On January 9, 2023 2:02:33 PM PST, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: >On Mon, Jan 9, 2023 at 10:29 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: >> >> I ran into a ton of casting trouble when compiling kernel/fork.c which >> uses this_cpu_cmpxchg() on a pointer type and the compiler hates casting >> pointers to an integer that is not the exact same size. > >Ahh. Yeah - not because that code needs or wants the 128-bit case, but >because the macro expands to all sizes in a switch statement, so the >compiler sees all the cases even if only one is then statically >picked. > >So the silly casts are for all the cases that never matter. > >Annoying. > >I wonder if the "this_cpu_cmpxchg()" macro could be made to use >_Generic() to pick out the pointer case first, and then only use >'sizeof()' for the integer types, so that we don't have this kind of >"every architecture needs to deal with the nasty situation" code. > >Ok, it's not actually the this_cpu_cmpxchg() macro, it's >__pcpu_size_call_return() and friends, but whatever. > >Another alternative is to try to avoid casting to "u64" as long as >humanly possible, and use only "typeof((*ptr))" everywhere. Then when >the type actually *is* 128-bit, it all works out fine, because it >won't be a pointer. That's the approach the uaccess macros tend to >take, and then they hit the reverse issue on clang, where using the >"byte register" constraints would cause warnings for non-byte >accesses, and we had to do > > unsigned char x_u8__; > __get_user_asm(x_u8__, ptr, "b", "=q", label); > (x) = x_u8__; > >because using '(x)' directly would then warn when 'x' wasn't a >char-sized thing - even if that asm case never actually was _used_ for >that case, since it was all inside a "switch (sizeof) case 1:" >statement. > > Linus I wrote a crazy macro for dealing with exactly this at one point, basically producing the "right type" to cast to. It would need to have 128-bit support added to it, but that should be trivial. It is called something like int_type() ... not in front of a computer right now so can't double check.
