linux-kernel - Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12)

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Date:   Mon, 2 Apr 2018 10:03:58 -0500 (CDT)
From:   Christopher Lameter <cl@...ux.com>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>
cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Andi Kleen <andi@...stfloor.org>, Ben Maurer <bmaurer@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences
 system call (v12)

On Sun, 1 Apr 2018, Alan Cox wrote:

> >        Restartable sequences are atomic  with  respect  to  preemption
> >        (making  it atomic with respect to other threads running on the
> >        same CPU), as well as  signal  delivery  (user-space  execution
> >        contexts nested over the same thread).
>
> CPU generally means 'big lump with legs on it'. You are not atomic to the
> same CPU, because that CPU may have 30+ cores with 8 threads per core.
>
> It could do with some better terminology (hardware thread, CPU context ?)

Well we call it a "CPU" in the scheduler context I think.  We could use
better terminology throughout the kernel tools and source.

Hardware Execution Context?

> >        In  a  typical  usage scenario, the thread registering the rseq
> >        structure will be performing  loads  and  stores  from/to  that
> >        structure.  It  is  however also allowed to read that structure
> >        from other threads.  The rseq field updates  performed  by  the
> >        kernel  provide  relaxed  atomicity  semantics, which guarantee
> >        that other threads performing relaxed atomic reads of  the  cpu
> >        number cache will always observe a consistent value.
>
> So what happens to your API if the kernel atomics get improved ? You are
> effectively exporting rseq behaviour from private to public.

There is already a pretty complex coherency model guiding kernel atomics.
Improvements/changes to that are difficult and the effect will ripple
throughout the kernel. So I would suggest that these areas of the kernel
are pretty "petrified" (or written in stone).