共享記憶體

/* src/os/unix/ngx_shmem.h */

typedef struct {
    u_char      *addr; // 地址
    size_t       size; // 長度
    ngx_str_t    name; // 名稱
    ngx_log_t   *log;
    ngx_uint_t   exists;   /* unsigned  exists:1;  */
} ngx_shm_t;

/* src/os/unix/ngx_shmem.c */

ngx_int_t
ngx_shm_alloc(ngx_shm_t *shm)
{
    // 匿名記憶體對映
 

    shm->addr = (u_char *) mmap(NULL, shm->size,
                                PROT_READ|PROT_WRITE,
                                MAP_ANON|MAP_SHARED, -1, 0);

    if (shm->addr == MAP_FAILED) {
        ngx_log_error(NGX_LOG_ALERT, shm->log, ngx_errno,
                      "mmap(MAP_ANON|MAP_SHARED, %uz) failed"
 
, shm->size);
        return NGX_ERROR;
    }

    return NGX_OK;
}

void
ngx_shm_free(ngx_shm_t *shm)
{
    // 刪除對映關係
    if (munmap((void *) shm->addr, shm->size) == -1) {
        ngx_log_error(NGX_LOG_ALERT, shm->log, ngx_errno,
                      "munmap(%p, %uz) failed", shm->addr, shm->
 
size);
    }
}

原子操作

不支援原子庫下的原子操作

/* src/os/unix/ngx_atomic.h */

typedef uint32_t                    ngx_atomic_uint_t;
typedef volatile ngx_atomic_uint_t  ngx_atomic_t;

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;
}

x86架構下的原子操作

/* src/os/unix/ngx_gcc_atomic_x86.h */

#if (NGX_SMP)
#define NGX_SMP_LOCK  "lock;"
#else
#define NGX_SMP_LOCK
#endif


/*
 * "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.
 */


#if !(( __GNUC__ == 2 && __GNUC_MINOR__ <= 7 ) || ( __INTEL_COMPILER >= 800 ))

/*
 * icc 8.1 and 9.0 compile broken code with -march=pentium4 option:
 * ngx_atomic_fetch_add() always return the input "add" value,
 * so we use the gcc 2.7 version.
 *
 * icc 8.1 and 9.0 with -march=pentiumpro option or icc 7.1 compile
 * correct code.
 */

static ngx_inline ngx_atomic_int_t
ngx_atomic_fetch_add(ngx_atomic_t *value, ngx_atomic_int_t add)
{
    __asm__ volatile (

         NGX_SMP_LOCK
    "    xaddl  %0, %1;   "

    : "+r" (add) : "m" (*value) : "cc", "memory");

    return add;
}


#else

/*
 * gcc 2.7 does not support "+r", so we have to use the fixed
 * %eax ("=a" and "a") and this adds two superfluous instructions in the end
 * of code, something like this: "mov %eax, %edx / mov %edx, %eax".
 */

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;
}

#endif

自旋鎖

/* src/core/ngx_spinlock.c */

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

    for ( ;; ) {

        // lock為0表示鎖是釋放的，lock不為0表示鎖是被某個程序持有的
        if (*lock == 0 && ngx_atomic_cmp_set(lock, 0, value)) {
            return;
        }

        if (ngx_ncpu > 1) {

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

                // 檢查鎖是否釋放的頻率會越來越小
                for (i = 0; i < n; i++) {
                    ngx_cpu_pause();
                }

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

        /* #if (NGX_HAVE_SCHED_YIELD)
           // sched_yield的定義：http://www.man7.org/linux/man-pages/man2/sched_yield.2.html
           #define ngx_sched_yield()  sched_yield()
           #else
           #define ngx_sched_yield()  usleep(1)
           #endif */
        ngx_sched_yield(); // 讓出CPU
    }
}