linux-kernel - Re: [RFC][PATCH] sched: Use lightweight hazard pointers to grab lazy mms

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Date:   Thu, 17 Jun 2021 11:13:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        Rik van Riel <riel@...riel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [RFC][PATCH] sched: Use lightweight hazard pointers to grab lazy
 mms

On Thu, Jun 17, 2021 at 11:08:03AM +0200, Peter Zijlstra wrote:
> +static void mmdrop_lazy(struct rq *rq)
> +{
> +	struct mm_struct *old_mm;
> +
> +	if (likely(!READ_ONCE(rq->drop_mm)))
> +		return;
> +
> +	/*
> +	 * Slow path.  This only happens when we recently stopped using
> +	 * an mm that is exiting.
> +	 */
> +	old_mm = xchg(&rq->drop_mm, NULL);
> +	if (old_mm)
> +		mmdrop(old_mm);
> +}

AFAICT if we observe a !NULL value on the load, the xchg() *MUST* also
see !NULL (although it might see a different !NULL value). So do we want
to write it something like so instead?

static void mmdrop_lazy(struct rq *rq)
{
	struct mm_struct *old_mm;

	if (likely(!READ_ONCE(rq->drop_mm)))
		return;

	/*
	 * Slow path.  This only happens when we recently stopped using
	 * an mm that is exiting.
	 */
	old_mm = xchg(&rq->drop_mm, NULL);
	if (WARN_ON_ONCE(!old_mm))
		return;

	mmdrop(old_mm);
}