Re: [PATCH] mm/page_alloc: Use write_seqlock_irqsave() instead write_seqlock() + local_irq_save().

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

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