linux-kernel - Re: [RFC] page-table walkers vs memory order

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Message-ID: <1343335169.32120.18.camel@twins>
Date:	Thu, 26 Jul 2012 22:39:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Andrea Arcangeli <aarcange@...hat.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [RFC] page-table walkers vs memory order

On Tue, 2012-07-24 at 14:51 -0700, Hugh Dickins wrote:
> I do love the status quo, but an audit would be welcome.  When
> it comes to patches, personally I tend to prefer ACCESS_ONCE() and
> smp_read_barrier_depends() and accompanying comments to be hidden away
> in the underlying macros or inlines where reasonable, rather than
> repeated all over; but I may have my priorities wrong on that.
> 
> 
Yeah, I was being lazy, and I totally forgot to actually look at the
alpha code.

How about we do a generic (cribbed from rcu_dereference):

#define page_table_deref(p)					\
({								\
	typeof(*p) *______p = (typeof(*p) __force *)ACCESS_ONCE(p);\
	smp_read_barrier_depends();				\
	((typeof(*p) __force __kernel *)(______p));		\
})

and use that all over to dereference page-tables. That way all this
lives in one place. Granted, I'll have to go edit all arch code, but I
seem to be doing that on a frequent basis anyway :/


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