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
  14 #ifndef QEMU_SEQLOCK_H
  15 #define QEMU_SEQLOCK_H
  16
  17 #include "qemu/atomic.h"
  18 #include "qemu/thread.h"
  19
  20 typedef struct QemuSeqLock QemuSeqLock;
  21
  22 struct QemuSeqLock {
  23     unsigned sequence;
  24 };
  25
  26 static inline void seqlock_init(QemuSeqLock *sl)
  27 {
  28     sl->sequence = 0;
  29 }
  30
  31 /* Lock out other writers and update the count.  */
  32 static inline void seqlock_write_begin(QemuSeqLock *sl)
  33 {
  34     atomic_set(&sl->sequence, sl->sequence + 1);
  35
  36     /* Write sequence before updating other fields.  */
  37     smp_wmb();
  38 }
  39
  40 static inline void seqlock_write_end(QemuSeqLock *sl)
  41 {
  42     /* Write other fields before finalizing sequence.  */
  43     smp_wmb();
  44
  45     atomic_set(&sl->sequence, sl->sequence + 1);
  46 }
  47
  48 static inline unsigned seqlock_read_begin(QemuSeqLock *sl)
  49 {
  50     /* Always fail if a write is in progress.  */
  51     unsigned ret = atomic_read(&sl->sequence);
  52
  53     /* Read sequence before reading other fields.  */
  54     smp_rmb();
  55     return ret & ~1;
  56 }
  57
  58 static inline int seqlock_read_retry(const QemuSeqLock *sl, unsigned start)
  59 {
  60     /* Read other fields before reading final sequence.  */
  61     smp_rmb();
  62     return unlikely(atomic_read(&sl->sequence) != start);
  63 }
  64
  65 #endif