On Fri, Jun 04, 2021 at 12:12:07PM +0200, Peter Zijlstra wrote: > Hi! > > With optimizing compilers becoming more and more agressive and C so far > refusing to acknowledge the concept of control-dependencies even while > we keep growing the amount of reliance on them, things will eventually > come apart. > > There have been talks with toolchain people on how to resolve this; one > suggestion was allowing the volatile qualifier on branch statements like > 'if', but so far no actual compiler has made any progress on this. > > Rather than waiting any longer, provide our own construct based on that > suggestion. The idea is by Alan Stern and refined by Paul and myself. > > Code generation is sub-optimal (for the weak architectures) since we're > forced to convert the condition into another and use a fixed conditional > branch instruction, but shouldn't be too bad. > > Usage of volatile_if requires the @cond to be headed by a volatile load > (READ_ONCE() / atomic_read() etc..) such that the compiler is forced to > emit the load and the branch emitted will have the required > data-dependency. Furthermore, volatile_if() is a compiler barrier, which > should prohibit the compiler from lifting anything out of the selection > statement. > > This construct should place control dependencies on a stronger footing > until such time that the compiler folks get around to accepting them :-) > > I've converted most architectures we care about, and the rest will get > an extra smp_mb() by means of the 'generic' fallback implementation (for > now). > > I've converted the control dependencies I remembered and those found > with a search for smp_acquire__after_ctrl_dep(), there might be more. > > Compile tested only (alpha, arm, arm64, x86_64, powerpc, powerpc64, s390 > and sparc64). > > Suggested-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Is there any interest in doing the same sort of thing for switch statements? A similar approach would probably work, but maybe people don't care about it. Alan
