UNREF(pi->fds[i], "polling_island");
    }

    err = epoll_ctl(pi->epoll_fd, EPOLL_CTL_DEL, pi->fds[i]->fd, NULL);
    if (err < 0 && errno != ENOENT) {
      gpr_log(GPR_ERROR,
              "epoll_ctl delete for fds[i]: %d failed with error: %s", i,
              pi->fds[i]->fd, strerror(errno));
      /* TODO: sreek - We need a better way to bubble up this error instead of
       * just logging a message */
      continue;
    }
  }

  pi->fd_cnt = 0;
}

/* The caller is expected to hold pi->mu lock before calling this function */
static void polling_island_remove_fd_locked(polling_island *pi, grpc_fd *fd,
                                            bool close_fd, bool remove_fd_ref) {
  int err;
  size_t i;

  /* Calling close() on the fd will automatically remove it from the epoll set.
     If not calling close(), the fd must be explicitly removed from the epoll
     set */
  if (close_fd) {
    close(fd->fd);
  } else {
    err = epoll_ctl(pi->epoll_fd, EPOLL_CTL_DEL, fd->fd, NULL);
    if (err < 0 && errno != ENOENT) {
      gpr_log(GPR_ERROR, "epoll_ctl delete for fd: %d failed with error; %s",
              fd->fd, strerror(errno));
    }
  }

  for (i = 0; i < pi->fd_cnt; i++) {
    if (pi->fds[i] == fd) {
      pi->fds[i] = pi->fds[--pi->fd_cnt];
      if (remove_fd_ref) {
        GRPC_FD_UNREF(fd, "polling_island");
      }
      break;
    }
  }
}

static polling_island *polling_island_create(grpc_fd *initial_fd,
                                             int initial_ref_cnt) {
  polling_island *pi = NULL;
  struct epoll_event ev;
  int err;

  /* Try to get one from the polling island freelist */
  gpr_mu_lock(&g_pi_freelist_mu);
  if (g_pi_freelist != NULL) {
    pi = g_pi_freelist;
    g_pi_freelist = g_pi_freelist->next_free;
    pi->next_free = NULL;
  }
  gpr_mu_unlock(&g_pi_freelist_mu);

  /* Create new polling island if we could not get one from the free list */
  if (pi == NULL) {
    pi = gpr_malloc(sizeof(*pi));
    gpr_mu_init(&pi->mu);
    pi->fd_cnt = 0;
    pi->fd_capacity = 0;
    pi->fds = NULL;
  }

  pi->epoll_fd = epoll_create1(EPOLL_CLOEXEC);
  if (pi->epoll_fd < 0) {
    gpr_log(GPR_ERROR, "epoll_create1() failed with error: %s",
            strerror(errno));
  }
  GPR_ASSERT(pi->epoll_fd >= 0);

  ev.events = (uint32_t)(EPOLLIN | EPOLLET);
  ev.data.ptr = NULL;
  err = epoll_ctl(pi->epoll_fd, EPOLL_CTL_ADD,
                  GRPC_WAKEUP_FD_GET_READ_FD(&grpc_global_wakeup_fd), &ev);
  if (err < 0) {
    gpr_log(GPR_ERROR,
            "Failed to add grpc_global_wake_up_fd (%d) to the epoll set "
            "(epoll_fd: %d) with error: %s",
            GRPC_WAKEUP_FD_GET_READ_FD(&grpc_global_wakeup_fd), pi->epoll_fd,
            strerror(errno));
  }

  pi->ref_cnt = initial_ref_cnt;
  pi->merged_to = NULL;
  pi->next_free = NULL;

  if (initial_fd != NULL) {
    /* It is not really needed to get the pi->mu lock here. If this is a newly
       created polling island (or one that we got from the freelist), no one
       else would be holding a lock to it anyway */
    gpr_mu_lock(&pi->mu);
    polling_island_add_fds_locked(pi, &initial_fd, 1, true);
    gpr_mu_unlock(&pi->mu);
  }

  return pi;
}

static void polling_island_delete(polling_island *pi) {
  GPR_ASSERT(pi->ref_cnt == 0);
  GPR_ASSERT(pi->fd_cnt == 0);

  close(pi->epoll_fd);
  pi->epoll_fd = -1;

  pi->merged_to = NULL;

  gpr_mu_lock(&g_pi_freelist_mu);
  pi->next_free = g_pi_freelist;
  g_pi_freelist = pi;
  gpr_mu_unlock(&g_pi_freelist_mu);
}

void polling_island_unref_and_unlock(polling_island *pi, int unref_by) {
  pi->ref_cnt -= unref_by;
  int ref_cnt = pi->ref_cnt;
  GPR_ASSERT(ref_cnt >= 0);

  gpr_mu_unlock(&pi->mu);

  if (ref_cnt == 0) {
    polling_island_delete(pi);
  }
}

polling_island *polling_island_update_and_lock(polling_island *pi, int unref_by,
                                               int add_ref_by) {
  polling_island *next = NULL;
  gpr_mu_lock(&pi->mu);
  while (pi->merged_to != NULL) {
    next = pi->merged_to;
    polling_island_unref_and_unlock(pi, unref_by);
    pi = next;
    gpr_mu_lock(&pi->mu);
  }

  pi->ref_cnt += add_ref_by;
  return pi;
}

void polling_island_pair_update_and_lock(polling_island **p,
                                         polling_island **q) {
  polling_island *pi_1 = *p;
  polling_island *pi_2 = *q;
  polling_island *temp = NULL;
  bool pi_1_locked = false;
  bool pi_2_locked = false;
  int num_swaps = 0;

  /* Loop until either pi_1 == pi_2 or until we acquired locks on both pi_1
     and pi_2 */
  while (pi_1 != pi_2 && !(pi_1_locked && pi_2_locked)) {
    /* The following assertions are true at this point:
       - pi_1 != pi_2  (else, the while loop would have exited)
       - pi_1 MAY be locked
       - pi_2 is NOT locked */

    /* To maintain lock order consistency, always lock polling_island node with
       lower address first.
       First, make sure pi_1 < pi_2 before proceeding any further. If it turns
       out that pi_1 > pi_2, unlock pi_1 if locked (because pi_2 is not locked
       at this point and having pi_1 locked would violate the lock order) and
       swap pi_1 and pi_2 so that pi_1 becomes less than pi_2 */
    if (pi_1 > pi_2) {
      if (pi_1_locked) {
        gpr_mu_unlock(&pi_1->mu);
        pi_1_locked = false;
      }

      GPR_SWAP(polling_island *, pi_1, pi_2);
      num_swaps++;
    }

    /* The following assertions are true at this point:
       - pi_1 != pi_2
       - pi_1 < pi_2  (address of pi_1 is less than that of pi_2)
       - pi_1 MAYBE locked
       - pi_2 is NOT locked */

    /* Lock pi_1 (if pi_1 is pointing to the terminal node in the list) */
    if (!pi_1_locked) {
      gpr_mu_lock(&pi_1->mu);
      pi_1_locked = true;

      /* If pi_1 is not terminal node (i.e pi_1->merged_to != NULL), we are not
         done locking this polling_island yet. Release the lock on this node and
         advance pi_1 to the next node in the list; and go to the beginning of
         the loop (we can't proceed to locking pi_2 unless we locked pi_1 first)
         */
      if (pi_1->merged_to != NULL) {
        temp = pi_1->merged_to;
        polling_island_unref_and_unlock(pi_1, 1);
        pi_1 = temp;
        pi_1_locked = false;

        continue;
      }
    }

    /* The following assertions are true at this point:
       - pi_1 is locked
       - pi_2 is unlocked
       - pi_1 != pi_2 */

    gpr_mu_lock(&pi_2->mu);
    pi_2_locked = true;

    /* If pi_2 is not terminal node, we are not done locking this polling_island
       yet. Release the lock and update pi_2 to the next node in the list */
    if (pi_2->merged_to != NULL) {
      temp = pi_2->merged_to;
      polling_island_unref_and_unlock(pi_2, 1);
      pi_2 = temp;
      pi_2_locked = false;
    }
  }

  /* At this point, either pi_1 == pi_2 AND/OR we got both locks */
  if (pi_1 == pi_2) {
    /* We may or may not have gotten the lock. If we didn't, walk the rest of
      the polling_island list and get the lock */
    GPR_ASSERT(pi_1_locked || (!pi_1_locked && !pi_2_locked));
    if (!pi_1_locked) {
      pi_1 = pi_2 = polling_island_update_and_lock(pi_1, 2, 0);
    }
  } else {
    GPR_ASSERT(pi_1_locked && pi_2_locked);
    /* If we swapped pi_1 and pi_2 odd number of times, do one more swap so that
       pi_1 and pi_2 point to the same polling_island lists they started off
       with at the beginning of this function (i.e *p and *q respectively) */
    if (num_swaps % 2 > 0) {
      GPR_SWAP(polling_island *, pi_1, pi_2);
    }
  }

  *p = pi_1;
  *q = pi_2;
}

polling_island *polling_island_merge(polling_island *p, polling_island *q) {
  /* Get locks on both the polling islands */
  polling_island_pair_update_and_lock(&p, &q);

  /* TODO: sreek: Think about this scenario some more. Is it possible ?. what
   * does it mean, when would this happen  */
  if (p == q) {
    /* Nothing needs to be done here */
    gpr_mu_unlock(&p->mu);
    return p;
  }

  /* Make sure that p points to the polling island with fewer fds than q */
  if (p->fd_cnt > q->fd_cnt) {
    GPR_SWAP(polling_island *, p, q);
  }

  /* "Merge" p with q i.e move all the fds from p (the polling_island with fewer
     fds) to q.
     Note: Not altering the ref counts on the affected fds here because they
     would effectively remain unchanged */
  polling_island_add_fds_locked(q, p->fds, p->fd_cnt, false);
  polling_island_remove_all_fds_locked(p, false);

  /* The merged polling island inherits all the ref counts of the island merging
     with it */
  q->ref_cnt += p->ref_cnt;

  gpr_mu_unlock(&p->mu);
  gpr_mu_unlock(&q->mu);

  return q;
}

static void polling_island_global_init() {
  gpr_mu_init(&g_pi_freelist_mu);
  g_pi_freelist = NULL;
}

/*******************************************************************************
 * Fd Definitions
 */

/* We need to keep a freelist not because of any concerns of malloc performance
 * but instead so that implementations with multiple threads in (for example)
 * epoll_wait deal with the race between pollset removal and incoming poll
 * notifications.
 *
 * The problem is that the poller ultimately holds a reference to this
 * object, so it is very difficult to know when is safe to free it, at least
 * without some expensive synchronization.
 *
 * If we keep the object freelisted, in the worst case losing this race just
 * becomes a spurious read notification on a reused fd.
 */

/* The alarm system needs to be able to wakeup 'some poller' sometimes
 * (specifically when a new alarm needs to be triggered earlier than the next
 * alarm 'epoch'). This wakeup_fd gives us something to alert on when such a
 * case occurs. */
/* TODO: sreek: Right now, this wakes up all pollers */
grpc_wakeup_fd grpc_global_wakeup_fd;

static grpc_fd *fd_freelist = NULL;
static gpr_mu fd_freelist_mu;

#ifdef GRPC_FD_REF_COUNT_DEBUG
#define REF_BY(fd, n, reason) ref_by(fd, n, reason, __FILE__, __LINE__)
#define UNREF_BY(fd, n, reason) unref_by(fd, n, reason, __FILE__, __LINE__)
static void ref_by(grpc_fd *fd, int n, const char *reason, const char *file,
                   int line) {
  gpr_log(GPR_DEBUG, "FD %d %p   ref %d %d -> %d [%s; %s:%d]", fd->fd, fd, n,
          gpr_atm_no_barrier_load(&fd->refst),
          gpr_atm_no_barrier_load(&fd->refst) + n, reason, file, line);
#else
#define REF_BY(fd, n, reason) ref_by(fd, n)
#define UNREF_BY(fd, n, reason) unref_by(fd, n)
static void ref_by(grpc_fd *fd, int n) {
#endif
  GPR_ASSERT(gpr_atm_no_barrier_fetch_add(&fd->refst, n) > 0);
}

#ifdef GRPC_FD_REF_COUNT_DEBUG
static void unref_by(grpc_fd *fd, int n, const char *reason, const char *file,
                     int line) {
  gpr_atm old;
  gpr_log(GPR_DEBUG, "FD %d %p unref %d %d -> %d [%s; %s:%d]", fd->fd, fd, n,
          gpr_atm_no_barrier_load(&fd->refst),
          gpr_atm_no_barrier_load(&fd->refst) - n, reason, file, line);
#else
static void unref_by(grpc_fd *fd, int n) {
  gpr_atm old;
#endif
  old = gpr_atm_full_fetch_add(&fd->refst, -n);
  if (old == n) {
    /* Add the fd to the freelist */
    gpr_mu_lock(&fd_freelist_mu);
    fd->freelist_next = fd_freelist;
    fd_freelist = fd;
    grpc_iomgr_unregister_object(&fd->iomgr_object);
    gpr_mu_unlock(&fd_freelist_mu);
  } else {
    GPR_ASSERT(old > n);
  }
}

/* Increment refcount by two to avoid changing the orphan bit */
#ifdef GRPC_FD_REF_COUNT_DEBUG
static void fd_ref(grpc_fd *fd, const char *reason, const char *file,
                   int line) {
  ref_by(fd, 2, reason, file, line);
}

static void fd_unref(grpc_fd *fd, const char *reason, const char *file,
                     int line) {
  unref_by(fd, 2, reason, file, line);
}
#else
static void fd_ref(grpc_fd *fd) { ref_by(fd, 2); }
static void fd_unref(grpc_fd *fd) { unref_by(fd, 2); }
#endif

static void fd_global_init(void) { gpr_mu_init(&fd_freelist_mu); }

static void fd_global_shutdown(void) {
  gpr_mu_lock(&fd_freelist_mu);
  gpr_mu_unlock(&fd_freelist_mu);
  while (fd_freelist != NULL) {
    grpc_fd *fd = fd_freelist;
    fd_freelist = fd_freelist->freelist_next;
    gpr_mu_destroy(&fd->mu);
    gpr_free(fd);
  }
  gpr_mu_destroy(&fd_freelist_mu);
}

static grpc_fd *fd_create(int fd, const char *name) {
  grpc_fd *new_fd = NULL;

  gpr_mu_lock(&fd_freelist_mu);
  if (fd_freelist != NULL) {
    new_fd = fd_freelist;
    fd_freelist = fd_freelist->freelist_next;
  }
  gpr_mu_unlock(&fd_freelist_mu);

  if (new_fd == NULL) {
    new_fd = gpr_malloc(sizeof(grpc_fd));
    gpr_mu_init(&new_fd->mu);
    gpr_mu_init(&new_fd->pi_mu);
  }

  /* Note: It is not really needed to get the new_fd->mu lock here. If this is a
     newly created fd (or an fd we got from the freelist), no one else would be
     holding a lock to it anyway. */
  gpr_mu_lock(&new_fd->mu);

  gpr_atm_rel_store(&new_fd->refst, 1);
  new_fd->shutdown = false;
  new_fd->read_closure = CLOSURE_NOT_READY;
  new_fd->write_closure = CLOSURE_NOT_READY;
  new_fd->fd = fd;
  new_fd->polling_island = NULL;
  new_fd->freelist_next = NULL;
  new_fd->on_done_closure = NULL;
  new_fd->closed = 0;
  new_fd->released = false;

  gpr_mu_unlock(&new_fd->mu);

  char *fd_name;
  gpr_asprintf(&fd_name, "%s fd=%d", name, fd);
  grpc_iomgr_register_object(&new_fd->iomgr_object, fd_name);
  gpr_free(fd_name);
#ifdef GRPC_FD_REF_COUNT_DEBUG
  gpr_log(GPR_DEBUG, "FD %d %p create %s", fd, r, fd_name);
#endif
  return new_fd;
}

static bool fd_is_orphaned(grpc_fd *fd) {
  return (gpr_atm_acq_load(&fd->refst) & 1) == 0;
}

static int fd_wrapped_fd(grpc_fd *fd) {
  int ret_fd = -1;
  gpr_mu_lock(&fd->mu);
  if (!fd->released && !fd->closed) {
    ret_fd = fd->fd;
  }
  gpr_mu_unlock(&fd->mu);

  return ret_fd;
}

static void fd_orphan(grpc_exec_ctx *exec_ctx, grpc_fd *fd,
                      grpc_closure *on_done, int *release_fd,
                      const char *reason) {
  /* TODO(sreek) In ev_poll_posix.c,the lock is acquired a little later. Why? */
  gpr_mu_lock(&fd->mu);
  fd->on_done_closure = on_done;

  /* If release_fd is not NULL, we should be relinquishing control of the file
     descriptor fd->fd (but we still own the grpc_fd structure). */
  fd->released = release_fd != NULL;
  if (!fd->released) {
    shutdown(fd->fd, SHUT_RDWR);
  } else {
    *release_fd = fd->fd;
  }

  REF_BY(fd, 1, reason); /* Remove active status, but keep referenced */
  fd->closed = 1;

  /* Remove the fd from the polling island:
     - Update the fd->polling_island to point to the latest polling island
     - Remove the fd from the polling island. Also, call close() on the file
       descriptor fd->fd ONLY if we haven't relinquised control (i.e
       fd->released is 'false')
     - Decrement the ref count on the polling island and det fd->polling_island
       to NULL */
  gpr_mu_lock(&fd->pi_mu);
  if (fd->polling_island != NULL) {
    fd->polling_island =
        polling_island_update_and_lock(fd->polling_island, 1, 0);
    polling_island_remove_fd_locked(fd->polling_island, fd, !fd->released,
                                    true);
    polling_island_unref_and_unlock(fd->polling_island, 1);
    fd->polling_island = NULL;
  }
  gpr_mu_unlock(&fd->pi_mu);

  grpc_exec_ctx_enqueue(exec_ctx, fd->on_done_closure, true, NULL);

  gpr_mu_unlock(&fd->mu);
  UNREF_BY(fd, 2, reason); /* Drop the reference */
}

static void notify_on_locked(grpc_exec_ctx *exec_ctx, grpc_fd *fd,
                             grpc_closure **st, grpc_closure *closure) {
  if (*st == CLOSURE_NOT_READY) {
    /* not ready ==> switch to a waiting state by setting the closure */
    *st = closure;
  } else if (*st == CLOSURE_READY) {
    /* already ready ==> queue the closure to run immediately */
    *st = CLOSURE_NOT_READY;
    grpc_exec_ctx_enqueue(exec_ctx, closure, !fd->shutdown, NULL);
  } else {
    /* upcallptr was set to a different closure.  This is an error! */
    gpr_log(GPR_ERROR,
            "User called a notify_on function with a previous callback still "
            "pending");
    abort();
  }
}

/* returns 1 if state becomes not ready */
static int set_ready_locked(grpc_exec_ctx *exec_ctx, grpc_fd *fd,
                            grpc_closure **st) {
  if (*st == CLOSURE_READY) {
    /* duplicate ready ==> ignore */
    return 0;
  } else if (*st == CLOSURE_NOT_READY) {
    /* not ready, and not waiting ==> flag ready */
    *st = CLOSURE_READY;
    return 0;
  } else {
    /* waiting ==> queue closure */
    grpc_exec_ctx_enqueue(exec_ctx, *st, !fd->shutdown, NULL);
    *st = CLOSURE_NOT_READY;
    return 1;
  }
}

static void fd_shutdown(grpc_exec_ctx *exec_ctx, grpc_fd *fd) {
  gpr_mu_lock(&fd->mu);
  GPR_ASSERT(!fd->shutdown);
  fd->shutdown = true;

  /* Flush any pending read and write closures. Since fd->shutdown is 'true' at
     this point, the closures would be called with 'success = false' */
  set_ready_locked(exec_ctx, fd, &fd->read_closure);
  set_ready_locked(exec_ctx, fd, &fd->write_closure);
  gpr_mu_unlock(&fd->mu);
}

static void fd_notify_on_read(grpc_exec_ctx *exec_ctx, grpc_fd *fd,
                              grpc_closure *closure) {
  gpr_mu_lock(&fd->mu);
  notify_on_locked(exec_ctx, fd, &fd->read_closure, closure);
  gpr_mu_unlock(&fd->mu);
}

static void fd_notify_on_write(grpc_exec_ctx *exec_ctx, grpc_fd *fd,
                               grpc_closure *closure) {
  gpr_mu_lock(&fd->mu);
  notify_on_locked(exec_ctx, fd, &fd->write_closure, closure);
  gpr_mu_unlock(&fd->mu);
}

/*******************************************************************************
 * Pollset Definitions
 */

static void sig_handler(int sig_num) {
  /* TODO: sreek - Remove this expensive log line */
  gpr_log(GPR_INFO, "Received signal %d", sig_num);
}

/* Global state management */
static void pollset_global_init(void) {
  grpc_wakeup_fd_init(&grpc_global_wakeup_fd);
  signal(SIGUSR1, sig_handler);
}

static void pollset_global_shutdown(void) {
  grpc_wakeup_fd_destroy(&grpc_global_wakeup_fd);
}

/* Return 1 if the pollset has active threads in pollset_work (pollset must
 * be locked) */
static int pollset_has_workers(grpc_pollset *p) {
  return p->root_worker.next != &p->root_worker;
}

static void remove_worker(grpc_pollset *p, grpc_pollset_worker *worker) {
  worker->prev->next = worker->next;
  worker->next->prev = worker->prev;
}

static grpc_pollset_worker *pop_front_worker(grpc_pollset *p) {
  if (pollset_has_workers(p)) {
    grpc_pollset_worker *w = p->root_worker.next;
    remove_worker(p, w);
    return w;
  } else {
    return NULL;
  }
}

static void push_back_worker(grpc_pollset *p, grpc_pollset_worker *worker) {
  worker->next = &p->root_worker;
  worker->prev = worker->next->prev;
  worker->prev->next = worker->next->prev = worker;
}

static void push_front_worker(grpc_pollset *p, grpc_pollset_worker *worker) {
  worker->prev = &p->root_worker;
  worker->next = worker->prev->next;
  worker->prev->next = worker->next->prev = worker;
}

/* p->mu must be held before calling this function */
static void pollset_kick(grpc_pollset *p,
                         grpc_pollset_worker *specific_worker) {
  GPR_TIMER_BEGIN("pollset_kick", 0);

  grpc_pollset_worker *worker = specific_worker;
  if (worker != NULL) {
    if (worker == GRPC_POLLSET_KICK_BROADCAST) {
      GPR_TIMER_BEGIN("pollset_kick.broadcast", 0);
      if (pollset_has_workers(p)) {
        for (worker = p->root_worker.next; worker != &p->root_worker;
             worker = worker->next) {
          pthread_kill(worker->pt_id, SIGUSR1);
        }
      } else {
        p->kicked_without_pollers = true;
      }
      GPR_TIMER_END("pollset_kick.broadcast", 0);
    } else {
      GPR_TIMER_MARK("kicked_specifically", 0);
      worker->kicked_specifically = true;
      pthread_kill(worker->pt_id, SIGUSR1);
    }
  } else {
    GPR_TIMER_MARK("kick_anonymous", 0);
    worker = pop_front_worker(p);
    if (worker != NULL) {
      GPR_TIMER_MARK("finally_kick", 0);
      push_back_worker(p, worker);
      pthread_kill(worker->pt_id, SIGUSR1);
    } else {
      GPR_TIMER_MARK("kicked_no_pollers", 0);
      p->kicked_without_pollers = true;
    }
  }

  GPR_TIMER_END("pollset_kick", 0);
}

static void kick_poller(void) { grpc_wakeup_fd_wakeup(&grpc_global_wakeup_fd); }

static void pollset_init(grpc_pollset *pollset, gpr_mu **mu) {
  gpr_mu_init(&pollset->mu);
  *mu = &pollset->mu;

  pollset->root_worker.next = pollset->root_worker.prev = &pollset->root_worker;
  pollset->kicked_without_pollers = false;

  pollset->shutting_down = false;
  pollset->finish_shutdown_called = false;
  pollset->shutdown_done = NULL;

  gpr_mu_init(&pollset->pi_mu);
  pollset->polling_island = NULL;
}

/* Convert a timespec to milliseconds:
   - Very small or negative poll times are clamped to zero to do a non-blocking
     poll (which becomes spin polling)
   - Other small values are rounded up to one millisecond
   - Longer than a millisecond polls are rounded up to the next nearest
     millisecond to avoid spinning
   - Infinite timeouts are converted to -1 */
static int poll_deadline_to_millis_timeout(gpr_timespec deadline,
                                           gpr_timespec now) {
  gpr_timespec timeout;
  static const int64_t max_spin_polling_us = 10;
  if (gpr_time_cmp(deadline, gpr_inf_future(deadline.clock_type)) == 0) {
    return -1;
  }

  if (gpr_time_cmp(deadline, gpr_time_add(now, gpr_time_from_micros(
                                                   max_spin_polling_us,
                                                   GPR_TIMESPAN))) <= 0) {
    return 0;
  }
  timeout = gpr_time_sub(deadline, now);
  return gpr_time_to_millis(gpr_time_add(
      timeout, gpr_time_from_nanos(GPR_NS_PER_MS - 1, GPR_TIMESPAN)));
}

static void set_ready(grpc_exec_ctx *exec_ctx, grpc_fd *fd, grpc_closure **st) {
  /* only one set_ready can be active at once (but there may be a racing
     notify_on) */
  gpr_mu_lock(&fd->mu);
  set_ready_locked(exec_ctx, fd, st);
  gpr_mu_unlock(&fd->mu);
}

static void fd_become_readable(grpc_exec_ctx *exec_ctx, grpc_fd *fd) {
  set_ready(exec_ctx, fd, &fd->read_closure);
}

static void fd_become_writable(grpc_exec_ctx *exec_ctx, grpc_fd *fd) {
  set_ready(exec_ctx, fd, &fd->write_closure);
}

#define GRPC_EPOLL_MAX_EVENTS 1000
static void pollset_work_and_unlock(grpc_exec_ctx *exec_ctx,
                                    grpc_pollset *pollset, int timeout_ms,
                                    sigset_t *sig_mask) {
  struct epoll_event ep_ev[GRPC_EPOLL_MAX_EVENTS];
  int epoll_fd = -1;
  int ep_rv;
  GPR_TIMER_BEGIN("pollset_work_and_unlock", 0);

  /* We need to get the epoll_fd to wait on. The epoll_fd is in inside the
     polling island pointed by pollset->polling_island.
     Acquire the following locks:
     - pollset->mu (which we already have)
     - pollset->pi_mu
     - pollset->polling_island->mu */
  gpr_mu_lock(&pollset->pi_mu);

  if (pollset->polling_island != NULL) {
    pollset->polling_island =
        polling_island_update_and_lock(pollset->polling_island, 1, 0);
    epoll_fd = pollset->polling_island->epoll_fd;
    gpr_mu_unlock(&pollset->polling_island->mu);
  }

  gpr_mu_unlock(&pollset->pi_mu);
  gpr_mu_unlock(&pollset->mu);

  /* If epoll_fd == -1, this is a blank pollset and does not have any fds yet */
  if (epoll_fd != -1) {
    do {
      ep_rv = epoll_pwait(epoll_fd, ep_ev, GRPC_EPOLL_MAX_EVENTS, timeout_ms,
                          sig_mask);

      if (ep_rv < 0) {
        if (errno != EINTR) {
          /* TODO (sreek) - Check for bad file descriptor error */
          gpr_log(GPR_ERROR, "epoll_pwait() failed: %s", strerror(errno));
        }
      } else {
        int i;
        for (i = 0; i < ep_rv; ++i) {
          grpc_fd *fd = ep_ev[i].data.ptr;
          int cancel = ep_ev[i].events & (EPOLLERR | EPOLLHUP);
          int read_ev = ep_ev[i].events & (EPOLLIN | EPOLLPRI);
          int write_ev = ep_ev[i].events & EPOLLOUT;
          if (fd == NULL) {
            grpc_wakeup_fd_consume_wakeup(&grpc_global_wakeup_fd);
          } else {
            if (read_ev || cancel) {
              fd_become_readable(exec_ctx, fd);
            }
            if (write_ev || cancel) {
              fd_become_writable(exec_ctx, fd);
            }
          }
        }
      }
    } while (ep_rv == GRPC_EPOLL_MAX_EVENTS);
  }

  GPR_TIMER_END("pollset_work_and_unlock", 0);
}

/* Release the reference to pollset->polling_island and set it to NULL.
   pollset->mu must be held */
static void pollset_release_polling_island_locked(grpc_pollset *pollset) {
  gpr_mu_lock(&pollset->pi_mu);
  if (pollset->polling_island) {
    pollset->polling_island =
        polling_island_update_and_lock(pollset->polling_island, 1, 0);
    polling_island_unref_and_unlock(pollset->polling_island, 1);
    pollset->polling_island = NULL;
  }
  gpr_mu_unlock(&pollset->pi_mu);
}

static void finish_shutdown_locked(grpc_exec_ctx *exec_ctx,
                                   grpc_pollset *pollset) {
  /* The pollset cannot have any workers if we are at this stage */
  GPR_ASSERT(!pollset_has_workers(pollset));

  pollset->finish_shutdown_called = true;
  pollset_release_polling_island_locked(pollset);

  grpc_exec_ctx_enqueue(exec_ctx, pollset->shutdown_done, true, NULL);
}

/* pollset->mu lock must be held by the caller before calling this */
static void pollset_shutdown(grpc_exec_ctx *exec_ctx, grpc_pollset *pollset,
                             grpc_closure *closure) {
  GPR_TIMER_BEGIN("pollset_shutdown", 0);
  GPR_ASSERT(!pollset->shutting_down);
  pollset->shutting_down = true;
  pollset->shutdown_done = closure;
  pollset_kick(pollset, GRPC_POLLSET_KICK_BROADCAST);

  /* If the pollset has any workers, we cannot call finish_shutdown_locked()
     because it would release the underlying polling island. In such a case, we
     let the last worker call finish_shutdown_locked() from pollset_work() */
  if (!pollset_has_workers(pollset)) {
    GPR_ASSERT(!pollset->finish_shutdown_called);
    GPR_TIMER_MARK("pollset_shutdown.finish_shutdown_locked", 0);
    finish_shutdown_locked(exec_ctx, pollset);
  }
  GPR_TIMER_END("pollset_shutdown", 0);
}

/* TODO(sreek) Is pollset_shutdown() guranteed to be called before this? */
static void pollset_destroy(grpc_pollset *pollset) {
  GPR_ASSERT(!pollset_has_workers(pollset));
  gpr_mu_destroy(&pollset->pi_mu);
  gpr_mu_destroy(&pollset->mu);
}

static void pollset_reset(grpc_pollset *pollset) {
  GPR_ASSERT(pollset->shutting_down);
  GPR_ASSERT(!pollset_has_workers(pollset));
  pollset->shutting_down = false;
  pollset->finish_shutdown_called = false;
  pollset->kicked_without_pollers = false;
  /* TODO(sreek) - Should pollset->shutdown closure be set to NULL here? */
  pollset_release_polling_island_locked(pollset);
}

/* pollset->mu lock must be held by the caller before calling this.
   The function pollset_work() may temporarily release the lock (pollset->mu)
   during the course of its execution but it will always re-acquire the lock and
   ensure that it is held by the time the function returns */
static void pollset_work(grpc_exec_ctx *exec_ctx, grpc_pollset *pollset,
                         grpc_pollset_worker **worker_hdl, gpr_timespec now,
                         gpr_timespec deadline) {
  GPR_TIMER_BEGIN("pollset_work", 0);

  int timeout_ms = poll_deadline_to_millis_timeout(deadline, now);

  sigset_t new_mask;
  sigset_t orig_mask;

  grpc_pollset_worker worker;
  worker.next = worker.prev = NULL;
  worker.kicked_specifically = 0;
  worker.pt_id = pthread_self();

  *worker_hdl = &worker;

  if (pollset->kicked_without_pollers) {
    /* If the pollset was kicked without pollers, pretend that the current
       worker got the kick and skip polling. A kick indicates that there is some
       work that needs attention like an event on the completion queue or an
       alarm */
    GPR_TIMER_MARK("pollset_work.kicked_without_pollers", 0);
    pollset->kicked_without_pollers = 0;
  } else if (!pollset->shutting_down) {
    sigemptyset(&new_mask);
    sigaddset(&new_mask, SIGUSR1);
    pthread_sigmask(SIG_BLOCK, &new_mask, &orig_mask);
    sigdelset(&orig_mask, SIGUSR1);

    push_front_worker(pollset, &worker);

    pollset_work_and_unlock(exec_ctx, pollset, timeout_ms, &orig_mask);
    grpc_exec_ctx_flush(exec_ctx);

    gpr_mu_lock(&pollset->mu);
    remove_worker(pollset, &worker);
  }

  /* If we are the last worker on the pollset (i.e pollset_has_workers() is
     false at this point) and the pollset is shutting down, we may have to
     finish the shutdown process by calling finish_shutdown_locked().
     See pollset_shutdown() for more details.

     Note: Continuing to access pollset here is safe; it is the caller's
     responsibility to not destroy a pollset when it has outstanding calls to
     pollset_work() */
  if (pollset->shutting_down && !pollset_has_workers(pollset) &&
      !pollset->finish_shutdown_called) {
    GPR_TIMER_MARK("pollset_work.finish_shutdown_locked", 0);
    finish_shutdown_locked(exec_ctx, pollset);

    gpr_mu_unlock(&pollset->mu);
    grpc_exec_ctx_flush(exec_ctx);
    gpr_mu_lock(&pollset->mu);
  }

  *worker_hdl = NULL;
  GPR_TIMER_END("pollset_work", 0);
}

static void pollset_add_fd(grpc_exec_ctx *exec_ctx, grpc_pollset *pollset,
                           grpc_fd *fd) {
  /* TODO sreek - Check if we need to get a pollset->mu lock here */
  gpr_mu_lock(&pollset->pi_mu);
  gpr_mu_lock(&fd->pi_mu);

  polling_island *pi_new = NULL;

  /* 1) If fd->polling_island and pollset->polling_island are both non-NULL and
   *    equal, do nothing.
   * 2) If fd->polling_island and pollset->polling_island are both NULL, create
   *    a new polling island (with a refcount of 2) and make the polling_island
   *    fields in both fd and pollset to point to the new island
   * 3) If one of fd->polling_island or pollset->polling_island is NULL, update
   *    the NULL polling_island field to point to the non-NULL polling_island
   *    field (ensure that the refcount on the polling island is incremented by
   *    1 to account for the newly added reference)
   * 4) Finally, if fd->polling_island and pollset->polling_island are non-NULL
   *    and different, merge both the polling islands and update the
   *    polling_island fields in both fd and pollset to point to the merged
   *    polling island.
   */
  if (fd->polling_island == pollset->polling_island) {
    pi_new = fd->polling_island;
    if (pi_new == NULL) {
      pi_new = polling_island_create(fd, 2);
    }
  } else if (fd->polling_island == NULL) {
    pi_new = polling_island_update_and_lock(pollset->polling_island, 1, 1);
    gpr_mu_unlock(&pi_new->mu);
  } else if (pollset->polling_island == NULL) {
    pi_new = polling_island_update_and_lock(fd->polling_island, 1, 1);
    gpr_mu_unlock(&pi_new->mu);
  } else {
    pi_new = polling_island_merge(fd->polling_island, pollset->polling_island);
  }

  fd->polling_island = pollset->polling_island = pi_new;

  gpr_mu_unlock(&fd->pi_mu);
  gpr_mu_unlock(&pollset->pi_mu);
}

/*******************************************************************************
 * Pollset-set Definitions
 */

static grpc_pollset_set *pollset_set_create(void) {
  grpc_pollset_set *pollset_set = gpr_malloc(sizeof(*pollset_set));
  memset(pollset_set, 0, sizeof(*pollset_set));
  gpr_mu_init(&pollset_set->mu);
  return pollset_set;
}

static void pollset_set_destroy(grpc_pollset_set *pollset_set) {
  size_t i;
  gpr_mu_destroy(&pollset_set->mu);
  for (i = 0; i < pollset_set->fd_count; i++) {
    GRPC_FD_UNREF(pollset_set->fds[i], "pollset_set");
  }
  gpr_free(pollset_set->pollsets);
  gpr_free(pollset_set->pollset_sets);
  gpr_free(pollset_set->fds);
  gpr_free(pollset_set);
}

static void pollset_set_add_fd(grpc_exec_ctx *exec_ctx,
                               grpc_pollset_set *pollset_set, grpc_fd *fd) {
  size_t i;
  gpr_mu_lock(&pollset_set->mu);
  if (pollset_set->fd_count == pollset_set->fd_capacity) {
    pollset_set->fd_capacity = GPR_MAX(8, 2 * pollset_set->fd_capacity);
    pollset_set->fds = gpr_realloc(
        pollset_set->fds, pollset_set->fd_capacity * sizeof(*pollset_set->fds));
  }
  GRPC_FD_REF(fd, "pollset_set");
  pollset_set->fds[pollset_set->fd_count++] = fd;
  for (i = 0; i < pollset_set->pollset_count; i++) {
    pollset_add_fd(exec_ctx, pollset_set->pollsets[i], fd);
  }
  for (i = 0; i < pollset_set->pollset_set_count; i++) {
    pollset_set_add_fd(exec_ctx, pollset_set->pollset_sets[i], fd);
  }
  gpr_mu_unlock(&pollset_set->mu);
}

static void pollset_set_del_fd(grpc_exec_ctx *exec_ctx,
                               grpc_pollset_set *pollset_set, grpc_fd *fd) {
  size_t i;
  gpr_mu_lock(&pollset_set->mu);
  for (i = 0; i < pollset_set->fd_count; i++) {
    if (pollset_set->fds[i] == fd) {
      pollset_set->fd_count--;
      GPR_SWAP(grpc_fd *, pollset_set->fds[i],
               pollset_set->fds[pollset_set->fd_count]);
      GRPC_FD_UNREF(fd, "pollset_set");
      break;
    }
  }
  for (i = 0; i < pollset_set->pollset_set_count; i++) {
    pollset_set_del_fd(exec_ctx, pollset_set->pollset_sets[i], fd);
  }
  gpr_mu_unlock(&pollset_set->mu);
}

static void pollset_set_add_pollset(grpc_exec_ctx *exec_ctx,
                                    grpc_pollset_set *pollset_set,
                                    grpc_pollset *pollset) {
  size_t i, j;
  gpr_mu_lock(&pollset_set->mu);
  if (pollset_set->pollset_count == pollset_set->pollset_capacity) {
    pollset_set->pollset_capacity =
        GPR_MAX(8, 2 * pollset_set->pollset_capacity);
    pollset_set->pollsets =
        gpr_realloc(pollset_set->pollsets, pollset_set->pollset_capacity *
                                               sizeof(*pollset_set->pollsets));
  }
  pollset_set->pollsets[pollset_set->pollset_count++] = pollset;
  for (i = 0, j = 0; i < pollset_set->fd_count; i++) {
    if (fd_is_orphaned(pollset_set->fds[i])) {
      GRPC_FD_UNREF(pollset_set->fds[i], "pollset_set");
    } else {
      pollset_add_fd(exec_ctx, pollset, pollset_set->fds[i]);
      pollset_set->fds[j++] = pollset_set->fds[i];
    }
  }
  pollset_set->fd_count = j;
  gpr_mu_unlock(&pollset_set->mu);
}

static void pollset_set_del_pollset(grpc_exec_ctx *exec_ctx,
                                    grpc_pollset_set *pollset_set,
                                    grpc_pollset *pollset) {
  size_t i;
  gpr_mu_lock(&pollset_set->mu);
  for (i = 0; i < pollset_set->pollset_count; i++) {
    if (pollset_set->pollsets[i] == pollset) {
      pollset_set->pollset_count--;
      GPR_SWAP(grpc_pollset *, pollset_set->pollsets[i],
               pollset_set->pollsets[pollset_set->pollset_count]);
      break;
    }
  }
  gpr_mu_unlock(&pollset_set->mu);
}

static void pollset_set_add_pollset_set(grpc_exec_ctx *exec_ctx,
                                        grpc_pollset_set *bag,
                                        grpc_pollset_set *item) {
  size_t i, j;
  gpr_mu_lock(&bag->mu);
  if (bag->pollset_set_count == bag->pollset_set_capacity) {
    bag->pollset_set_capacity = GPR_MAX(8, 2 * bag->pollset_set_capacity);
    bag->pollset_sets =
        gpr_realloc(bag->pollset_sets,
                    bag->pollset_set_capacity * sizeof(*bag->pollset_sets));
  }
  bag->pollset_sets[bag->pollset_set_count++] = item;
  for (i = 0, j = 0; i < bag->fd_count; i++) {
    if (fd_is_orphaned(bag->fds[i])) {
      GRPC_FD_UNREF(bag->fds[i], "pollset_set");
    } else {
      pollset_set_add_fd(exec_ctx, item, bag->fds[i]);
      bag->fds[j++] = bag->fds[i];
    }
  }
  bag->fd_count = j;
  gpr_mu_unlock(&bag->mu);
}

static void pollset_set_del_pollset_set(grpc_exec_ctx *exec_ctx,
                                        grpc_pollset_set *bag,
                                        grpc_pollset_set *item) {
  size_t i;
  gpr_mu_lock(&bag->mu);
  for (i = 0; i < bag->pollset_set_count; i++) {
    if (bag->pollset_sets[i] == item) {
      bag->pollset_set_count--;
      GPR_SWAP(grpc_pollset_set *, bag->pollset_sets[i],
               bag->pollset_sets[bag->pollset_set_count]);
      break;
    }
  }
  gpr_mu_unlock(&bag->mu);
}

/*******************************************************************************
 * Event engine binding
 */

static void shutdown_engine(void) {
  fd_global_shutdown();
  pollset_global_shutdown();
}

static const grpc_event_engine_vtable vtable = {
    .pollset_size = sizeof(grpc_pollset),

    .fd_create = fd_create,
    .fd_wrapped_fd = fd_wrapped_fd,
    .fd_orphan = fd_orphan,
    .fd_shutdown = fd_shutdown,
    .fd_notify_on_read = fd_notify_on_read,
    .fd_notify_on_write = fd_notify_on_write,

    .pollset_init = pollset_init,
    .pollset_shutdown = pollset_shutdown,
    .pollset_reset = pollset_reset,
    .pollset_destroy = pollset_destroy,
    .pollset_work = pollset_work,
    .pollset_kick = pollset_kick,
    .pollset_add_fd = pollset_add_fd,

    .pollset_set_create = pollset_set_create,
    .pollset_set_destroy = pollset_set_destroy,
    .pollset_set_add_pollset = pollset_set_add_pollset,
    .pollset_set_del_pollset = pollset_set_del_pollset,
    .pollset_set_add_pollset_set = pollset_set_add_pollset_set,
    .pollset_set_del_pollset_set = pollset_set_del_pollset_set,
    .pollset_set_add_fd = pollset_set_add_fd,
    .pollset_set_del_fd = pollset_set_del_fd,

    .kick_poller = kick_poller,

    .shutdown_engine = shutdown_engine,
};

const grpc_event_engine_vtable *grpc_init_epoll_linux(void) {
  fd_global_init();
  pollset_global_init();
  polling_island_global_init();
  return &vtable;
}

#endif