linux-kernel - Re: [PATCH v3 1/2] epoll: add nsec timeout support with epoll

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Date:   Wed, 18 Nov 2020 11:21:19 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     linux-fsdevel@...r.kernel.org
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Soheil Hassas Yeganeh <soheil.kdev@...il.com>,
        Arnd Bergmann <arnd@...db.de>, Shuo Chen <shuochen@...gle.com>,
        linux-man@...r.kernel.org
Subject: Re: [PATCH v3 1/2] epoll: add nsec timeout support with epoll_pwait2

On Wed, Nov 18, 2020 at 9:46 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> From: Willem de Bruijn <willemb@...gle.com>
>
> Add syscall epoll_pwait2, an epoll_wait variant with nsec resolution
> that replaces int timeout with struct timespec. It is equivalent
> otherwise.
>
>     int epoll_pwait2(int fd, struct epoll_event *events,
>                      int maxevents,
>                      const struct timespec *timeout,
>                      const sigset_t *sigset);
>
> The underlying hrtimer is already programmed with nsec resolution.
> pselect and ppoll also set nsec resolution timeout with timespec.
>
> The sigset_t in epoll_pwait has a compat variant. epoll_pwait2 needs
> the same.
>
> For timespec, only support this new interface on 2038 aware platforms
> that define __kernel_timespec_t. So no CONFIG_COMPAT_32BIT_TIME.
>
> Changes
>   v3:
>   - rewrite: add epoll_pwait2 syscall instead of epoll_create1 flag
>   v2:
>   - cast to s64: avoid overflow on 32-bit platforms (Shuo Chen)
>   - minor commit message rewording
>
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
>
> ---

> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index 109e6681b8fa..9a4e8ec207fc 100644
> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> @@ -447,3 +447,4 @@
>  440    i386    process_madvise         sys_process_madvise
>  441    i386    watch_mount             sys_watch_mount
>  442    i386    memfd_secret            sys_memfd_secret
> +443    i386    epoll_pwait2            sys_epoll_pwait2                compat_sys_epoll_pwait2

I should have caught this sooner, but this does not work as intended.

x86 will still call epoll_pwait2 with old_timespec32.

One approach is a separate epoll_pwait2_time64 syscall, similar to
ppoll_time64. But that was added to work around legacy 32-bit ppoll.
Not needed for a new API.

In libc, ppoll_time64 is declared with type struct __timespec64. That
type is not defined in Linux uapi. Will need to look at this some
more.