Date: Tue, 21 Oct 2014 21:00:19 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: torvalds@linux-foundation.org, paulmck@linux.vnet.ibm.com,
        tglx@linutronix.de, akpm@linux-foundation.org, riel@redhat.com,
        mgorman@suse.de, oleg@redhat.com, mingo@redhat.com, minchan@kernel.org,
        kamezawa.hiroyu@jp.fujitsu.com, viro@zeniv.linux.org.uk,
        laijs@cn.fujitsu.com, dave@stgolabs.net
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [RFC][PATCH 5/6] mm: Provide speculative fault infrastructure
Message-ID: <20141021190019.GJ3219@twins.programming.kicks-ass.net>
References: <20141020215633.717315139@infradead.org>
 <20141020222841.490529442@infradead.org>
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Sender: linux-kernel-owner@vger.kernel.org

On Mon, Oct 20, 2014 at 11:56:38PM +0200, Peter Zijlstra wrote:
>  static bool pte_map_lock(struct fault_env *fe)
>  {
> +	bool ret = false;
> +
> +	if (!(fe->flags & FAULT_FLAG_SPECULATIVE)) {
> +		fe->pte = pte_offset_map_lock(fe->mm, fe->pmd, fe->address, &fe->ptl);
> +		return true;
> +	}
> +
> +	/*
> +	 * The first vma_is_dead() guarantees the page-tables are still valid,
> +	 * having IRQs disabled ensures they stay around, hence the second
> +	 * vma_is_dead() to make sure they are still valid once we've got the
> +	 * lock. After that a concurrent zap_pte_range() will block on the PTL
> +	 * and thus we're safe.
> +	 */
> +	local_irq_disable();
> +	if (vma_is_dead(fe->vma, fe->sequence))
> +		goto out;
> +
>  	fe->pte = pte_offset_map_lock(fe->mm, fe->pmd, fe->address, &fe->ptl);

Yeah, so this deadlocks just fine, I found we still do TLB flushes while
holding the PTL. Bugger that, the alternative is either force everybody
to do RCU freed page-tables or put back the ugly code :/

A well..

> +
> +	if (vma_is_dead(fe->vma, fe->sequence)) {
> +		pte_unmap_unlock(fe->pte, fe->ptl);
> +		goto out;
> +	}
> +
> +	ret = true;
> +out:
> +	local_irq_enable();
> +	return ret;
>  }
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