linux-kernel - Re: Current mainline git (24e700e291d52bd2) hangs when building e.g. perf

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Date:   Fri, 8 Sep 2017 16:23:11 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        Markus Trippelsdorf <markus@...ppelsdorf.de>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: Current mainline git (24e700e291d52bd2) hangs when building e.g. perf

On Fri, Sep 8, 2017 at 4:07 PM, Andy Lutomirski <luto@...nel.org> wrote:
>
> I *think* this is impossible because CPU A's mm_cpumask manipulations
> are atomic and should therefore force out the streaming write buffers,
> but maybe there's some other scenario where this matters.

I don't think atomic memops do that.

They enforce globally visible ordering, but since they happen in the
cache and is not actually visible to outside, that doesn't actually
affect any streaming write buffers.

Then, if somebody else requests a cacheline that we have exclusive
ownership to, the write buffers just need to flush before we give up
that cacheline.

So a locked memory op is *not* serializing, it only enforces memory
ordering. Big difference.

Only fully serializing instructions will serialize with the write
buffers, and they are expensive as hell (partly exactly _due_ to these
kinds of issues).

So this change to delay invalidation does sound fairly scary..

              Linus