SIGSEGV in remove_workers_node (mod_proxy_cluster.so) when using LoadBalancingGroup ----------------------------------------------------------------------------------- Key: MODCLUSTER-526 URL: https://issues.jboss.org/browse/MODCLUSTER-526 Project: mod_cluster Issue Type: Bug Components: Native (httpd modules) Affects Versions: 1.3.3.Final Environment: Fedora 20, x86_64, httpd 2.4.20 mpm_event Reporter: Michal Karm Babacek Assignee: Michal Karm Babacek Priority: Blocker Fix For: 1.3.6.CR3 h3. Setup * 3 tomcats * 2 load balancing groups * 1 request every 3 seconds (no load at all) * shutdown and kill of various nodes * no later than third kill/start iteration causes SIGSEGV h3. SIGSEGV {code} #if AP_MODULE_MAGIC_AT_LEAST(20101223,1) /* Here that is tricky the worker needs shared memory but we don't and CONFIG will reset it */ helper->index = 0; /* mark it removed */ worker->s = helper->shared; crash---> memcpy(worker->s, stat, sizeof(proxy_worker_shared)); #else worker->id = 0; /* mark it removed */ #endif {code} h3. Behavior {code} 957 helper = (proxy_cluster_helper *) worker->context; 961 if (helper) { 962 i = helper->count_active; 963 } 968 if (i == 0) { 971 proxy_worker_shared *stat = worker->s; 972 proxy_cluster_helper *helper = (proxy_cluster_helper *) worker->context; {code} At this point, {{helper->shared}} points to a {{proxy_worker_shared}} structure that appears to be properly filled. {code} 999 if (worker->cp->pool) { 1000 apr_pool_destroy(worker->cp->pool); 1001 worker->cp->pool = NULL; 1002 } {code} Regardless of the aforementioned block being there or nor (stuffed after 1010), {{helper->shared}} suddenly points to {{NULL}}. {code} 1008 helper->index = 0; 1009 worker->s = helper->shared; {code} Above assignment makes {{worker->s}} pointing to NULL. {code} 1010 memcpy(worker->s, stat, sizeof(proxy_worker_shared)); {code} And here we go :( IMHO, _other thread_ already cleared that memory and nulled the pointer, because it absolutely doesn't happen if I run 1 process and 1 thread. The [workaround that prevents the core|https://github.com/modcluster/mod_cluster/pull/207] looks like this: {code} if (helper->shared) { worker->s = helper->shared; memcpy(worker->s, stat, sizeof(proxy_worker_shared)); } {code} h3. How do we fix it? Any ideas? [~jfclere]