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

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Date:	Sun, 14 Jun 2015 10:13:52 +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 06/13, 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.

True.

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

True.

So since we have a spin_lock() there already, I tried to find the right primitive 
to turn it into a full memory barrier but gave up. Is there a simple primitive for 
that?

Also, since this is x86 specific code we could rely on the fact that 
spinlock-acquire is a full memory barrier?

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

I'm afraid lockless is rarely simple, but I'm open to suggestions ...

Thanks,

	Ingo
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