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Date: Tue, 14 Nov 2017 16:05:21 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Michal Hocko <mhocko@kernel.org>
cc: Andrew Morton <akpm@linux-foundation.org>, Tejun Heo <tj@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Johannes Weiner <hannes@cmpxchg.org>, Mel Gorman <mgorman@suse.de>,
        linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm: drop hotplug lock from lru_add_drain_all
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References: <20171114135348.28704-1-mhocko@kernel.org> <alpine.DEB.2.20.1711141512180.2044@nanos> <20171114142347.syzyd6tlnpe2afur@dhcp22.suse.cz> <20171114143200.brmgskoqxjlrhrzx@dhcp22.suse.cz>
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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
>