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
