linux-kernel - Re: Do we need to correct barriering in circular-buffers.rst?

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Message-ID: <CAKwvOd=LAdy5vMMfiadkHBmySG3BySzjpzQtKjadu7x6NEx8vQ@mail.gmail.com>
Date:   Fri, 27 Sep 2019 14:58:13 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andrea Parri <parri.andrea@...il.com>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Mark Rutland <mark.rutland@....com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        jose.marchesi@...cle.com
Subject: Re: Do we need to correct barriering in circular-buffers.rst?

On Fri, Sep 27, 2019 at 1:43 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Fri, Sep 27, 2019 at 5:49 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > Barring LTO the above works for perf because of inter-translation-unit
> > function calls, which imply a compiler barrier.
> >
> > Now, when the compiler inlines, it looses that sync point (and thereby
> > subtlely changes semantics from the non-inline variant). I suspect LTO
> > does the same and can cause subtle breakage through this transformation.
>
> Do you have a bug report or godbolt link for the above?  I trust that
> you're familiar enough with the issue to be able to quickly reproduce
> it?  These descriptions of problems are difficult for me to picture in
> code or generated code, and when I try to read through
> memory-barriers.txt my eyes start to glaze over (then something else
> catches fire and I have to go put that out).  Having a concise test
> case I think would better illustrate potential issues with LTO that
> we'd then be able to focus on trying to fix/support.
>

Further, if we identified a case were the existing invariants were
broken under LTO, such a case could be added to
CONFIG_DEBUG_LOCKING_API_SELFTESTS (or whatever the most appropriate
kself test is).

-- 
Thanks,
~Nick Desaulniers