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

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Date:	Sat, 13 Jun 2015 19:45:27 +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 06/13, Ingo Molnar wrote:
>
> * Ingo Molnar <mingo@...nel.org> wrote:
>
> > * Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > >
> > > 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.

Hmm. that write must be visible before we start to _read_ *pgd_src,
afaics.

> > 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.

I agree,

> Ah, there's another detail I forgot. This might handle the fork case, but in
> exec() we have:
>
>         tsk->mm = mm;
>         arch_pgd_init_late(mm);

Yes, this too.

But wmb() can't help. At least we need the full mb() to serialize the
STORE above (or list addition in copy_process) with the LOAD which
reads *pgd_src.

Plus we need another mb() in sync_global_pgds(), say, before the main
for_each_process() loop.


it would be nice to make this more simple/clear somehow...

Oleg.

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