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.
@@ -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)
