linux-kernel - Re: [PATCH] io_uring: use __kernel

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Message-ID: <CAK8P3a3AAFXNmpQwuirzM+jgEQGj9tMC_5oaSs4CfiEVGmTkZg@mail.gmail.com>
Date:   Tue, 1 Oct 2019 17:49:43 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     y2038 Mailman List <y2038@...ts.linaro.org>,
        Linux API <linux-api@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Stefan Bühler <source@...uehler.de>,
        Hannes Reinecke <hare@...e.com>,
        Jackie Liu <liuyun01@...inos.cn>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hristo Venev <hristo@...ev.name>,
        linux-block <linux-block@...r.kernel.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] io_uring: use __kernel_timespec in timeout ABI

On Tue, Oct 1, 2019 at 5:38 PM Jens Axboe <axboe@...nel.dk> wrote:
>
> On 10/1/19 8:09 AM, Jens Axboe wrote:
> > On 9/30/19 2:20 PM, Arnd Bergmann wrote:
> >> All system calls use struct __kernel_timespec instead of the old struct
> >> timespec, but this one was just added with the old-style ABI. Change it
> >> now to enforce the use of __kernel_timespec, avoiding ABI confusion and
> >> the need for compat handlers on 32-bit architectures.
> >>
> >> Any user space caller will have to use __kernel_timespec now, but this
> >> is unambiguous and works for any C library regardless of the time_t
> >> definition. A nicer way to specify the timeout would have been a less
> >> ambiguous 64-bit nanosecond value, but I suppose it's too late now to
> >> change that as this would impact both 32-bit and 64-bit users.
> >
> > Thanks for catching that, Arnd. Applied.
>
> On second thought - since there appears to be no good 64-bit timespec
> available to userspace, the alternative here is including on in liburing.

What's wrong with using __kernel_timespec? Just the name?
I suppose liburing could add a macro to give it a different name
for its users.

> That seems kinda crappy in terms of API, so why not just use a 64-bit nsec
> value as you suggest? There's on released kernel with this feature yet, so
> there's nothing stopping us from just changing the API to be based on
> a single 64-bit nanosecond timeout.

Certainly fine with me.

> +       timeout = READ_ONCE(sqe->addr);
>         hrtimer_init(&req->timeout.timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
>         req->timeout.timer.function = io_timeout_fn;
> -       hrtimer_start(&req->timeout.timer, timespec_to_ktime(ts),
> +       hrtimer_start(&req->timeout.timer, ns_to_ktime(timeout),

It seems a little odd to use the 'addr' field as something that's not
an address,
and I'm not sure I understand the logic behind when you use a READ_ONCE()
as opposed to simply accessing the sqe the way it is done a few lines
earlier.

The time handling definitely looks good to me.

       Arnd