LXR qemu/include/qemu/seqlock.h

   1/*
   2 * Seqlock implementation for QEMU
   3 *
   4 * Copyright Red Hat, Inc. 2013
   5 *
   6 * Author:
   7 *  Paolo Bonzini <pbonzini@redhat.com>
   8 *
   9 * This work is licensed under the terms of the GNU GPL, version 2 or later.
  10 * See the COPYING file in the top-level directory.
  11 *
  12 */
  13#ifndef QEMU_SEQLOCK_H
  14#define QEMU_SEQLOCK_H 1
  15
  16#include <qemu/atomic.h>
  17#include <qemu/thread.h>
  18
  19typedef struct QemuSeqLock QemuSeqLock;
  20
  21struct QemuSeqLock {
  22    QemuMutex *mutex;
  23    unsigned sequence;
  24};
  25
  26static inline void seqlock_init(QemuSeqLock *sl, QemuMutex *mutex)
  27{
  28    sl->mutex = mutex;
  29    sl->sequence = 0;
  30}
  31
  32/* Lock out other writers and update the count.  */
  33static inline void seqlock_write_lock(QemuSeqLock *sl)
  34{
  35    if (sl->mutex) {
  36        qemu_mutex_lock(sl->mutex);
  37    }
  38    ++sl->sequence;
  39
  40    /* Write sequence before updating other fields.  */
  41    smp_wmb();
  42}
  43
  44static inline void seqlock_write_unlock(QemuSeqLock *sl)
  45{
  46    /* Write other fields before finalizing sequence.  */
  47    smp_wmb();
  48
  49    ++sl->sequence;
  50    if (sl->mutex) {
  51        qemu_mutex_unlock(sl->mutex);
  52    }
  53}
  54
  55static inline unsigned seqlock_read_begin(QemuSeqLock *sl)
  56{
  57    /* Always fail if a write is in progress.  */
  58    unsigned ret = atomic_read(&sl->sequence);
  59
  60    /* Read sequence before reading other fields.  */
  61    smp_rmb();
  62    return ret & ~1;
  63}
  64
  65static inline int seqlock_read_retry(const QemuSeqLock *sl, unsigned start)
  66{
  67    /* Read other fields before reading final sequence.  */
  68    smp_rmb();
  69    return unlikely(atomic_read(&sl->sequence) != start);
  70}
  71
  72#endif
  73