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

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

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.