On 2023-06-23 00:43:00 [+0900], Tetsuo Handa wrote:
> On 2023/06/23 0:04, Petr Mladek wrote:
> > AFAIK, rt_spin_lock(lock) fulfills exactly the above requirements.
> > The owner could schedule. The waiter could schedule as well so that
> > they could be running on the same CPU. Also the current owner gets
> > higher priority when the is a waiter with the higher priority to avoid
> > the priority inversion.
>
> Excuse me, but that is about multiple threads trying to acquire the same lock, isn't it?
>
> Our case is that one thread which makes zonelist_update_seq.seqcount odd acquires
> zonelist_update_seq.lock but threads spinning at
> read_seqbegin(zonelist_update_seq.seqcount) from zonelist_iter_begin() do nothing but
> cpu_relax() busy loop. There is no way to teach that these threads need to give
> CPU to the thread which made zonelist_update_seq.seqcount odd...
For !RT there is no spinning because interrupts are disabled.
For RT there is no spinning because the reader blocks on lock owned by
writer.
> > The result would look like:
> >
> > in kernel/linux/printk.h:
> >
> > static inline void printk_deferred_enter(void)
> > {
> > if (!defined(CONFIG_PREEMPT_RT))
> > preempt_disable();
> > else
> > migrate_disable();
> >
> > __printk_safe_enter();
> > }
> >
> > in mm/page_alloc.c
> >
> > printk_deferred_enter();
> > write_seqlock_irqsafe(&zonelist_update_seq, flags);
>
> OK. But for stable,
>
> + if (defined(CONFIG_PREEMPT_RT))
> + migrate_disable();
> /*
> * Explicitly disable this CPU's interrupts before taking seqlock
> * to prevent any IRQ handler from calling into the page allocator
> * (e.g. GFP_ATOMIC) that could hit zonelist_iter_begin and livelock.
> */
> local_irq_save(flags);
> /*
> * Explicitly disable this CPU's synchronous printk() before taking
> * seqlock to prevent any printk() from trying to hold port->lock, for
> * tty_insert_flip_string_and_push_buffer() on other CPU might be
> * calling kmalloc(GFP_ATOMIC | __GFP_NOWARN) with port->lock held.
> */
> printk_deferred_enter();
> write_seqlock(&zonelist_update_seq);
>
> will be easier to apply.
I would prefer not to worry about stable-RT but about upstream and then
we backport what is needed into the stable-RT trees once we settled on
something. This does not affect !RT.
Sebastian
