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 #include "qemu/lockable.h"
  20
  21 typedef struct QemuSeqLock QemuSeqLock;
  22
  23 struct QemuSeqLock {
  24     unsigned sequence;
  25 };
  26
  27 static inline void seqlock_init(QemuSeqLock *sl)
  28 {
  29     sl->sequence = 0;
  30 }
  31
  32 /* Lock out other writers and update the count.  */
  33 static inline void seqlock_write_begin(QemuSeqLock *sl)
  34 {
  35     qatomic_set(&sl->sequence, sl->sequence + 1);
  36
  37     /* Write sequence before updating other fields.  */
  38     smp_wmb();
  39 }
  40
  41 static inline void seqlock_write_end(QemuSeqLock *sl)
  42 {
  43     /* Write other fields before finalizing sequence.  */
  44     smp_wmb();
  45
  46     qatomic_set(&sl->sequence, sl->sequence + 1);
  47 }
  48
  49 /* Lock out other writers and update the count.  */
  50 static inline void seqlock_write_lock_impl(QemuSeqLock *sl, QemuLockable *lock)
  51 {
  52     qemu_lockable_lock(lock);
  53     seqlock_write_begin(sl);
  54 }
  55 #define seqlock_write_lock(sl, lock) \
  56     seqlock_write_lock_impl(sl, QEMU_MAKE_LOCKABLE(lock))
  57
  58 /* Update the count and release the lock.  */
  59 static inline void seqlock_write_unlock_impl(QemuSeqLock *sl, QemuLockable *lock)
  60 {
  61     seqlock_write_end(sl);
  62     qemu_lockable_unlock(lock);
  63 }
  64 #define seqlock_write_unlock(sl, lock) \
  65     seqlock_write_unlock_impl(sl, QEMU_MAKE_LOCKABLE(lock))
  66
  67
  68 static inline unsigned seqlock_read_begin(const QemuSeqLock *sl)
  69 {
  70     /* Always fail if a write is in progress.  */
  71     unsigned ret = qatomic_read(&sl->sequence);
  72
  73     /* Read sequence before reading other fields.  */
  74     smp_rmb();
  75     return ret & ~1;
  76 }
  77
  78 static inline int seqlock_read_retry(const QemuSeqLock *sl, unsigned start)
  79 {
  80     /* Read other fields before reading final sequence.  */
  81     smp_rmb();
  82     return unlikely(qatomic_read(&sl->sequence) != start);
  83 }
  84
  85 #endif