Re: [PATCH] bpf: task_group_seq_get_next: cleanup the usage of get/put_task_struct

[Kui-Feng Lee]

On 8/21/23 13:32, Kui-Feng Lee wrote:
> On 8/21/23 13:03, Oleg Nesterov wrote:
> > get_pid_task() makes no sense, the code does put_task_struct() soon 
> > after.
> > Use find_task_by_pid_ns() instead of find_pid_ns + get_pid_task and kill
> > kill put_task_struct(), this allows to do get_task_struct() only once
> > before return.
> >
> > While at it, kill the unnecessary "if (!pid)" check in the "if (!*tid)"
> > block, this matches the next usage of find_pid_ns() + get_pid_task() in
> > this function.
> >
> > Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> > ---
> >   kernel/bpf/task_iter.c | 12 ++----------
> >   1 file changed, 2 insertions(+), 10 deletions(-)
> >
> > diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> > index 4d1125108014..1589ec3faded 100644
> > --- a/kernel/bpf/task_iter.c
> > +++ b/kernel/bpf/task_iter.c
> > @@ -42,9 +42,6 @@ static struct task_struct 
> > *task_group_seq_get_next(struct bpf_iter_seq_task_comm
> >       if (!*tid) {
> >           /* The first time, the iterator calls this function. */
> >           pid = find_pid_ns(common->pid, common->ns);
> > -        if (!pid)
> > -            return NULL;
> > -
> >           task = get_pid_task(pid, PIDTYPE_TGID);
> >           if (!task)
> >               return NULL;
> > @@ -66,17 +63,12 @@ static struct task_struct 
> > *task_group_seq_get_next(struct bpf_iter_seq_task_comm
> >           return task;
> >       }
> > -    pid = find_pid_ns(common->pid_visiting, common->ns);
> > -    if (!pid)
> > -        return NULL;
> > -
> > -    task = get_pid_task(pid, PIDTYPE_PID);
> > +    task = find_task_by_pid_ns(common->pid_visiting, common->ns);
> >       if (!task)
> >           return NULL;
> >   retry:
> >       next_task = next_thread(task);
> > -    put_task_struct(task);
>
> It called get_task_struct() against this task to hold a refcount at the
> previous time calling this function. When will it release the refcount?


Oh! I missed the fact that the caller will handle it.

> >       saved_tid = *tid;
> >       *tid = __task_pid_nr_ns(next_task, PIDTYPE_PID, common->ns);
> > @@ -88,7 +80,6 @@ static struct task_struct 
> > *task_group_seq_get_next(struct bpf_iter_seq_task_comm
> >           return NULL;
> >       }
> > -    get_task_struct(next_task);
> >       common->pid_visiting = *tid;
> >       if (skip_if_dup_files && task->files == 
> > task->group_leader->files) {
> > @@ -96,6 +87,7 @@ static struct task_struct 
> > *task_group_seq_get_next(struct bpf_iter_seq_task_comm
> >           goto retry;
> >       }
> > +    get_task_struct(next_task);
> >       return next_task;
> >   }