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:
o smp_store_release() -> smp_load_acquire() (locally transitive)
o smp_store_release() -> lockless_dereference() (???)
o smp_store_release() -> READ_ONCE(); if
o rcu_assign_pointer() -> rcu_dereference()
o smp_mb(); WRITE_ONCE() -> READ_ONCE(); (globally transitive)
o synchronize_rcu(); WRITE_ONCE() -> READ_ONCE(); (globally transitive)
o synchronize_rcu(); WRITE_ONCE() -> rcu_read_lock(); READ_ONCE()
(strange and wonderful properties)
Seem reasonable, or am I missing some?
Thanx, Paul
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