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Subject: Re: [PATCH] mm, slub: add cpus_read_lock/unlock() for
 slab_mem_going_offline_callback()
To:     David Hildenbrand <david@redhat.com>, qiang.zhang@windriver.com,
        akpm@linux-foundation.org, sfr@canb.auug.org.au
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <20210816074605.30336-1-qiang.zhang@windriver.com>
 <ef944ea5-c3cb-24e5-1ff8-b8e1008fa6ed@redhat.com>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <dd5785b6-3f96-3af9-586b-1d962228bed8@suse.cz>
Date:   Tue, 17 Aug 2021 09:57:10 +0200
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On 8/16/21 10:04 AM, David Hildenbrand wrote:
> On 16.08.21 09:46, qiang.zhang@windriver.com wrote:
>> From: "Qiang.Zhang" <qiang.zhang@windriver.com>
>>
>> The flush_all_cpus_locked() should be called with cpus_read_lock/unlock(),
>> ensure flush_cpu_slab() can be executed on schedule_on CPU.
>>
>> Fixes: 1c84f3c91640 ("mm, slub: fix memory and cpu hotplug related lock ordering issues")
> Memory notifiers are getting called from online_pages()/offline_pages(), 
> where we call memory_notify(MEM_GOING_OFFLINE, &arg) under 
> mem_hotplug_begin().
> 
> mem_hotplug_begin() does a cpus_read_lock().

Exactly. Also flush_all_cpus_locked() has a lockdep assert for
cpus_read_lock() which doesn't trigger in testing.

> How does this even work or against which branch is this?
>