netdev - Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access

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Message-ID: <1381360675.4971.45.camel@edumazet-glaptop.roam.corp.google.com>
Date:	Wed, 09 Oct 2013 16:17:55 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, sbw@....edu,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply
 rcu_access_pointer() to avoid sparse false positive

On Wed, 2013-10-09 at 15:56 -0700, Josh Triplett wrote:

> I'd be really hesitant to introduce that type of direct assignment to an
> __rcu pointer without wrapping it in some appropriately named macro, or
> at the very least adding a comment.

Well, there is no special magic here, in this specific case :

- deleting an item in an rcu list

Check list_del_rcu(), and you'll notice there is no _barrier_

Adding correct barriers is good, but please do not add them when not
needed.

It makes code hard to understand.


ACCESS(*ptr) = value;

is clear and autodocumented, because it highlights the potential
problem, that is *ptr can be read without any barrier from another cpu.
So we ask the compiler to not write temporary garbage in it.

rcu_assign_pointer(*ptr, rcu_access_pointer(value)) 

is very confusing, because it hides the _real_ problem and add defensive
programming tricks.




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