linux-kernel - Re: [PATCH] mm/slub: mark racy accesses on slab->slabs

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Date: Thu, 21 Mar 2024 10:48:11 +0800
From: linke li <lilinke99@...com>
To: chengming.zhou@...ux.dev
Cc: 42.hyeyoo@...il.com,
	akpm@...ux-foundation.org,
	cl@...ux.com,
	iamjoonsoo.kim@....com,
	lilinke99@...com,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	penberg@...nel.org,
	rientjes@...gle.com,
	roman.gushchin@...ux.dev,
	vbabka@...e.cz
Subject: Re: [PATCH] mm/slub: mark racy accesses on slab->slabs

Sorry for a late reply, I just found this because of my bad email client.

> There is another unmarked access of "slab->slabs" in the show_slab_objects(),
> which you can change too.

Yes, I think show_slab_objects() has a similar situation. Should I
consider to submit a V2 patch for this?

> I'm not sure that it's really safe to access "slab->slabs" here without any protection?
> Although it should be no problem in practice, alternative choice maybe putting partial
> slabs count in the kmem_cache_cpu struct.

I think it is ok, because it seems that slab->slabs in slub_percpu_partial
and show_slab_objects() are just used for showing some infomation.

I noticed Paul summarized some of these strategies in access-marking.txt[1]

Quote from it:

"Use of the data_race() Macro
----------------------------

Here are some situations where data_race() should be used instead of
READ_ONCE() and WRITE_ONCE():

1.	Data-racy loads from shared variables whose values are used only
	for diagnostic purposes.

2.	Data-racy reads whose values are checked against marked reload.

3.	Reads whose values feed into error-tolerant heuristics.

4.	Writes setting values that feed into error-tolerant heuristics.
"

Thanks,
Linke

[1]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt