linux-kernel - Re: [PATCH V8 07/10] riscv: Add qspinlock support

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Date:   Thu, 28 Jul 2022 16:34:27 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Palmer Dabbelt <palmer@...osinc.com>,
        Heiko Stübner <heiko@...ech.de>,
        Christoph Hellwig <hch@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Philipp Tomsich <philipp.tomsich@...ll.eu>,
        Christoph Muellner <cmuellner@...ux.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Laight <David.Laight@...lab.com>,
        clang-built-linux <llvm@...ts.linux.dev>,
        kbuild-all@...ts.01.org,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-csky@...r.kernel.org, Guo Ren <guoren@...ux.alibaba.com>,
        Niklas Schnelle <schnelle@...ux.ibm.com>
Subject: Re: [PATCH V8 07/10] riscv: Add qspinlock support

On Thu, Jul 28, 2022 at 4:14 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Thu, Jul 28, 2022 at 5:35 AM Guo Ren <guoren@...nel.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.
Okay, I would try that style.

> 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.
C-SKY hasn't any plan to support clang, as I know.

>
>         Arnd



-- 
Best Regards
 Guo Ren