util/stats64.c

   1 /*
   2  * Atomic operations on 64-bit quantities.
   3  *
   4  * Copyright (C) 2017 Red Hat, Inc.
   5  *
   6  * Author: Paolo Bonzini <pbonzini@redhat.com>
   7  *
   8  * This work is licensed under the terms of the GNU GPL, version 2 or later.
   9  * See the COPYING file in the top-level directory.
  10  */
  11
  12 #include "qemu/osdep.h"
  13 #include "qemu/atomic.h"
  14 #include "qemu/stats64.h"
  15 #include "qemu/processor.h"
  16
  17 #ifndef CONFIG_ATOMIC64
  18 static inline void stat64_rdlock(Stat64 *s)
  19 {
  20     /* Keep out incoming writers to avoid them starving us. */
  21     atomic_add(&s->lock, 2);
  22
  23     /* If there is a concurrent writer, wait for it.  */
  24     while (atomic_read(&s->lock) & 1) {
  25         cpu_relax();
  26     }
  27 }
  28
  29 static inline void stat64_rdunlock(Stat64 *s)
  30 {
  31     atomic_sub(&s->lock, 2);
  32 }
  33
  34 static inline bool stat64_wrtrylock(Stat64 *s)
  35 {
  36     return atomic_cmpxchg(&s->lock, 0, 1) == 0;
  37 }
  38
  39 static inline void stat64_wrunlock(Stat64 *s)
  40 {
  41     atomic_dec(&s->lock);
  42 }
  43
  44 uint64_t stat64_get(const Stat64 *s)
  45 {
  46     uint32_t high, low;
  47
  48     stat64_rdlock((Stat64 *)s);
  49
  50     /* 64-bit writes always take the lock, so we can read in
  51      * any order.
  52      */
  53     high = atomic_read(&s->high);
  54     low = atomic_read(&s->low);
  55     stat64_rdunlock((Stat64 *)s);
  56
  57     return ((uint64_t)high << 32) | low;
  58 }
  59
  60 bool stat64_add32_carry(Stat64 *s, uint32_t low, uint32_t high)
  61 {
  62     uint32_t old;
  63
  64     if (!stat64_wrtrylock(s)) {
  65         cpu_relax();
  66         return false;
  67     }
  68
  69     /* 64-bit reads always take the lock, so they don't care about the
  70      * order of our update.  By updating s->low first, we can check
  71      * whether we have to carry into s->high.
  72      */
  73     old = atomic_fetch_add(&s->low, low);
  74     high += (old + low) < old;
  75     atomic_add(&s->high, high);
  76     stat64_wrunlock(s);
  77     return true;
  78 }
  79
  80 bool stat64_min_slow(Stat64 *s, uint64_t value)
  81 {
  82     uint32_t high, low;
  83     uint64_t orig;
  84
  85     if (!stat64_wrtrylock(s)) {
  86         cpu_relax();
  87         return false;
  88     }
  89
  90     high = atomic_read(&s->high);
  91     low = atomic_read(&s->low);
  92
  93     orig = ((uint64_t)high << 32) | low;
  94     if (orig < value) {
  95         /* We have to set low before high, just like stat64_min reads
  96          * high before low.  The value may become higher temporarily, but
  97          * stat64_get does not notice (it takes the lock) and the only ill
  98          * effect on stat64_min is that the slow path may be triggered
  99          * unnecessarily.
 100          */
 101         atomic_set(&s->low, (uint32_t)value);
 102         smp_wmb();
 103         atomic_set(&s->high, value >> 32);
 104     }
 105     stat64_wrunlock(s);
 106     return true;
 107 }
 108
 109 bool stat64_max_slow(Stat64 *s, uint64_t value)
 110 {
 111     uint32_t high, low;
 112     uint64_t orig;
 113
 114     if (!stat64_wrtrylock(s)) {
 115         cpu_relax();
 116         return false;
 117     }
 118
 119     high = atomic_read(&s->high);
 120     low = atomic_read(&s->low);
 121
 122     orig = ((uint64_t)high << 32) | low;
 123     if (orig > value) {
 124         /* We have to set low before high, just like stat64_max reads
 125          * high before low.  The value may become lower temporarily, but
 126          * stat64_get does not notice (it takes the lock) and the only ill
 127          * effect on stat64_max is that the slow path may be triggered
 128          * unnecessarily.
 129          */
 130         atomic_set(&s->low, (uint32_t)value);
 131         smp_wmb();
 132         atomic_set(&s->high, value >> 32);
 133     }
 134     stat64_wrunlock(s);
 135     return true;
 136 }
 137 #endif