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 18:45, Sebastian Andrzej Siewior wrote:
> 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.

My understanding is that zonelist_iter_begin() accesses only zonelist_update_seq.seqcount .
 From where is the zonelist_update_seq.lock accessed when zonelist_iter_begin() is called?