Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Applyrcu_access_pointer() to avoid sparse false positive

[Eric Dumazet]

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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