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stats64.c (3371B)

      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     qatomic_add(&s->lock, 2);
     22 
     23     /* If there is a concurrent writer, wait for it.  */
     24     while (qatomic_read(&s->lock) & 1) {
     25         cpu_relax();
     26     }
     27 }
     28 
     29 static inline void stat64_rdunlock(Stat64 *s)
     30 {
     31     qatomic_sub(&s->lock, 2);
     32 }
     33 
     34 static inline bool stat64_wrtrylock(Stat64 *s)
     35 {
     36     return qatomic_cmpxchg(&s->lock, 0, 1) == 0;
     37 }
     38 
     39 static inline void stat64_wrunlock(Stat64 *s)
     40 {
     41     qatomic_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 = qatomic_read(&s->high);
     54     low = qatomic_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 = qatomic_fetch_add(&s->low, low);
     74     high += (old + low) < old;
     75     qatomic_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 = qatomic_read(&s->high);
     91     low = qatomic_read(&s->low);
     92 
     93     orig = ((uint64_t)high << 32) | low;
     94     if (value < orig) {
     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         qatomic_set(&s->low, (uint32_t)value);
    102         smp_wmb();
    103         qatomic_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 = qatomic_read(&s->high);
    120     low = qatomic_read(&s->low);
    121 
    122     orig = ((uint64_t)high << 32) | low;
    123     if (value > orig) {
    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         qatomic_set(&s->low, (uint32_t)value);
    131         smp_wmb();
    132         qatomic_set(&s->high, value >> 32);
    133     }
    134     stat64_wrunlock(s);
    135     return true;
    136 }
    137 #endif