[PATCH V8 07/10] riscv: Add qspinlock support

[Arnd Bergmann]

On Thu, Jul 28, 2022 at 5:35 AM Guo Ren <guoren at kernel.org> wrote:
>
> Hi Palmer,
>
> The warning is from a clang problem.
>
> drivers/net/wireguard/queueing.c:
> static void __wg_prev_queue_enqueue(struct prev_queue *queue, struct
> sk_buff *skb)
> {
>         WRITE_ONCE(NEXT(skb), NULL);
>         WRITE_ONCE(NEXT(xchg_release(&queue->head, skb)), skb);
> }
>
> The queue->head is 64bit pointer size.
>
> #define __xchg_relaxed(ptr, new, size)                                  \
> ({                                                                      \
>         __typeof__(ptr) __ptr = (ptr);                                  \
>         __typeof__(new) __new = (new);                                  \
>         __typeof__(*(ptr)) __ret;                                       \
>         switch (size) {                                                 \
>         case 2: {                                                       \
> ... Clang shouldn't give warning from here, because code won't enter the path.
>                 break;                                                  \
>         }                                                               \
>         case 4:                                                         \
> ...
>                 break;                                                  \
>         case 8:                                                         \
> ... The case would enter this path.
>                 break;                                                  \
>         default:                                                        \
>                 BUILD_BUG();                                            \
>         }                                                               \
>         __ret;                                                          \
> })

I assume it's this warning you are referring to?

>> drivers/net/wireguard/queueing.c:68:18: warning: cast to 'typeof (*((__ai_ptr)))' (aka 'struct sk_buff *') from smaller integer type 'unsigned int' [-Wint-to-pointer-cast]
           WRITE_ONCE(NEXT(xchg_release(&queue->head, skb)), skb);

I don't consider this a bug in clang, it just performs the normal type checking
before dead code elimination and complains about code that clearly violates
the type rules.

I would suggest you split out the 16-bit xchg() into a properly typed inline
function and add type casts when calling it. In fact, I would love to
completely eliminate the 8-bit and 16-bit cases from the regular xchg()
and cmpxchg() interface and require all callers to explicitly call the
xchg16()/cmpxchg16() instead, as we require for cmpxchg64() on 32-bit
architectures already. This is something to do for another time though.

> >    include/asm-generic/io.h:547:31: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
> >            val = __raw_readb(PCI_IOBASE + addr);
> >                              ~~~~~~~~~~ ^
> >    include/asm-generic/io.h:560:61: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
> >            val = __le16_to_cpu((__le16 __force)__raw_readw(PCI_IOBASE + addr));

Not your bug, but I see that CONFIG_MMU=n risc-v has the same bug that s390 has
with missing I/O space support.  The correct workaround for this is to mark all
drivers using PCI I/O space as 'depends on HAS_IO_PORT' or similar and then
leaving out the definitions from the asm-generic header. Niklas Schnelle has
spent a lot of time working on patches for this, but they are somewhat stuck
in review. If RISC-V has the same problem, I hope we can get more people
interested in it. I think OpenRISC and C-Sky have this as well, but I'm not
sure if there is any plan to upstream clang support for those.

        Arnd