Hi Paul, On Mon, Feb 01, 2016 at 07:54:58PM -0800, Paul E. McKenney wrote: > 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
Just want to make sure, this one is actually: o smp_store_release() -> READ_ONCE(); if ;<WRITE_ONCE()> right? Because control dependency only orders READ->WRITE. If so, do we also need to take the following pairing into consideration? o smp_store_release() -> READ_ONCE(); if ;smp_rmb(); <ACCESS_ONCE()> > > 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. > Hmm... I don't think we should differ rcu_dereference() and lockless_dereference(). One reason: list_for_each_entry_rcu() are using lockless_dereference() right now, which means we used to think rcu_dereference() and lockless_dereference() are interchangeable, right? Besides, Will, what's the reason of having a locally transitive chain termination? Because on some architectures RELEASE->DEPENDENCY pairs may not be locally transitive? Regards, Boqun > > 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 >
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