On Thu 2023-06-22 22:36:27, Tetsuo Handa wrote:
> On 2023/06/22 8:24, Tetsuo Handa wrote:
> > By the way, given
> >
> > write_seqlock_irqsave(&zonelist_update_seq, flags);
> > <<IRQ>>
> > some_timer_function() {
> > kmalloc(GFP_ATOMIC);
> > }
> > <</IRQ>>
> > printk_deferred_enter();
> >
> > scenario in CONFIG_PREEMPT_RT=y case is handled by executing some_timer_function()
> > on a dedicated kernel thread for IRQs, what guarantees that the kernel thread for
> > IRQs gives up CPU and the user thread which called write_seqlock() gains CPU until
> > write_sequnlock() is called? How can the kernel figure out that executing the user
> > thread needs higher priority than the kernel thread?
>
> I haven't got response on this question.
>
> Several years ago, I demonstrated that a SCHED_IDLE priority userspace thread holding
> oom_lock causes other concurrently allocating !SCHED_IDLE priority threads to
> misunderstand that mutex_trylock(&oom_lock) failure implies we are making forward
> progress (despite the SCHED_IDLE priority userspace thread was unable to wake up for
> minutes).
>
> If a SCHED_IDLE priority thread which called write_seqlock_irqsave() is preempted by
> some other !SCHED_IDLE priority threads (especially realtime priority threads), and
> such !SCHED_IDLE priority thread calls kmalloc(GFP_ATOMIC) or printk(), a similar thing
> (misunderstand that spinning on read_seqbegin() from zonelist_iter_begin() can make
> forward progress despite a thread which called write_seqlock_irqsave() cannot make
> progress due to preemption) can happen.
>
> Question to Sebastian:
> To make sure that such thing cannot happen, we should make sure that
> a thread which entered write_seqcount_begin(&zonelist_update_seq.seqcount) from
> write_seqlock_irqsave(&zonelist_update_seq, flags) can continue using CPU until
> write_seqcount_end(&zonelist_update_seq.seqcount) from
> write_seqlock_irqrestore(&zonelist_update_seq, flags).
> Does adding preempt_disable() before write_seqlock(&zonelist_update_seq, flags) help?
>
>
>
> Question to Peter:
> Even if local_irq_save(flags) disables IRQ, NMI context can enqueue message via printk().
> When does the message enqueued from NMI context gets printed?
They are flushed to the console either by irq_work or by another
printk(). The irq_work could not be proceed when IRQs are disabled.
But another non-deferred printk() would try to flush them immediately.
> If there is a possibility
> that the message enqueued from NMI context gets printed between
> "write_seqlock_irqsave(&zonelist_update_seq, flags) and printk_deferred_enter()" or
> "printk_deferred_exit() and write_sequnlock_irqrestore(&zonelist_update_seq, flags)" ?
> If yes, we can't increment zonelist_update_seq.seqcount before printk_deferred_enter()...
It might happen when a printk() is called in these holes.
Best Regards,
Petr
