On 05/15, Christian Brauner wrote: > > On Wed, May 15, 2019 at 04:38:58PM +0200, Oleg Nesterov wrote: > > > > it seems that you can do a single check > > > > tsk = pid_task(p, PIDTYPE_TGID); > > if (!tsk) > > ret = -ESRCH; > > > > this even looks more correct if we race with exec changing the leader. > > The logic here being that you can only reach the thread_group leader > from struct pid if PIDTYPE_PID == PIDTYPE_TGID for this struct pid? Not exactly... it is not that PIDTYPE_PID == PIDTYPE_TGID for this pid, struct pid has no "type" or something like this. The logic is that pid->tasks[PIDTYPE_XXX] is the list of task which use this pid as "XXX" type. For example, clone(CLONE_THREAD) creates a pid which has a single non- empty list, pid->tasks[PIDTYPE_PID]. This pid can't be used as TGID or SID. So if pid_task(PIDTYPE_TGID) returns non-NULL we know that this pid was used for a group-leader, see copy_process() which does if (thread_group_leader(p)) attach_pid(p, PIDTYPE_TGID); If we race with exec which changes the leader pid_task(TGID) can return the old leader. We do not care, but this means that we should not check thread_group_leader(). Oleg.
