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
  19 typedef struct QemuSeqLock QemuSeqLock;
  20
  21 struct QemuSeqLock {
  22     QemuMutex *mutex;
  23     unsigned sequence;
  24 };
  25
  26 static 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.  */
  33 static 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
  44 static 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
  55 static inline unsigned seqlock_read_begin(QemuSeqLock *sl)
  56 {
  57     /* Always fail if a write is in progress.  */
  58     unsigned ret = sl->sequence & ~1;
  59
  60     /* Read sequence before reading other fields.  */
  61     smp_rmb();
  62     return ret;
  63 }
  64
  65 static int seqlock_read_retry(const QemuSeqLock *sl, unsigned start)
  66 {
  67     /* Read other fields before reading final sequence.  */
  68     smp_rmb();
  69     return unlikely(sl->sequence != start);
  70 }
  71
  72 #endif