linux-kernel - Re: [PATCH] perf bench: Add support for 32-bit systems with 64-bit time

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Date:   Fri, 10 Sep 2021 10:45:23 +1000
From:   Alistair Francis <alistair23@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Alistair Francis <alistair.francis@...nsource.wdc.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-perf-users@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Darren Hart <dvhart@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Atish Patra <atish.patra@....com>,
        Alistair Francis <alistair.francis@....com>
Subject: Re: [PATCH] perf bench: Add support for 32-bit systems with 64-bit time_t

On Thu, Sep 9, 2021 at 6:20 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Thu, Sep 9, 2021 at 6:25 AM Alistair Francis
> <alistair.francis@...nsource.wdc.com> wrote:
> >
> > From: Alistair Francis <alistair.francis@....com>
> >
> > Some 32-bit architectures (such are 32-bit RISC-V) only have a 64-bit
> > time_t and as such don't have the SYS_futex syscall. This patch will
> > allow us to use the SYS_futex_time64 syscall on those platforms.
> >
> > This patch does not attempt to gracefully allow 32-bit architectures
> > with both SYS_futex and SYS_futex_time64 to support a 64-bit time_t.
> > This patch only applies to 32-bit architectures with a 64-bit time_t.
> >
> > Signed-off-by: Alistair Francis <alistair.francis@....com>
>
> Hi Alistair,
>
> I know you've made this mistake before and I've pointed it out
> several times. Please don't do this again, and try to fix up the
> ones you already broke!
>
> > +/**
> > + * Some newer 32-bit architectures (such as RISC-V 32-bit) don't have
> > + * the SYS_futex syscall and instead only have the SYS_futex_time64 call.
> > + * Let's ensure that those still compile and run by just using the
> > + * SYS_futex_time64 syscall. On these systems `struct timespec` will use a
> > + * 64-bit time_t so the SYS_futex_time64 call will work.
> > + */
> > +#if !defined(SYS_futex) && defined(SYS_futex_time64)
> > + #define SYS_futex SYS_futex_time64
> > +#endif
>
> This cannot work, as two system calls take different arguments: futex() takes
> a __kernel_old_timespec and futex_time64() takes a __kernel_timespec.
>
> You cannot derive anything about the ABI of the C library based on whether
> the macros are defined or not. Either you convert the arguments passed into
> the system call into the format expected by the kernel, or you pick the
> correct system call based on sizeof(struct timespec).

Thanks Arnd. Sorry I hadn't looked at this in a while and forgot that
it's more complex.

I have some patches to fix this up. I'll send them later today.

Alistair

>
>        Arnd