Re: [RFC][PATCH] mips: Fix arch_spin_unlock()

From: Paul E. McKenney
Date: Mon Feb 01 2016 - 22:55:22 EST


On Mon, Feb 01, 2016 at 01:56:22PM +0000, Will Deacon wrote:
> On Fri, Jan 29, 2016 at 02:22:53AM -0800, Paul E. McKenney wrote:
> > On Fri, Jan 29, 2016 at 09:59:59AM +0000, Will Deacon wrote:
> > > On Thu, Jan 28, 2016 at 02:31:31PM -0800, Paul E. McKenney wrote:
> >
> > [ . . . ]
> >
> > > > For Linux in general, this is a question: How strict do we want to be
> > > > about matching the type of write with the corresponding read? My
> > > > default approach is to initially be quite strict and loosen as needed.
> > > > Here "quite strict" might mean requiring an rcu_assign_pointer() for
> > > > the write and rcu_dereference() for the read, as opposed to (say)
> > > > ACCESS_ONCE() for the read. (I am guessing that this would be too
> > > > tight, but it makes a good example.)
> > > >
> > > > Thoughts?
> > >
> > > That sounds broadly sensible to me and allows rcu_assign_pointer and
> > > rcu_dereference to be used as drop-in replacements for release/acquire
> > > where local transitivity isn't required. However, I don't think we can
> > > rule out READ_ONCE/WRITE_ONCE interactions as they seem to be used
> > > already in things like the osq_lock (albeit without the address
> > > dependency).
> >
> > Agreed. So in the most strict case that I can imagine anyone putting
> > up with, we have the following pairings:
>
> I think we can group these up:
>
> Locally transitive:
>
> > o smp_store_release() -> smp_load_acquire() (locally transitive)
>
> Locally transitive chain termination:
>
> (i.e. these can't be used to extend a chain)

Agreed.

> > o smp_store_release() -> lockless_dereference() (???)
> > o rcu_assign_pointer() -> rcu_dereference()
> > o smp_store_release() -> READ_ONCE(); if

I am OK with the first and last, but I believe that the middle one
has real use cases. So the rcu_assign_pointer() -> rcu_dereference()
case needs to be locally transitive.

> Globally transitive:
>
> > o smp_mb(); WRITE_ONCE() -> READ_ONCE(); (globally transitive)
> > o synchronize_rcu(); WRITE_ONCE() -> READ_ONCE(); (globally transitive)
>
> RCU:
>
> > o synchronize_rcu(); WRITE_ONCE() -> rcu_read_lock(); READ_ONCE()
> > (strange and wonderful properties)

Agreed.

> > Seem reasonable, or am I missing some?
>
> Looks alright to me.

So I have some litmus tests to generate. ;-)

Thnax, Paul