include/qemu/stats64.h

   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 #ifndef QEMU_STATS64_H
  13 #define QEMU_STATS64_H
  14
  15 #include "qemu/atomic.h"
  16
  17 /* This provides atomic operations on 64-bit type, using a reader-writer
  18  * spinlock on architectures that do not have 64-bit accesses.  Even on
  19  * those architectures, it tries hard not to take the lock.
  20  */
  21
  22 typedef struct Stat64 {
  23 #ifdef CONFIG_ATOMIC64
  24     uint64_t value;
  25 #else
  26     uint32_t low, high;
  27     uint32_t lock;
  28 #endif
  29 } Stat64;
  30
  31 #ifdef CONFIG_ATOMIC64
  32 static inline void stat64_init(Stat64 *s, uint64_t value)
  33 {
  34     /* This is not guaranteed to be atomic! */
  35     *s = (Stat64) { value };
  36 }
  37
  38 static inline uint64_t stat64_get(const Stat64 *s)
  39 {
  40     return atomic_read__nocheck(&s->value);
  41 }
  42
  43 static inline void stat64_add(Stat64 *s, uint64_t value)
  44 {
  45     atomic_add(&s->value, value);
  46 }
  47
  48 static inline void stat64_min(Stat64 *s, uint64_t value)
  49 {
  50     uint64_t orig = atomic_read__nocheck(&s->value);
  51     while (orig > value) {
  52         orig = atomic_cmpxchg__nocheck(&s->value, orig, value);
  53     }
  54 }
  55
  56 static inline void stat64_max(Stat64 *s, uint64_t value)
  57 {
  58     uint64_t orig = atomic_read__nocheck(&s->value);
  59     while (orig < value) {
  60         orig = atomic_cmpxchg__nocheck(&s->value, orig, value);
  61     }
  62 }
  63 #else
  64 uint64_t stat64_get(const Stat64 *s);
  65 bool stat64_min_slow(Stat64 *s, uint64_t value);
  66 bool stat64_max_slow(Stat64 *s, uint64_t value);
  67 bool stat64_add32_carry(Stat64 *s, uint32_t low, uint32_t high);
  68
  69 static inline void stat64_init(Stat64 *s, uint64_t value)
  70 {
  71     /* This is not guaranteed to be atomic! */
  72     *s = (Stat64) { .low = value, .high = value >> 32, .lock = 0 };
  73 }
  74
  75 static inline void stat64_add(Stat64 *s, uint64_t value)
  76 {
  77     uint32_t low, high;
  78     high = value >> 32;
  79     low = (uint32_t) value;
  80     if (!low) {
  81         if (high) {
  82             atomic_add(&s->high, high);
  83         }
  84         return;
  85     }
  86
  87     for (;;) {
  88         uint32_t orig = s->low;
  89         uint32_t result = orig + low;
  90         uint32_t old;
  91
  92         if (result < low || high) {
  93             /* If the high part is affected, take the lock.  */
  94             if (stat64_add32_carry(s, low, high)) {
  95                 return;
  96             }
  97             continue;
  98         }
  99
 100         /* No carry, try with a 32-bit cmpxchg.  The result is independent of
 101          * the high 32 bits, so it can race just fine with stat64_add32_carry
 102          * and even stat64_get!
 103          */
 104         old = atomic_cmpxchg(&s->low, orig, result);
 105         if (orig == old) {
 106             return;
 107         }
 108     }
 109 }
 110
 111 static inline void stat64_min(Stat64 *s, uint64_t value)
 112 {
 113     uint32_t low, high;
 114     uint32_t orig_low, orig_high;
 115
 116     high = value >> 32;
 117     low = (uint32_t) value;
 118     do {
 119         orig_high = atomic_read(&s->high);
 120         if (orig_high < high) {
 121             return;
 122         }
 123
 124         if (orig_high == high) {
 125             /* High 32 bits are equal.  Read low after high, otherwise we
 126              * can get a false positive (e.g. 0x1235,0x0000 changes to
 127              * 0x1234,0x8000 and we read it as 0x1234,0x0000). Pairs with
 128              * the write barrier in stat64_min_slow.
 129              */
 130             smp_rmb();
 131             orig_low = atomic_read(&s->low);
 132             if (orig_low <= low) {
 133                 return;
 134             }
 135
 136             /* See if we were lucky and a writer raced against us.  The
 137              * barrier is theoretically unnecessary, but if we remove it
 138              * we may miss being lucky.
 139              */
 140             smp_rmb();
 141             orig_high = atomic_read(&s->high);
 142             if (orig_high < high) {
 143                 return;
 144             }
 145         }
 146
 147         /* If the value changes in any way, we have to take the lock.  */
 148     } while (!stat64_min_slow(s, value));
 149 }
 150
 151 static inline void stat64_max(Stat64 *s, uint64_t value)
 152 {
 153     uint32_t low, high;
 154     uint32_t orig_low, orig_high;
 155
 156     high = value >> 32;
 157     low = (uint32_t) value;
 158     do {
 159         orig_high = atomic_read(&s->high);
 160         if (orig_high > high) {
 161             return;
 162         }
 163
 164         if (orig_high == high) {
 165             /* High 32 bits are equal.  Read low after high, otherwise we
 166              * can get a false positive (e.g. 0x1234,0x8000 changes to
 167              * 0x1235,0x0000 and we read it as 0x1235,0x8000). Pairs with
 168              * the write barrier in stat64_max_slow.
 169              */
 170             smp_rmb();
 171             orig_low = atomic_read(&s->low);
 172             if (orig_low >= low) {
 173                 return;
 174             }
 175
 176             /* See if we were lucky and a writer raced against us.  The
 177              * barrier is theoretically unnecessary, but if we remove it
 178              * we may miss being lucky.
 179              */
 180             smp_rmb();
 181             orig_high = atomic_read(&s->high);
 182             if (orig_high > high) {
 183                 return;
 184             }
 185         }
 186
 187         /* If the value changes in any way, we have to take the lock.  */
 188     } while (!stat64_max_slow(s, value));
 189 }
 190
 191 #endif
 192
 193 #endif