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...
> 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.
