From de12a7878c11f3b282d640888aa635e0711d0b5e Mon Sep 17 00:00:00 2001 From: "Eric W. Biederman" Date: Mon, 10 Apr 2006 17:16:49 -0600 Subject: [PATCH] [PATCH] de_thread: Don't confuse users do_each_thread. Oleg Nesterov spotted two interesting bugs with the current de_thread code. The simplest is a long standing double decrement of __get_cpu_var(process_counts) in __unhash_process. Caused by two processes exiting when only one was created. The other is that since we no longer detach from the thread_group list it is possible for do_each_thread when run under the tasklist_lock to see the same task_struct twice. Once on the task list as a thread_group_leader, and once on the thread list of another thread. The double appearance in do_each_thread can cause a double increment of mm_core_waiters in zap_threads resulting in problems later on in coredump_wait. To remedy those two problems this patch takes the simple approach of changing the old thread group leader into a child thread. The only routine in release_task that cares is __unhash_process, and it can be trivially seen that we handle cleaning up a thread group leader properly. Since de_thread doesn't change the pid of the exiting leader process and instead shares it with the new leader process. I change thread_group_leader to recognize group leadership based on the group_leader field and not based on pids. This should also be slightly cheaper then the existing thread_group_leader macro. I performed a quick audit and I couldn't see any user of thread_group_leader that cared about the difference. Signed-off-by: Eric W. Biederman Signed-off-by: Linus Torvalds --- fs/exec.c | 7 ++++++- include/linux/sched.h | 3 ++- 2 files changed, 8 insertions(+), 2 deletions(-) diff --git a/fs/exec.c b/fs/exec.c index 0291a68a3626..4d38ad0b70d6 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -723,7 +723,12 @@ static int de_thread(struct task_struct *tsk) current->parent = current->real_parent = leader->real_parent; leader->parent = leader->real_parent = child_reaper; current->group_leader = current; - leader->group_leader = leader; + leader->group_leader = current; + + /* Reduce leader to a thread */ + detach_pid(leader, PIDTYPE_PGID); + detach_pid(leader, PIDTYPE_SID); + list_del_init(&leader->tasks); add_parent(current); add_parent(leader); diff --git a/include/linux/sched.h b/include/linux/sched.h index 541f4828f5e7..a3e4f6b503a3 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1203,7 +1203,8 @@ extern void wait_task_inactive(task_t * p); #define while_each_thread(g, t) \ while ((t = next_thread(t)) != g) -#define thread_group_leader(p) (p->pid == p->tgid) +/* de_thread depends on thread_group_leader not being a pid based check */ +#define thread_group_leader(p) (p == p->group_leader) static inline task_t *next_thread(task_t *p) {