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Date:   Thu, 10 Jun 2021 01:29:15 +0300
From:   Cyrill Gorcunov <gorcunov@gmail.com>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Pekka Enberg <penberg@kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mel Gorman <mgorman@techsingularity.net>,
        Jesper Dangaard Brouer <brouer@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Jann Horn <jannh@google.com>
Subject: Re: [RFC v2 29/34] mm: slub: Move flush_cpu_slab() invocations
 __free_slab() invocations out of IRQ context
Message-ID: <YMFAu5kuhZbZquiI@grain>
References: <20210609113903.1421-1-vbabka@suse.cz>
 <20210609113903.1421-30-vbabka@suse.cz>
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On Wed, Jun 09, 2021 at 01:38:58PM +0200, Vlastimil Babka wrote:
> +static DEFINE_MUTEX(flush_lock);
> +static DEFINE_PER_CPU(struct slub_flush_work, slub_flush);
> +
>  static void flush_all(struct kmem_cache *s)
>  {
> -	on_each_cpu_cond(has_cpu_slab, flush_cpu_slab, s, 1);
> +	struct slub_flush_work *sfw;
> +	unsigned int cpu;
> +
> +	cpus_read_lock();
> +	mutex_lock(&flush_lock);
> +

Hi, Vlastimil! Could you please point why do you lock cpus first and
mutex only after? Why not mutex_lock + cpus_read_lock instead?