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 2023/06/22 23:11, Petr Mladek wrote:
>> 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.

Is that rule same for both COMFIG_PREEMPT_RT=y and COMFIG_PREEMPT_RT=n ?

If yes, when Sebastian's patch is applied, IRQs will not be disabled for
COMFIG_PREEMPT_RT=y kernels because write_seqlock_irqsave() is equivalent
with write_seqlock() because spin_lock_irqsave(lock, flags) is equivalent
with spin_lock(lock), and


> 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.

printk() can happen between these holes for COMFIG_PREEMPT_RT=y kernels.
We will need to call printk_deferred_enter() before incrementing
zonelist_update_seq.seqcount in order to close these holes.