src/core/ngx_rwlock.c

/*
 * Copyright (C) Ruslan Ermilov
 * Copyright (C) Nginx, Inc.
 */


#include <ngx_config.h>
#include <ngx_core.h>


#if (NGX_HAVE_ATOMIC_OPS)


#define NGX_RWLOCK_SPIN   2048
#define NGX_RWLOCK_WLOCK  ((ngx_atomic_uint_t) -1)


void
ngx_rwlock_wlock(ngx_atomic_t *lock)
{
    ngx_uint_t  i, n;

    for ( ;; ) {

        if (ngx_trylock(lock, NGX_RWLOCK_WLOCK)) {
            return;
        }

        if (ngx_ncpu > 1) {

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

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

                if (ngx_trylock(lock, NGX_RWLOCK_WLOCK)) {
                    return;
                }
            }
        }

        ngx_sched_yield();
    }
}


void
ngx_rwlock_rlock(ngx_atomic_t *lock)
{
    ngx_uint_t         i, n;
    ngx_atomic_uint_t  readers;

    for ( ;; ) {
        readers = ngx_atomic_load(lock);

        if (readers != NGX_RWLOCK_WLOCK
            && ngx_atomic_cmp_set(lock, readers, readers + 1))
        {
            return;
        }

        if (ngx_ncpu > 1) {

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

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

                readers = ngx_atomic_load(lock);

                if (readers != NGX_RWLOCK_WLOCK
                    && ngx_atomic_cmp_set(lock, readers, readers + 1))
                {
                    return;
                }
            }
        }

        ngx_sched_yield();
    }
}


void
ngx_rwlock_unlock(ngx_atomic_t *lock)
{
    ngx_atomic_uint_t  readers;

    readers = ngx_atomic_load(lock);

    if (readers == NGX_RWLOCK_WLOCK) {
        (void) ngx_atomic_cmp_set(lock, NGX_RWLOCK_WLOCK, 0);
        return;
    }

    for ( ;; ) {

        if (ngx_atomic_cmp_set(lock, readers, readers - 1)) {
            return;
        }

        readers = ngx_atomic_load(lock);
    }
}


void
ngx_rwlock_downgrade(ngx_atomic_t *lock)
{
    ngx_atomic_uint_t  readers;

    readers = ngx_atomic_load(lock);

    if (readers == NGX_RWLOCK_WLOCK) {
        (void) ngx_atomic_cmp_set(lock, NGX_RWLOCK_WLOCK, 1);
    }
}


#else

#if (NGX_HTTP_UPSTREAM_ZONE || NGX_STREAM_UPSTREAM_ZONE || NGX_COMPAT)

#error ngx_atomic_cmp_set() is not defined!

#endif

#endif