From: Chris Darroch <chrisd@apache.org>
Date: Wed, 5 Mar 2008 22:08:25 +0000 (+0000)
Subject: add comment from event MPM's fdqueue ap_pop_pool() regarding
X-Git-Tag: 2.2.9~325
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=c3634fd742f0a1acb3303fd77498b10f8d6bcf54;p=thirdparty%2Fapache%2Fhttpd.git

add comment from event MPM's fdqueue ap_pop_pool() regarding
ABA problem and avoidance because only caller is single listener thread


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@634050 13f79535-47bb-0310-9956-ffa450edef68
---

diff --git a/server/mpm/worker/fdqueue.c b/server/mpm/worker/fdqueue.c
index 8a75d24d1ed..d46dd5361bd 100644
--- a/server/mpm/worker/fdqueue.c
+++ b/server/mpm/worker/fdqueue.c
@@ -188,6 +188,14 @@ apr_status_t ap_queue_info_wait_for_idler(fd_queue_info_t *queue_info,
     apr_atomic_dec32(&(queue_info->idlers));
 
     /* Atomically pop a pool from the recycled list */
+
+    /* This function is safe only as long as it is single threaded because
+     * it reaches into the queue and accesses "next" which can change.
+     * We are OK today because it is only called from the listener thread.
+     * cas-based pushes do not have the same limitation - any number can
+     * happen concurrently with a single cas-based pop.
+     */
+
     for (;;) {
         struct recycled_pool *first_pool = queue_info->recycled_pools;
         if (first_pool == NULL) {