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

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Date:   Sat, 21 Nov 2020 10:27:38 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        David Laight <David.Laight@...lab.com>,
        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>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH v3 1/2] epoll: add nsec timeout support with epoll_pwait2

On Fri, Nov 20, 2020 at 11:28 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
> On Fri, Nov 20, 2020 at 2:23 PM Arnd Bergmann <arnd@...nel.org> wrote:
> > On Fri, Nov 20, 2020 at 5:01 PM Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
>
> I think it'll be better to split the patchsets:
>
> epoll: convert internal api to timespec64
> epoll: add syscall epoll_pwait2
> epoll: wire up syscall epoll_pwait2
> selftests/filesystems: expand epoll with epoll_pwait2
>
> and
>
> select: compute slack based on relative time
> epoll: compute slack based on relative time
>
> and judge the slack conversion on its own merit.

Yes, makes sense.

> I also would rather not tie this up with the compat deduplication.
> Happy to take a stab at that though. On that note, when combining
> functions like
>
>   int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
>                            fd_set __user *exp, struct timespec64 *end_time,
>                            u64 slack)
>
> and
>
>   static int compat_core_sys_select(int n, compat_ulong_t __user *inp,
>         compat_ulong_t __user *outp, compat_ulong_t __user *exp,
>         struct timespec64 *end_time, u64 slack)
>
> by branching on in_compat_syscall() inside get_fd_set/set_fd_set and
> deprecating their compat_.. counterparts, what would the argument
> pointers look like? Or is that not the approach you have in mind?

In this case, the top-level entry points becomes unified, and you get
the prototype from core_sys_select() with the native arguments.

I would imagine this can be done like the way I proposed
for get_bitmap() in sys_migrate_pages:

https://lore.kernel.org/lkml/20201102123151.2860165-4-arnd@kernel.org/

        Arnd