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

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Date:   Wed, 18 Nov 2020 22:22:20 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        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 <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 11:50 AM Arnd Bergmann <arnd@...nel.org> wrote:
>
> On Wed, Nov 18, 2020 at 5:21 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> > > 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.
>
> The libc __timespec64 corresponds to the __kernel_timespec64
> structure in uapi. It is defined to only have 'long' nanoseconds
> member because that's what c99 and posix require, but the bits
> are in the position that matches the lower 32 bits of the 64-bit
> tv_nsec in the kernel, and get_timespec64() performs the
> necessary conversion to either check or zero the upper bits.
>
> I think all you need in user space is to pass the timeout as a
> __timespec64 structure and add a conversion in the exported
> library interface.

Indeed, that resolves it, thanks.

I'll define that struct in the selftest and update the definition of
sys_epoll_pwait2 there.