Nginx でのアトミック操作

アトミック操作をサポートしないプロセッサでは、Nginx は独自のアトミック操作を提供しますが、アトミック操作は C 言語を使用してシミュレートされます:

static ngx_inline ngx_atomic_uint_t
ngx_atomic_cmp_set(ngx_atomic_t *lock, ngx_atomic_uint_t old,
     ngx_atomic_uint_t set)
{
     if (*lock == old) {
         *lock = set;
         return 1;
     }

     return 0;
}

static ngx_inline ngx_atomic_int_t
ngx_atomic_fetch_add(ngx_atomic_t *value, ngx_atomic_int_t add)
{
     ngx_atomic_int_t  old;

     old = *value;
     *value += add;

     return old;
}

アトミック操作をサポートするプロセッサ (x86 を例にします) :

#define NGX_SMP_LOCK  "lock;"

/*
* "cmpxchgl  r, [m]":
*
*     if (eax == [m]) {
*         zf = 1;
*         [m] = r;
*     } else {
*         zf = 0;
*         eax = [m];
*     }
*
*
* The "r" means the general register.
* The "=a" and "a" are the %eax register.
* Although we can return result in any register, we use "a" because it is
* used in cmpxchgl anyway.  The result is actually in %al but not in %eax,
* however, as the code is inlined gcc can test %al as well as %eax,
* and icc adds "movzbl %al, %eax" by itself.
*
* The "cc" means that flags were changed.
*/

static ngx_inline ngx_atomic_uint_t
ngx_atomic_cmp_set(ngx_atomic_t *lock, ngx_atomic_uint_t old,
    ngx_atomic_uint_t set)
{
    u_char  res;

    __asm__ volatile (

         NGX_SMP_LOCK
    "    cmpxchgl  %3, %1;   "
    "    sete      %0;       "

    : "=a" (res) : "m" (*lock), "a" (old), "r" (set) : "cc", "memory");

    return res;
}

/*
* "xaddl  r, [m]":
*
*     temp = [m];
*     [m] += r;
*     r = temp;
*
*
* The "+r" means the general register.
* The "cc" means that flags were changed.
*/
static ngx_inline ngx_atomic_int_t
ngx_atomic_fetch_add(ngx_atomic_t *value, ngx_atomic_int_t add)
{
    ngx_atomic_uint_t  old;

    __asm__ volatile (

         NGX_SMP_LOCK
    "    xaddl  %2, %1;   "

    : "=a" (old) : "m" (*value), "a" (add) : "cc", "memory");

    return old;
}

スピンロック:

/* old "as" does not support "pause" opcode */
#define ngx_cpu_pause()         __asm__ (".byte 0xf3, 0x90")
#define ngx_sched_yield()  sched_yield()

void
ngx_spinlock(ngx_atomic_t *lock, ngx_atomic_int_t value, ngx_uint_t spin)
{

#if (NGX_HAVE_ATOMIC_OPS)

    ngx_uint_t  i, n;

    for ( ;; ) {

        if (*lock == 0 && ngx_atomic_cmp_set(lock, 0, value)) {
            return;
        }

        if (ngx_ncpu > 1) {

            for (n = 1; n < spin; n <<= 1) {

                for (i = 0; i < n; i++) {
                    ngx_cpu_pause();
                }

                if (*lock == 0 && ngx_atomic_cmp_set(lock, 0, value)) {
                    return;
                }
            }
        }

        ngx_sched_yield();
    }

信号:

ngx_int_t
ngx_init_signals(ngx_log_t *log)
{
    ngx_signal_t      *sig;
    struct sigaction   sa;

    for (sig = signals; sig->signo != 0; sig++) {
        ngx_memzero(&sa, sizeof(struct sigaction));
        sa.sa_handler = sig->handler;
        sigemptyset(&sa.sa_mask);
        if (sigaction(sig->signo, &sa, NULL) == -1) {
#if (NGX_VALGRIND)
            ngx_log_error(NGX_LOG_ALERT, log, ngx_errno,
                          "sigaction(%s) failed, ignored", sig->signame);
#else
            ngx_log_error(NGX_LOG_EMERG, log, ngx_errno,
                          "sigaction(%s) failed", sig->signame);
            return NGX_ERROR;
#endif
        }
    }

    return NGX_OK;
}

上記では、Nginx のアトミック操作をその側面も含めて紹介しましたが、PHP チュートリアルに興味のある友人に役立つことを願っています。