linux-kernel - Re: [PATCH] mm: drop hotplug lock from lru_add_drain

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Date:   Tue, 14 Nov 2017 16:05:21 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: drop hotplug lock from lru_add_drain_all

On Tue, 14 Nov 2017, Michal Hocko wrote:

> On Tue 14-11-17 15:23:47, Michal Hocko wrote:
> [...]
> > +/*
> > + * Doesn't need any cpu hotplug locking because we do rely on per-cpu
> > + * kworkers being shut down before our page_alloc_cpu_dead callback is
> > + * executed on the offlined cpu
> > + */
> >  void lru_add_drain_all(void)
> >  {
> >  	static DEFINE_MUTEX(lock);
> 
> Ble the part of the comment didn't go through

Looks good.

Thanks,

	tglx

> diff --git a/mm/swap.c b/mm/swap.c
> index 8bfdcab9f83e..1ab8122d2d0c 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -688,6 +688,13 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy)
>  
>  static DEFINE_PER_CPU(struct work_struct, lru_add_drain_work);
>  
> +/*
> + * Doesn't need any cpu hotplug locking because we do rely on per-cpu
> + * kworkers being shut down before our page_alloc_cpu_dead callback is
> + * executed on the offlined cpu.
> + * Calling this function with cpu hotplug locks held can actually lead
> + * to obscure indirect dependencies via WQ context.
> + */
>  void lru_add_drain_all(void)
>  {
>  	static DEFINE_MUTEX(lock);
> -- 
> Michal Hocko
> SUSE Labs
>