linux-kernel - Re: [PATCH 06/12] x86/mm: Enable and use the arch_pgd_init

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Message-ID: <20150612225000.GA24699@redhat.com>
Date:	Sat, 13 Jun 2015 00:50:00 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	linux-kernel@...r.kernel.org, linux-mml@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: [PATCH 06/12] x86/mm: Enable and use the arch_pgd_init_late()
	method

On 06/11, Ingo Molnar wrote:
>
> +void arch_pgd_init_late(struct mm_struct *mm, pgd_t *pgd)
> +{
> +	/*
> +	 * This is called after a new MM has been made visible
> +	 * in fork() or exec().
> +	 *
> +	 * This barrier makes sure the MM is visible to new RCU
> +	 * walkers before we initialize it, so that we don't miss
> +	 * updates:
> +	 */
> +	smp_wmb();

I can't understand the comment and the barrier...

Afaics, we need to ensure that:

> +			if (pgd_val(*pgd_src))
> +				WRITE_ONCE(*pgd_dst, *pgd_src);

either we notice the recent update of this PGD, or (say) the subsequent
sync_global_pgds() can miss the child.

How the write barrier can help?

Oleg.

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