On Wed, Oct 11, 2017 at 11:56:54AM -0700, Linus Torvalds wrote: > On Wed, Oct 11, 2017 at 11:43 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: > > > > A long thread and I lost track somewhat, but, yes, from KTSAN (data > > race detector) point of view we will need a way to understand when > > things are ordered due to data and control dependencies > > (i.e.effectively acquire but only wrt the dependent stuff). > > There is a logic level and physical level, it's perfectly fine if on > > physical level all these _DEP/_CTRL variants are no-op (the same as > > READ_ONCE, at least on todays archs with todays compilers). But on > > logical level they represent a well defined, meaningful thing. > > No, they do not. > > Do you believe in fairies and Santa Claus? > > Because quite frankly, the likelihood of either of those being true is > _way_ higher than the likelihood of any normal human ever getting > those things right. > > So asking a programmer to annotate whether two memory accesses have a > data dependency or a control dependency is completely inappropriate. > You won't get people understanding it to begin with, much less then > figure out subtle things like whether a control dependency is an > actual branch, or might be turned into a data dependency through > select, or whatever. > > We've had really smart people who wrote core code that couldn't get it > right, and that weren't sure if a control dependency was really > guaranteed or not. > > That is *exactly* the kinds of thing that _automation_ should handle. > Not some human. Figure the data/control dependencies out from the > code, not from some logical level. > > I saw the contortions that the ISO C people tried to go through just > to describe control and data dependencies. It was awful. It should > have never been described on that kind of level to begin with, when it > would have been much easier to just describe it to compiler people as > "this is a consume relationship". The same rules apply here. Don't > make it about some high-level thing and humans annontating things. > Make it about the actual generated code. Speaking as one of the ISO C people... What exactly do you have in mind when you say to just describe it to compiler people as "this is a consume relationship"? Thanx, Paul
