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

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Message-ID: <20150613064705.GA13835@gmail.com>
Date:	Sat, 13 Jun 2015 08:47:05 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
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


* Oleg Nesterov <oleg@...hat.com> wrote:

> 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?

So the real thing this pairs with is the earlier:

	tsk->mm = mm;

plus the linking of the new task in the task list.

_that_ write must become visible to others before we do the (conditional) copy 
ourselves.

Granted, it happens quite a bit earlier, and the task linking's use of locking is 
a natural barrier - but since this is lockless I didn't want to leave a silent 
assumption in.

Perhaps remove the barrier and just leave a comment in that describes the 
assumption on task-linking being a full barrier?

Thanks,

	Ingo
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