On 2023-06-21 23:50:02 [+0900], Tetsuo Handa wrote:
> On 2023/06/21 23:34, Sebastian Andrzej Siewior wrote:
> >> Also, if local_irq_save() is hidden due to RT, what guarantees that
> >>
> >> write_seqlock_irqsave(&zonelist_update_seq, flags);
> >> <<IRQ>>
> >> some_timer_function() {
> >> printk();
> >> }
> >> <<IRQ>>
> >> printk_deferred_enter();
> >>
> >> does not happen because write_seqlock_irqsave() does not disable IRQ?
> >
> > I don't see how zonelist_update_seq and printk here are connected
> > without the port lock/ or memory allocation. But there are two things
> > that are different on RT which probably answer your question:
>
> It is explained as the first deadlock scenario in commit 1007843a9190
> ("mm/page_alloc: fix potential deadlock on zonelist_update_seq seqlock").
> We have to disable IRQ before making zonelist_update_seq.seqcount odd.
You did explain the relation zonelist_update_seq -> kmalloc(, GFP_ATOMIC) and
tty_insert_flip_string_and_push_buffer() -> kmalloc(, GFP_ATOMIC) with
printk() on another CPU.
As far as PREEMPT_RT goes, it is not an issue because the port-lock is
not used or printing is deferred.
Sebastian
