On Wed, May 15, 2019 at 05:19:13PM +0200, Oleg Nesterov wrote: > 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 Ah, this was what I was asking myself when I worked on thread-specific signal sending. This clarifies quite a lot of things! Though I wonder how one reliably gets a the PGID or SID from a PIDTYPE_PID. > > 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(). Nice! Thank you, Oleg! :) Christian
