Hi Paul, On Thu, Jan 28, 2016 at 02:31:31PM -0800, Paul E. McKenney wrote: > On Thu, Jan 28, 2016 at 09:57:19AM +0000, Will Deacon wrote: > > On Wed, Jan 27, 2016 at 03:38:36PM -0800, Paul E. McKenney wrote: > > > On Wed, Jan 27, 2016 at 03:21:58PM +0000, Will Deacon wrote: > > Yes, sorry for the shorthand: > > > > - Each paragraph is a separate thread > > - Wx=1 means WRITE_ONCE(x, 1), Rx=1 means READ_ONCE(x) returns 1 > > - WxRel means smp_store_release(x,1), RxAcq=1 means smp_load_acquire(x) > > returns 1 > > - Everything is initially zero > > > > > > and I suppose a variant of that: > > > > > > > > > > > > Wx=1 > > > > WyRel=1 > > > > > > > > RyAcq=1 > > > > Wz=1 > > > > > > > > Rz=1 > > > > <address dependency> > > > > Rx=0 > > > > > > Agreed, this would be needed as well, along with the read-read and > > > read-write variants. I picked the write-read version (Will's first > > > test above) because write-read reordering is the most likely on > > > hardware that I am aware of. > > > > Question: if you replaced "Wz=1" with "WzRel=1" in my second test, would > > it then be forbidden? > > On Power, yes. I would guess on ARM as well.
Indeed. > 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). Will
