On Tue, May 19, 2015 at 5:55 PM, Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > http://www.rdrop.com/users/paulmck/RCU/consume.2015.05.18a.pdf >From a very quick read-through, the restricted dependency chain in 7.9 seems to be reasonable, and essentially covers "thats' what hardware gives us anyway", making compiler writers happy. I would clarify the language somewhat: - it says that the result of a cast of a pointer is a dependency. You need to make an exception for casting to bool, methinks (because that's effectively just a test-against-NULL, which you later describe as terminating the dependency). Maybe get rid of the "any type", and just limit it to casts to types of size intptr_t, ie ones that don't drop significant bits. All the other rules talk about [u]intptr_t anyway. - you clarify that the trivial "& 0" and "| ~0" kill the dependency chain, but if you really want to be a stickler, you might want to extend it to a few more cases. Things like "& 1" (to extract a tag from the lot bit of a tagged pointer) should likely also drop the dependency, since a compiler would commonly end up using the end result as a conditional even if the code was written to then use casting etc to look like a dereference. - the "you can add/subtract integral values" still opens you up to language lawyers claiming "(char *)ptr - (intptr_t)ptr" preserving the dependency, which it clearly doesn't. But language-lawyering it does, since all those operations (cast to pointer, cast to integer, subtracting an integer) claim to be dependency-preserving operations. So I think you want to limit the logical operators to things that don't mask off too many bits, and you should probably limit the add/subtract operations some way (maybe specify that the integer value you add/subtract cannot be related to the pointer). But I think limiting it to mostly pointer ops (and a _few_ integer operations to do offsets and remove tag bits) is otherwise a good approach. Linus -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
