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

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2023-06-23 18:51:24 [+0900], Tetsuo Handa wrote:
> 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?

zonelist_iter_begin()
 -> read_seqbegin()
   -> read_seqcount_begin()
     -> raw_read_seqcount_begin()
       -> __read_seqcount_begin()

Now. The inner part looks like
         while ((__seq = seqprop_sequence(s)) & 1)
                 cpu_relax();

but with RT enabled (in-tree, not out-of-tree, in-tree) seqprop_sequence()
goes from:

| __seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s)       \
| {                                                                       \
|         unsigned seq = READ_ONCE(s->seqcount.sequence);                 \
|                                                                         \
|         if (!IS_ENABLED(CONFIG_PREEMPT_RT))                             \
|                 return seq;                                             \
|                                                                         \
|         if (preemptible && unlikely(seq & 1)) {                         \
|                 __SEQ_LOCK(lock_acquire);                               \
|                 __SEQ_LOCK(lockbase##_unlock(s->lock));                 \
|                                                                         \
|                 /*                                                      \
|                  * Re-read the sequence counter since the (possibly     \
|                  * preempted) writer made progress.                     \
|                  */                                                     \
|                 seq = READ_ONCE(s->seqcount.sequence);                  \
|         }                                                               \
|                                                                         \
|         return seq;                                                     \

to

| unsigned __seqprop_spinlock_sequence(const seqcount_spinlock_t *s)
| { 
| 	unsigned seq = READ_ONCE(s->seqcount.sequence);
| 
| 	if (unlikely(seq & 1)) {
| 		spin_lock(s->lock); 
| 		spin_unlock(s->lock);
| 		seq = READ_ONCE(s->seqcount.sequence);
| 	}
| 	return seq;
| }

tada \o/

Sebastian