Revert "Implement pthread_rwlock functions for NaCl newlib."

src/untrusted/pthread/nc_rwlock.c

-/*
- * Copyright 2015 The Native Client Authors. All rights reserved.
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file.
- */
-
-/*
- * Native Client rwlock implementation
- *
- * This implementation is a 'write-preferring' reader-writer lock which
- * avoids writer starvation by preventing readers from acquiring the lock
- * while there are waiting writers (with an exception to prevent deadlocks
- * in the case of recursive read lock (see read_lock_available)). See:
- * http://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock
- *
- * The thundering herd problem is avoided by only waking a single
- * waiter (either a single writer or a single reader) when the
- * lock is released.
- */
-
-#include <errno.h>
-
-#include "native_client/src/untrusted/pthread/pthread.h"
-#include "native_client/src/untrusted/pthread/pthread_internal.h"
-#include "native_client/src/untrusted/pthread/pthread_types.h"
-
-int pthread_rwlockattr_init(pthread_rwlockattr_t *attr) {
-  attr->type = PTHREAD_PROCESS_PRIVATE;
-  return 0;
-}
-
-int pthread_rwlockattr_destroy(pthread_rwlockattr_t *attr) {
-  return 0;
-}
-
-int pthread_rwlockattr_getpshared(const pthread_rwlockattr_t *attr,
-                                  int *pshared) {
-  *pshared = attr->type;
-  return 0;
-}
-
-int pthread_rwlockattr_setpshared(pthread_rwlockattr_t *attr, int pshared) {
-  if (pshared != PTHREAD_PROCESS_PRIVATE && pshared != PTHREAD_PROCESS_SHARED)
-    return EINVAL;
-  attr->type = pshared;
-  return 0;
-}
-
-int pthread_rwlock_init(pthread_rwlock_t *rwlock,
-                        const pthread_rwlockattr_t *attr) {
-  if (attr != NULL && attr->type != PTHREAD_PROCESS_PRIVATE)
-    return EINVAL;
-  rwlock->writer_thread_id = NACL_PTHREAD_ILLEGAL_THREAD_ID;
-  rwlock->writers_waiting = 0;
-  rwlock->reader_count = 0;
-  int rc = pthread_mutex_init(&rwlock->mutex, NULL);
-  if (rc != 0)
-    return rc;
-  rc = pthread_cond_init(&rwlock->write_possible, NULL);
-  if (rc != 0)
-    return rc;
-  return pthread_cond_init(&rwlock->read_possible, NULL);
-}
-
-/*
- * Helper function used by waiting writers to determine if they can take
- * the lock. The rwlock->mutex must be held when calling this function.
- * Returns 1 if the write lock can be taken, 0 if it can't.
- */
-static inline int write_lock_available(pthread_rwlock_t *rwlock) {
-  /*
-   * Write lock is available if there is no current writer and no current
-   * readers.
-   */
-  if (rwlock->writer_thread_id != NACL_PTHREAD_ILLEGAL_THREAD_ID)
-    return 0;
-  if (rwlock->reader_count > 0)
-    return 0;
-  return 1;
-}
-
-/*
- * Helper function used by waiting readers to determine if they can take
- * the lock. The rwlock->mutex must be held when calling this function.
- * Returns 1 if the write lock can be taken, 0 if it can't.
- */
-static inline int read_lock_available(pthread_rwlock_t *rwlock) {
-  /*
-   * Read lock is unavailable if there is a current writer.
-   */
-  if (rwlock->writer_thread_id != NACL_PTHREAD_ILLEGAL_THREAD_ID)
-    return 0;
-
-  /*
-   * Attempt to reduce writer starvation by blocking readers when there
-   * is a waiting writer.  However don't do this if the current thread
-   * already holds one or more rdlocks in order to allow for recursive
-   * rdlocks. See: http://stackoverflow.com/questions/2190090/
-   * how-to-prevent-writer-starvation-in-a-read-write-lock-in-pthreads
-   */
-  if (rwlock->writers_waiting > 0 && __nc_get_tdb()->rdlock_count == 0)
-    return 0;
-  return 1;
-}
-
-/*
- * Internal function used to acquire the read lock.
- * This operates in three different ways in order to implement the three public
- * functions:
- *  pthread_rwlock_rdlock
- *  pthread_rwlock_tryrdlock
- *  pthread_rwlock_timedrdlock
- */
-static int rdlock_internal(pthread_rwlock_t *rwlock,
-                           const struct timespec *abs_timeout,
-                           int try_only) {
-  int rc2;
-  int rc = pthread_mutex_lock(&rwlock->mutex);
-  if (rc != 0)
-    return rc;
-
-  /*
-   * Wait repeatedly until the write preconditions are met.
-   * In theory this loop should only execute once because the preconditions
-   * should always be true when the condition is signaled.
-   */
-  while (!read_lock_available(rwlock)) {
-    if (try_only) {
-      rc = EBUSY;
-    } else if (abs_timeout != NULL) {
-      rc = pthread_cond_timedwait(&rwlock->read_possible,
-                                  &rwlock->mutex,
-                                  abs_timeout);
-    } else {
-      rc = pthread_cond_wait(&rwlock->read_possible, &rwlock->mutex);
-    }
-    if (rc != 0)
-      goto done;
-  }
-
-  /* Acquire the read lock. */
-  rwlock->reader_count++;
-  __nc_get_tdb()->rdlock_count++;
-done:
-  rc2 = pthread_mutex_unlock(&rwlock->mutex);
-  return rc == 0 ? rc2 : rc;
-}
-
-/*
- * Internal function used to acquire the write lock.
- * This operates in three different ways in order to implement the three public
- * functions:
- *  pthread_rwlock_wrlock
- *  pthread_rwlock_trywrlock
- *  pthread_rwlock_timedwrlock
- */
-static int rwlock_internal(pthread_rwlock_t *rwlock,
-                           const struct timespec *abs_timeout,
-                           int try_only) {
-  int rc2;
-  int rc = pthread_mutex_lock(&rwlock->mutex);
-  if (rc != 0)
-    return rc;
-
-  /* Wait repeatedly until the write preconditions are met */
-  while (!write_lock_available(rwlock)) {
-    if (try_only) {
-      rc = EBUSY;
-    } else {
-      /*
-       * Before waiting (and releasing the lock) we increment the
-       * waiting_writers count so the unlocking code knows to wake
-       * a writer first (before any waiting readers).
-       */
-      rwlock->writers_waiting++;
-      if (abs_timeout != NULL) {
-        rc = pthread_cond_timedwait(&rwlock->write_possible,
-                                    &rwlock->mutex,
-                                    abs_timeout);
-      } else {
-        rc = pthread_cond_wait(&rwlock->write_possible,
-                               &rwlock->mutex);
-      }
-      rwlock->writers_waiting--;
-    }
-    if (rc != 0)
-      goto done;
-  }
-
-  /* Acquire the write lock. */
-  rwlock->writer_thread_id = pthread_self();
-done:
-  rc2 = pthread_mutex_unlock(&rwlock->mutex);
-  return rc == 0 ? rc2 : rc;
-}
-
-int pthread_rwlock_timedrdlock(pthread_rwlock_t *rwlock,
-                               const struct timespec *abs_timeout) {
-  return rdlock_internal(rwlock, abs_timeout, 0);
-}
-
-int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock) {
-  return rdlock_internal(rwlock, NULL, 0);
-}
-
-int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock) {
-  return rdlock_internal(rwlock, NULL, 1);
-}
-
-int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock) {
-  return rwlock_internal(rwlock, NULL, 0);
-}
-
-int pthread_rwlock_timedwrlock(pthread_rwlock_t *rwlock,
-                               const struct timespec *abs_timeout) {
-  return rwlock_internal(rwlock, abs_timeout, 0);
-}
-
-int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock) {
-  return rwlock_internal(rwlock, NULL, 1);
-}
-
-int pthread_rwlock_unlock(pthread_rwlock_t *rwlock) {
-  int rc2;
-  int rc = pthread_mutex_lock(&rwlock->mutex);
-  if (rc != 0)
-    return rc;
-
-  if (rwlock->writer_thread_id != NACL_PTHREAD_ILLEGAL_THREAD_ID) {
-    /* The write lock is held. Ensure it's the current thread that holds it. */
-    if (rwlock->writer_thread_id != pthread_self()) {
-      rc = EPERM;
-      goto done;
-    }
-
-    /* Release write lock. */
-    rwlock->writer_thread_id = NACL_PTHREAD_ILLEGAL_THREAD_ID;
-    if (rwlock->writers_waiting > 0) {
-      /* Wake a waiting writer if there is one. */
-      rc = pthread_cond_signal(&rwlock->write_possible);
-    } else {
-      /* Otherwise wake a waiting reader. */
-      rc = pthread_cond_signal(&rwlock->read_possible);
-    }
-  } else {
-    if (rwlock->reader_count == 0) {
-      rc = EPERM;
-      goto done;
-    }
-
-    /* Release read lock. */
-    rwlock->reader_count--;
-    __nc_get_tdb()->rdlock_count--;
-    if (rwlock->reader_count == 0 && rwlock->writers_waiting > 0) {
-      /* Wake a waiting writer. */
-      rc = pthread_cond_signal(&rwlock->write_possible);
-    }
-  }
-
-done:
-  rc2 = pthread_mutex_unlock(&rwlock->mutex);
-  return rc == 0 ? rc2 : rc;
-}
-
-int pthread_rwlock_destroy(pthread_rwlock_t *rwlock) {
-  /* Return EBUSY if another thread holds the mutex. */
-  int rc = pthread_mutex_trylock(&rwlock->mutex);
-  if (rc != 0) {
-    return rc;
-  }
-
-  /* Return EBUSY if there are active readers or an active writer. */
-  if (rwlock->reader_count != 0) {
-    pthread_mutex_unlock(&rwlock->mutex);
-    return EBUSY;
-  }
-  if (rwlock->writer_thread_id != NACL_PTHREAD_ILLEGAL_THREAD_ID) {
-    pthread_mutex_unlock(&rwlock->mutex);
-    return EBUSY;
-  }
-
-  int rc1 = pthread_cond_destroy(&rwlock->write_possible);
-  int rc2 = pthread_cond_destroy(&rwlock->read_possible);
-
-  /* Finally unlock the mutex and destroy it. */
-  pthread_mutex_unlock(&rwlock->mutex);
-  int rc3 = pthread_mutex_destroy(&rwlock->mutex);
-  if (rc1 != 0)
-    return rc1;
-  if (rc2 != 0)
-    return rc2;
-  return rc3;
-}