Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20200612185122.327860-1-andrealmeid@collabora.com>
In-Reply-To: <20200612185122.327860-1-andrealmeid@collabora.com>
From:   "H.J. Lu" <hjl.tools@gmail.com>
Date:   Fri, 12 Jun 2020 12:35:19 -0700
Message-ID: <CAMe9rOqnBRzXv4xnhFvOgdVpDo0oRc1SYq38zcJWo9BPZseagg@mail.gmail.com>
Subject: Re: [RFC 0/4] futex2: Add new futex interface
To:     =?UTF-8?Q?Andr=C3=A9_Almeida?= <andrealmeid@collabora.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Florian Weimer <fweimer@redhat.com>, malteskarupke@web.de,
        GNU C Library <libc-alpha@sourceware.org>,
        Linux API <linux-api@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>, dvhart@infradead.org,
        kernel@collabora.com, krisman@collabora.com,
        pgriffais@valvesoftware.com
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Jun 12, 2020 at 11:53 AM Andr=C3=A9 Almeida via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> Hello,
>
> This RFC is a followup to the previous discussion initiated from my last
> patch "futex: Implement mechanism to wait on any of several futexes"[1].
> As stated in the thread, the correct approach to move forward with the
> wait multiple operation would be to create a new syscall that would have
> all new cool features.
>
> The first patch adds the new interface and just translate the call for
> the old interface, without implementing new features. The goal here is
> to establish the interface and to check if everyone is happy with this
> API. The rest of patches are selftests to show the interface in action.
> I have the following questions:
>
> - Has anyone stared worked on a implementation of this interface? If
>   yes, it would be nice to share the progress so we don't have duplicated
>   work.
>
> - What suggestions do you have to implement this? Start from scratch or
>   reuse the most code possible?
>
> - The interface seems correct and implements the requirements asked by yo=
u?
>
> - The proposed interface uses ktime_t type for absolute timeout, and I
>   assumed that it should use values in a nsec resolution. If this is true=
,
>   we have some problems with i386 ABI, please check out the
>   COMPAT_32BIT_TIME implementation in patch 1 for more details. I
>   haven't added a time64 implementation yet, until this is clarified.
>
> - Is expected to have a x32 ABI implementation as well? In the case of
>   wait and wake, we could use the same as x86_64 ABI. However, for the
>   waitv (aka wait on multiple futexes) we would need a proper x32 entry
>   since we are dealing with 32bit pointers.

x32 should be able to use the same i386 compat systcall entry.   Will it be
problem?

> Those are the cool new features that this syscall should address some
> day:
>
> - Operate with variable bit size futexes, not restricted to 32:
>   8, 16 and 64
>
> - Wait on multiple futexes, using the following semantics:
>
>   struct futex_wait {
>         void *uaddr;
>         unsigned long val;
>         unsigned long flags;
>   };
>
>   sys_futex_waitv(struct futex_wait *waiters, unsigned int nr_waiters,
>                   unsigned long flags, ktime_t *timo);
>
> - Have NUMA optimizations: if FUTEX_NUMA_FLAG is present, the `void *uadd=
r`
>   argument won't be a u{8, 16, 32, 64} value anymore, but a struct
>   containing a NUMA node hint:
>
>   struct futex32_numa {
>           u32 value __attribute__ ((aligned (8)));
>           u32 hint;
>   };
>
>   struct futex64_numa {
>           u64 value __attribute__ ((aligned (16)));
>           u64 hint;
>   };
>

H.J.