Re: [GIT PULL rcu/next] RCU commits for 4.13

[Linus Torvalds]

On Wed, Jun 28, 2017 at 4:54 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>
> Linus, are you dead-set against defining spin_unlock_wait() to be
> spin_lock + spin_unlock?  For example, is the current x86 implementation
> of spin_unlock_wait() really a non-negotiable hard requirement?  Or
> would you be willing to live with the spin_lock + spin_unlock semantics?

So I think the "same as spin_lock + spin_unlock" semantics are kind of insane.

One of the issues is that the same as "spin_lock + spin_unlock" is
basically now architecture-dependent. Is it really the
architecture-dependent ordering you want to define this as?

So I just think it's a *bad* definition. If somebody wants something
that is exactly equivalent to spin_lock+spin_unlock, then dammit, just
do *THAT*. It's completely pointless to me to define
spin_unlock_wait() in those terms.

And if it's not equivalent to the *architecture* behavior of
spin_lock+spin_unlock, then I think it should be descibed in terms
that aren't about the architecture implementation (so you shouldn't
describe it as "spin_lock+spin_unlock", you should describe it in
terms of memory barrier semantics.

And if we really have to use the spin_lock+spinunlock semantics for
this, then what is the advantage of spin_unlock_wait at all, if it
doesn't fundamentally avoid some locking overhead of just taking the
spinlock in the first place?

And if we can't use a cheaper model, maybe we should just get rid of
it entirely?

Finally: if the memory barrier semantics are exactly the same, and
it's purely about avoiding some nasty contention case, I think the
concept is broken - contention is almost never an actual issue, and if
it is, the problem is much deeper than spin_unlock_wait().

                  Linus