On Thu 22-06-23 08:24:30, Tetsuo Handa wrote:
> On 2023/06/21 23:50, 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.
> >
>
> Since we must replace local_irq_save() + write_seqlock() with write_seqlock_irqsave() for
> CONFIG_PREEMPT_RT=y case but we must not replace local_irq_save() + write_seqlock() with
> write_seqlock_irqsave() for CONFIG_PREEMPT_RT=n case, the proper fix is something like below?
Now, I am confused. Why write_seqlock_irqsave is not allowed for !RT?
Let me quote the changelog and he scenario 1:
write_seqlock(&zonelist_update_seq); // makes zonelist_update_seq.seqcount odd
// e.g. timer interrupt handler runs at this moment
some_timer_func() {
kmalloc(GFP_ATOMIC) {
__alloc_pages_slowpath() {
read_seqbegin(&zonelist_update_seq) {
// spins forever because zonelist_update_seq.seqcount is odd
}
}
}
}
// e.g. timer interrupt handler finishes
write_sequnlock(&zonelist_update_seq); // makes zonelist_update_seq.seqcount even
This is clearly impossible with write_seqlock_irqsave as interrupts are
disabled before the lock is taken.
--
Michal Hocko
SUSE Labs
