Re: [RFC PATCH v2] posix-timers: Support delivery of signals to the current thread

Dmitry Vyukov
2023-01-26T10:58:35Z

On Wed, 25 Jan 2023 at 18:07, Oleg Nesterov <oleg@redhat.com> wrote:
>
> On 01/25, Oleg Nesterov wrote:
> >
> > > > int posix_timer_event(struct k_itimer *timr, int si_private)
> > > > {
> > > > enum pid_type type;
> > > > + struct pid *pid;
> > > > int ret;
> > > > /*
> > > > * FIXME: if ->sigq is queued we can race with
> > > > @@ -350,8 +351,9 @@ int posix_timer_event(struct k_itimer *timr, int si_private)
> > > > */
> > > > timr->sigq->info.si_sys_private = si_private;
> > > >
> > > > - type = !(timr->it_sigev_notify & SIGEV_THREAD_ID) ? PIDTYPE_TGID : PIDTYPE_PID;
> > > > - ret = send_sigqueue(timr->sigq, timr->it_pid, type);
> > > > + type = (timr->it_sigev_notify & SIGEV_THREAD_ID) ? PIDTYPE_PID : PIDTYPE_TGID;
> > > > + pid = (type == PIDTYPE_PID) ? timr->it_pid : task_pid(current);
> > > > + ret = send_sigqueue(timr->sigq, pid, type);
> > > > /* If we failed to send the signal the timer stops. */
> > > > return ret > 0;
> > > > }
>
> ...
>
> > But! I just noticed send_sigqueue() does pid_task(pid, type), so the patch
> > above needs another change
> >
> > --- a/kernel/signal.c
> > +++ b/kernel/signal.c
> > @@ -1970,7 +1970,8 @@ int send_sigqueue(struct sigqueue *q, struct pid *pid, enum pid_type type)
> >
> > ret = -1;
> > rcu_read_lock();
> > - t = pid_task(pid, type);
> > + // comment to explain why don't we use "type"
> > + t = pid_task(pid, PIDTYPE_PID);
> > if (!t || !likely(lock_task_sighand(t, &flags)))
> > goto ret;
>
> So. Unless I missed something (quite possibly) we do not even need the patch
> above. The one liner change below can work just fine.
>
> Oleg.
>
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1970,7 +1970,8 @@ int send_sigqueue(struct sigqueue *q, struct pid *pid, enum pid_type type)
>
> ret = -1;
> rcu_read_lock();
> - t = pid_task(pid, type);
> + /* GOOD COMMENT */
> + t = type == PIDTYPE_PID ? pid_task(pid, type) : current;
> if (!t || !likely(lock_task_sighand(t, &flags)))
> goto ret;

This works for my use-case.

However, this is a bit trickier since current may be completely
unrelated to the original timer/process (think of monotonic timers).

Please take a look at v3: