On Tue, May 19, 2015 at 06:57:02PM -0700, Linus Torvalds wrote: > 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. Excellent point! I now say: If a pointer is part of a dependency chain, then casting it (either explicitly or implicitly) to any pointer-sized type extends the chain to the result. If this approach works out, the people in the Core Working Group will come up with alternative language-lawyer-proof wording, but this informal version will hopefully do for the moment. > - 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. Ah, how about the following? If a value of type intptr_t or uintptr_t is part of a dependency chain, and if that value is one of the operands to an & or | infix operator whose result has too few or too many bits set, then the resulting value will not be part of any dependency chain. For example, on a 64-bit system, if p is part of a dependency chain, then (p & 0x7) provides just the tag bits, and normally cannot even be legally dereferenced. Similarly, (p | ~0) normally cannot be legally dereferenced. > - 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. My thought was that the result of "(char *)ptr - (intptr_t)ptr" is a NULL pointer in most environments, and dereferencing a NULL pointer is undefined behavior. So it becomes irrelevant whether or not the NULL pointer carries a dependency. There are some stranger examples, such as "(char *)ptr - ((intptr_t)ptr)/7", but in that case, if the resulting pointer happens by chance to reference valid memory, I believe a dependency would still be carried. Of course, if you are producing code like that, I am guessing that dependencies are the very least of your concerns. However, I will give this some more thought. > 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. Glad you mostly like it! ;-) Thanx, Paul -- 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
