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