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 :-)
