Hi Oleg,
I am sorry for the late reply. I wanted to think more before answering
all the mails.
On Mon 2015-06-08 23:13:36, Oleg Nesterov wrote:
> I do not. Contrary, I think this needs more code in the likely case.
> Anyway, this API won't have too many users, so I don't even this this
> is that important.
>
> > > > + if (sig_kernel_stop(signr)) {
> > > > + __set_current_state(TASK_STOPPED);
> > > > + spin_unlock_irqrestore(&sighand->siglock, flags);
> > > > + /* Don't run again until woken by SIGCONT or SIGKILL */
> > > > + freezable_schedule();
> > > > + goto relock;
> > >
> > > Yes this avoids the race with SIGCONT. But as I said we can add another
> > > trivial helper which checks JOBCTL_STOP_DEQUEUED. So a kthread can do
> > > this itself.
> >
> > Hmm, the helper would have a strange semantic. You need to take
> > sighand->siglock, dequeue the signal (SIGSTOP), and call
> > __set_current_state(TASK_STOPPED) before you release the lock.
> > But what would happen if the dequeued signal is _not_ SIGSTOP?
>
> Perhaps I missed your point, but no. If you want to handle SIGSTOP
> you can do
>
I think that we need to add:
spin_lock_irq(&sighand->siglock);
> signr = kthread_signal_dequeue();
> switch (signr) {
> case SIGSTOP:
> something_else();
> kthread_do_signal_stop();
> ...
> }
And if we want to avoid any race, kthread_do_signal_stop() should look like:
void kthread_do_signal_stop(unsigned long flags)
{
struct sighand_struct *sighand = current->sighand;
__set_current_state(TASK_STOPPED);
spin_unlock_irqrestore(&sighand->siglock, flags);
/* Don't run again until woken by SIGCONT or SIGKILL */
freezable_schedule();
}
It means that we will have spin_lock() in one function and
spin_unlock() in another one. This is what I meant with
the strange semantic. This is why I think that it might be
cleaner to implement some generic kthread_do_signal() or so
and allow to (re)define/add sigactions via callbacks.
Note that I am not aware of any kthread that would use SIGSTOP
non-standard way.
Anyway, I am going to concentrate on the main structure of the kthread
API and will put the controversial signal handling a side for now.
I will get back to it when converting the few kthreads that use
signals. I will think more about your feedback in the meantime.
Best Regards,
Petr
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