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

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Date:	Mon, 15 Jun 2015 00:10:55 +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 06/14, Oleg Nesterov wrote:
>
> So, unless you are going to remove pgd_lock altogether perhaps we can
> rely on it the same way
>
> 	mb();
> 	spin_unlock_wait(&pgd_lock);
> 	rmb();
>
>
> Avoids the barriers (and comments) on another side, but I can't say
> I really like this...
>
>
> So I won't argue with 2 mb's on both sides.

Or we can add

	// A new child created before can miss the PGD updates,
	// but we must see that child on the process list

	read_lock(tasklist_lock);
	read_unlock(tasklist_lock);

	// We can miss a new child forked after read_unlock(),
	// but then its parent must see all PGD updates right
	// after it does write_unlock(tasklist);

	for_each_process(p) {

before main for_each_process() loop in sync_global_pgds().

As for exec_mmap() we can rely on task_lock(), sync_global_pgds()
takes it too. The corner case is when exec changes the leader, so
exec_mmap/sync_global_pgds can take different locks. But in this
case we can rely on de_thread() (which takes tasklist for write)
by the same reason: either sync_global_pgds() will see the new
leader, or the new leader must see the updates.

Oleg.

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