Re: mainline build failure for loongarch allmodconfig with gcc-12

[Huacai Chen]

Hi, Linus,

On Wed, Aug 24, 2022 at 2:09 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Aug 23, 2022 at 1:34 AM Sudip Mukherjee (Codethink)
> <sudipm.mukherjee@xxxxxxxxx> wrote:
> >
> > I have been trying to build loongarch as part of my nightly builds, and
> > I can build loongson3_defconfig without any error. But allmodconfig fails
> > with the error:
> >
> > In function '__cmpxchg',
> >     inlined from 'ssh_seq_next' at drivers/platform/surface/aggregator/controller.c:61:9,
>
> Looks like ssh_seq_next() wants to do an atomic cmpxchg() on a single
> byte value, and the Loongarch implementation only does 4- and 8-byte
> versions.
>
> It looks like loongarch - from its MIPS heritage - inherited the "we
> can't do atomics on byte variables", so that it needs the same strange
> "do bytes as word accesses with mask-and-shifts".
>
> For MIPS, the code is in __xchg_small() in arch/mips/kernel/cmpxchg.c.
>
> Alpha has something similar, except it's all done in inline asm in
> arch/alpha/include/asm/xchg.h (look for "____cmpxchg(_u8," in there.
>
> Of course, we could just add a Kconfig variable like
> "ARCH_LACKS_BYTE_ATOMICS" and make that driver depend on it not being
> true, and just have Loongarch set it.
>
> But I think loongarch should just implement the byte masking stuff.
> Particularly since I suspect it can just copy the MIPS code as-is.
Yes, I agree. There is already a proposed patch to do this [1]. It was
discussed for a long time about the "forward progress", but I think
that problem is solved because LoongArch has "exclusive access (with
timeout) of ll" and "random delay of sc" in hardware. So I hope this
patch can be accepted now.

https://lore.kernel.org/loongarch/CAJF2gTQBjetiA1eDaXhBsiEmRYCdOAAWWAGcwVjhZTBYH5BpGQ@xxxxxxxxxxxxxx/T/#m9df7f86d9509cec23a66a0304152377e1070b62e

Huacai
>
>                   Linus