Commit 5194f42f authored by Yann Ylavic's avatar Yann Ylavic
Browse files

mpm_event: ap_queue_info_try_get_idler() may atomically decrement and then 

re-increment the number idlers if it went under or to zero.  We can avoid
this by switching to a compare-and-swap scheme.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1802535 13f79535-47bb-0310-9956-ffa450edef68
parent d16ac821

server/mpm/event/fdqueue.c

+16 −16
Original line number Diff line number Diff line
@@ -27,10 +27,10 @@ struct recycled_pool 


struct fd_queue_info_t

{

    apr_uint32_t idlers;     /**

    apr_uint32_t volatile idlers; /**

                                   * >= zero_pt: number of idle worker threads

                              * < zero_pt:  number of threads blocked waiting

                              *             for an idle worker

                                   * <  zero_pt: number of threads blocked,

                                   *             waiting for an idle worker

                                   */

    apr_thread_mutex_t *idlers_mutex;

    apr_thread_cond_t *wait_for_idler;
@@ -38,7 +38,7 @@ struct fd_queue_info_t 
    int max_idlers;

    int max_recycled_pools;

    apr_uint32_t recycled_pools_count;

    struct recycled_pool *recycled_pools;

    struct recycled_pool *volatile recycled_pools;

};



static apr_status_t queue_info_cleanup(void *data_)
@@ -123,18 +123,18 @@ apr_status_t ap_queue_info_set_idle(fd_queue_info_t * queue_info, 


apr_status_t ap_queue_info_try_get_idler(fd_queue_info_t * queue_info)

{

    /* Don't block if there isn't any idle worker.

     * apr_atomic_add32(x, -1) does the same as dec32(x), except

     * that it returns the previous value (unlike dec32's bool).

     *

     * XXX: why don't we consume the last idler?

     */

    if (apr_atomic_add32(&(queue_info->idlers), -1) <= zero_pt + 1) {

        apr_atomic_inc32(&(queue_info->idlers));    /* back out dec */

    /* Don't block if there isn't any idle worker. */

    for (;;) {

        apr_uint32_t idlers = queue_info->idlers;

        if (idlers <= zero_pt) {

            return APR_EAGAIN;

        }

        if (apr_atomic_cas32(&queue_info->idlers, idlers - 1,

                             idlers) == idlers) {

            return APR_SUCCESS;

        }

    }

}



apr_status_t ap_queue_info_wait_for_idler(fd_queue_info_t * queue_info,

                                          int *had_to_block)