Re: act_mirred: remove spinlock in fast path

On Sat, 2016-06-18 at 11:24 -0400, Jamal Hadi Salim wrote:

On 16-06-18 11:16 AM, Eric Dumazet wrote:

quoted

quoted

Given an update/replace of an action is such a rare occassion, what
is wrong with init doing a spin lock on existing action?
Sure, there is performance impact on fast path at that point - but:
as established update/replace is _a rare occassion_ ;->

The potential 'problem' is not the write side, but the read side.

If you read say 3 values <A, B, C>  you might want to read them in a
consistent way, instead of <new_A, old_B, old_C>

That part i get.
What i meant is: while the fast path is doing rcu_read_lock()
of <A, B, C>  and on the rare occassion that _init() is doing a
write to <A,B,C> then if it should spin lock it would not corrupt
what fast path sees as <A, B, C> during the transition.
Am i misunderstanding?

Yes, I do not see how a change in the write side can help.

You probably need a bit of coffee ;)