summaryrefslogtreecommitdiff
path: root/util/stats64.c
blob: 897613c94965dee4783c8ef38fc6dc309a57a766 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
/*
 * Atomic operations on 64-bit quantities.
 *
 * Copyright (C) 2017 Red Hat, Inc.
 *
 * Author: Paolo Bonzini <pbonzini@redhat.com>
 *
 * This work is licensed under the terms of the GNU GPL, version 2 or later.
 * See the COPYING file in the top-level directory.
 */

#include "qemu/osdep.h"
#include "qemu/atomic.h"
#include "qemu/stats64.h"
#include "qemu/processor.h"

#ifndef CONFIG_ATOMIC64
static inline void stat64_rdlock(Stat64 *s)
{
    /* Keep out incoming writers to avoid them starving us. */
    qatomic_add(&s->lock, 2);

    /* If there is a concurrent writer, wait for it.  */
    while (qatomic_read(&s->lock) & 1) {
        cpu_relax();
    }
}

static inline void stat64_rdunlock(Stat64 *s)
{
    qatomic_sub(&s->lock, 2);
}

static inline bool stat64_wrtrylock(Stat64 *s)
{
    return qatomic_cmpxchg(&s->lock, 0, 1) == 0;
}

static inline void stat64_wrunlock(Stat64 *s)
{
    qatomic_dec(&s->lock);
}

uint64_t stat64_get(const Stat64 *s)
{
    uint32_t high, low;

    stat64_rdlock((Stat64 *)s);

    /* 64-bit writes always take the lock, so we can read in
     * any order.
     */
    high = qatomic_read(&s->high);
    low = qatomic_read(&s->low);
    stat64_rdunlock((Stat64 *)s);

    return ((uint64_t)high << 32) | low;
}

bool stat64_add32_carry(Stat64 *s, uint32_t low, uint32_t high)
{
    uint32_t old;

    if (!stat64_wrtrylock(s)) {
        cpu_relax();
        return false;
    }

    /* 64-bit reads always take the lock, so they don't care about the
     * order of our update.  By updating s->low first, we can check
     * whether we have to carry into s->high.
     */
    old = qatomic_fetch_add(&s->low, low);
    high += (old + low) < old;
    qatomic_add(&s->high, high);
    stat64_wrunlock(s);
    return true;
}

bool stat64_min_slow(Stat64 *s, uint64_t value)
{
    uint32_t high, low;
    uint64_t orig;

    if (!stat64_wrtrylock(s)) {
        cpu_relax();
        return false;
    }

    high = qatomic_read(&s->high);
    low = qatomic_read(&s->low);

    orig = ((uint64_t)high << 32) | low;
    if (value < orig) {
        /* We have to set low before high, just like stat64_min reads
         * high before low.  The value may become higher temporarily, but
         * stat64_get does not notice (it takes the lock) and the only ill
         * effect on stat64_min is that the slow path may be triggered
         * unnecessarily.
         */
        qatomic_set(&s->low, (uint32_t)value);
        smp_wmb();
        qatomic_set(&s->high, value >> 32);
    }
    stat64_wrunlock(s);
    return true;
}

bool stat64_max_slow(Stat64 *s, uint64_t value)
{
    uint32_t high, low;
    uint64_t orig;

    if (!stat64_wrtrylock(s)) {
        cpu_relax();
        return false;
    }

    high = qatomic_read(&s->high);
    low = qatomic_read(&s->low);

    orig = ((uint64_t)high << 32) | low;
    if (value > orig) {
        /* We have to set low before high, just like stat64_max reads
         * high before low.  The value may become lower temporarily, but
         * stat64_get does not notice (it takes the lock) and the only ill
         * effect on stat64_max is that the slow path may be triggered
         * unnecessarily.
         */
        qatomic_set(&s->low, (uint32_t)value);
        smp_wmb();
        qatomic_set(&s->high, value >> 32);
    }
    stat64_wrunlock(s);
    return true;
}
#endif