On Fri, Jun 04, 2021 at 01:44:37PM +0200, Peter Zijlstra wrote: > On Fri, Jun 04, 2021 at 12:12:07PM +0200, Peter Zijlstra wrote: > > +/** > > + * volatile_if() - Provide a control-dependency > > + * > > + * volatile_if(READ_ONCE(A)) > > + * WRITE_ONCE(B, 1); > > + * > > + * will ensure that the STORE to B happens after the LOAD of A. Normally a > > + * control dependency relies on a conditional branch having a data dependency > > + * on the LOAD and an architecture's inability to speculate STOREs. IOW, this > > + * provides a LOAD->STORE order. > > + * > > + * Due to optimizing compilers extra care is needed; as per the example above > > + * the LOAD must be 'volatile' qualified in order to ensure the compiler > > + * actually emits the load, such that the data-dependency to the conditional > > + * branch can be formed. > > + * > > + * Secondly, the compiler must be prohibited from lifting anything out of the > > + * selection statement, as this would obviously also break the ordering. > > + * > > + * Thirdly, and this is the tricky bit, architectures that allow the > > + * LOAD->STORE reorder must ensure the compiler actually emits the conditional > > + * branch instruction, this isn't possible in generic. > > + * > > + * See the volatile_cond() wrapper. > > + */ > > +#define volatile_if(cond) if (volatile_cond(cond)) > > On naming (sorry Paul for forgetting that in the initial mail); while I > think using the volatile qualifier for the language feature (can we haz > plz, kthxbai) makes perfect sense, Paul felt that we might use a > 'better' name for the kernel use, ctrl_dep_if() was proposed. > > Let us pain bike sheds :-) I have felt that pain many times... ;-) Here is what I see thus far from these two threads: 1. volatile_if() as above. Nice ease of use, but might be suboptimal on architectures where a branch is slower than an acquire load. 2. #1, but with my preferred name of ctrl_dep_if() instead of volatile_if(). 3. READ_ONCE_CTRL() like back in the old days. This has the advantage of giving the compiler more information, but has problems with relaxed atomic RMW operations. 4. A full (fool?) solution based on #3 would also include _ctrl suffixed atomic RMW operations. 5. Your bikeshed color here! Thanx, Paul
