On Wed, Jun 05, 2024 at 01:56:23PM +0200, Frederic Weisbecker wrote: > Le Tue, Jun 04, 2024 at 03:21:55PM -0700, Paul E. McKenney a écrit : > > This commit expands on the ordering properties of rcu_assign_pointer() > > and rcu_dereference(), outlining their constraints on CPUs and compilers. > > > > Reported-by: Rao Shoaib <rao.shoaib@xxxxxxxxxx> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > --- > > Documentation/RCU/whatisRCU.rst | 30 +++++++++++++++++++----------- > > 1 file changed, 19 insertions(+), 11 deletions(-) > > > > diff --git a/Documentation/RCU/whatisRCU.rst b/Documentation/RCU/whatisRCU.rst > > index 94838c65c7d97..d585a5490aeec 100644 > > --- a/Documentation/RCU/whatisRCU.rst > > +++ b/Documentation/RCU/whatisRCU.rst > > @@ -250,21 +250,25 @@ rcu_assign_pointer() > > ^^^^^^^^^^^^^^^^^^^^ > > void rcu_assign_pointer(p, typeof(p) v); > > > > - Yes, rcu_assign_pointer() **is** implemented as a macro, though it > > - would be cool to be able to declare a function in this manner. > > - (Compiler experts will no doubt disagree.) > > + Yes, rcu_assign_pointer() **is** implemented as a macro, though > > + it would be cool to be able to declare a function in this manner. > > + (And there has been some discussion of adding overloaded functions > > + to the C language, so who knows?) > > > > The updater uses this spatial macro to assign a new value to an > > RCU-protected pointer, in order to safely communicate the change > > in value from the updater to the reader. This is a spatial (as > > opposed to temporal) macro. It does not evaluate to an rvalue, > > - but it does execute any memory-barrier instructions required > > - for a given CPU architecture. Its ordering properties are that > > - of a store-release operation. > > - > > - Perhaps just as important, it serves to document (1) which > > - pointers are protected by RCU and (2) the point at which a > > - given structure becomes accessible to other CPUs. That said, > > + but it does provide any compiler directives and memory-barrier > > + instructions required for a given compile or CPU architecture. > > + Its ordering properties are that of a store-release operation, > > + that is, any prior loads and stores required to initialize the > > + structure are ordered before the store that publishes the pointer > > + to that structure. > > About that, why rcu_dereference() isn't a matching load-acquire? Here is an example showing the difference: p = rcu_dereference(gp); r1 = READ_ONCE(x); r2 = p->a; The READ_ONCE() is not ordered against the rcu_dereference(), only the read from p->a. In contrast, if that rcu_dereference() was instead an smp_load_acquire(), both of the two later statements would be ordered. Ah. You are suggesting that this be added to the description of rcu_dereference()? Or are you asking that this documentation state that an rcu_dereference() memory-barrier-pairs with an rcu_assign_pointer()? Or something else completely? Thanx, Paul
