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) > o smp_store_release() -> lockless_dereference() (???) > o rcu_assign_pointer() -> rcu_dereference() > o smp_store_release() -> READ_ONCE(); if 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) > > Seem reasonable, or am I missing some? Looks alright to me. Will
