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| author | Yann Ylavic <ylavic@apache.org> | 2017-07-20 23:34:47 +0000 |
|---|---|---|
| committer | Yann Ylavic <ylavic@apache.org> | 2017-07-20 23:34:47 +0000 |
| commit | 7ea6d8b113bc4e3e02c982ce2ad77249beada007 (patch) | |
| tree | d815af62b3e7520df64a495e07aae0d10c93def5 | |
| parent | 7157b8a35eb64a659ad6a44feec824af2155f05f (diff) | |
| download | httpd-7ea6d8b113bc4e3e02c982ce2ad77249beada007.tar.gz | |
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
| -rw-r--r-- | server/mpm/event/fdqueue.c | 32 |
1 files changed, 16 insertions, 16 deletions
diff --git a/server/mpm/event/fdqueue.c b/server/mpm/event/fdqueue.c index 7775a157d0..5b0192b2ee 100644 --- a/server/mpm/event/fdqueue.c +++ b/server/mpm/event/fdqueue.c @@ -27,18 +27,18 @@ struct recycled_pool struct fd_queue_info_t { - apr_uint32_t idlers; /** - * >= zero_pt: number of idle worker threads - * < zero_pt: number of threads blocked waiting - * for an idle worker - */ + apr_uint32_t volatile idlers; /** + * >= zero_pt: number of idle worker threads + * < zero_pt: number of threads blocked, + * waiting for an idle worker + */ apr_thread_mutex_t *idlers_mutex; apr_thread_cond_t *wait_for_idler; int terminated; 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_) @@ -123,17 +123,17 @@ 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 */ - return APR_EAGAIN; + /* 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; + } } - return APR_SUCCESS; } apr_status_t ap_queue_info_wait_for_idler(fd_queue_info_t * queue_info, |