linux-kernel - Re: [RFC v2 29/34] mm: slub: Move flush_cpu_slab() invocations __free

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Message-ID: <YMFAu5kuhZbZquiI@grain>
Date:   Thu, 10 Jun 2021 01:29:15 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [RFC v2 29/34] mm: slub: Move flush_cpu_slab() invocations
 __free_slab() invocations out of IRQ context

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?