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

[Will Deacon <will.deacon@arm.com>]

[turns out I've not been on cc for this thread, but Jade pointed me to it
 and I see my name came up at some point!]

On Wed, Jun 28, 2017 at 05:05:46PM -0700, Linus Torvalds wrote:
> On Wed, Jun 28, 2017 at 4:54 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> 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?

Just on this point -- the arm64 code provides the same ordering semantics
as you would get from a lock;unlock sequence, but we can optimise that
when compared to an actual lock;unlock sequence because we don't need to
wait in turn for our ticket. I suspect something similar could be done
if/when we move to qspinlocks.

Whether or not this is actually worth optimising is another question, but
it is worth noting that unlock_wait can be implemented more cheaply than
lock;unlock, whilst providing the same ordering guarantees (if that's
really what we want -- see my reply to Paul).

Simplicity tends to be my preference, so ripping this out would suit me
best ;)

Will