* Re: arch/riscv doesn't support xchg() on bool
2019-10-22 1:23 ` arch/riscv doesn't support xchg() on bool Paul Walmsley
@ 2019-10-22 1:45 ` Eric Biggers
0 siblings, 0 replies; 2+ messages in thread
From: Eric Biggers @ 2019-10-22 1:45 UTC (permalink / raw)
To: Paul Walmsley; +Cc: linux-riscv, linux-kernel, linux-fscrypt
Hi Paul,
On Mon, Oct 21, 2019 at 06:23:11PM -0700, Paul Walmsley wrote:
> Hi Eric,
>
> On Mon, 21 Oct 2019, Eric Biggers wrote:
>
> > The kbuild test robot reported a build error on RISC-V in this patch:
> >
> > https://patchwork.kernel.org/patch/11182389/
> >
> > ... because of the line:
> >
> > if (!xchg(&mode->logged_impl_name, true)) {
> >
> > where logged_impl_name is a 'bool'. The problem is that unlike most (or
> > all?) other kernel architectures, arch/riscv/ doesn't support xchg() on
> > bytes.
>
> When I looked at this in August, it looked like several Linux other
> architectures - SPARC, Microblaze, C-SKY, and Hexagon - also didn't
> support xchg() on anything other than 32-bit types:
>
> https://lore.kernel.org/lkml/alpine.DEB.2.21.9999.1908161931110.32497@viisi.sifive.com/
>
> Examples:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/sparc/include/asm/cmpxchg_32.h#n18
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/sparc/include/asm/cmpxchg_32.h#n41
>
> > Is there any chance this could be implemented, to avoid this
> > architecture-specific quirk?
>
> It is certainly possible. I wonder whether it is wise. Several of the
> other architectures implement a software workaround for this operation,
> and I guess you're advocating that we do the same. We could copy one
> these implementations. However, the workarounds balloon into quite a lot
> of code. Here is an example from MIPS:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/mips/kernel/cmpxchg.c#n10
>
> I could be wrong, but I think this expansion would be pretty surprising
> for most users of xchg(). I suspect most xchg() users are looking for
> something performant, and would be better served by simply using a
> variable with a 32-bit type.
>
> In the case of your patch, it appears that struct
> fscrypt_mode.logged_impl_name is only used in the patched function. It
> looks like it could be promoted into a u32 without much difficulty.
> Would you be willing to consider that approach of solving the problem?
> Then the code would be able to take advantage of the fast hardware
> implementation that's available on many architectures (including RISC-V).
Yes, I already sent a new version of the patch, which changes the variable to an
int: https://patchwork.kernel.org/patch/11203003/. I was wondering more about
how to stop other people from running into this.
>
> > Note, there's at least one other place in the kernel that also uses
> > xchg() on a bool.
>
> Given the nasty compatibility code, I wonder if we'd be better served by
> removing most of this compatibility code across the kernel, and just
> requiring callers to use a 32-bit type? For most callers that I've seen,
> this doesn't seem to be much of an issue; and it would avoid the nasty
> code involved in software emulations of xchg().
>
It's possible that's the better approach; someone would need to go through all
the xchg() users and check whether any truly need the 8 or 16-bit support. My
main concern was just the annoyance of code that only fails to compile on
certain architectures. It should either be one way or the other everywhere.
- Eric
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