linux-kernel - Re: [PATCH 3/3] compiler: inline does not imply notrace

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Date:   Tue, 22 Nov 2022 21:09:08 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Nadav Amit" <nadav.amit@...il.com>,
        "Thomas Gleixner" <tglx@...utronix.de>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-um@...ts.infradead.org,
        Linux-Arch <linux-arch@...r.kernel.org>, linux-mm@...ck.org,
        "Andy Lutomirski" <luto@...nel.org>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "Richard Weinberger" <richard@....at>,
        "Anton Ivanov" <anton.ivanov@...bridgegreys.com>,
        "Johannes Berg" <johannes@...solutions.net>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Nadav Amit" <namit@...are.com>,
        "Steven Rostedt" <rostedt@...dmis.org>
Subject: Re: [PATCH 3/3] compiler: inline does not imply notrace

On Tue, Nov 22, 2022, at 20:53, Nadav Amit wrote:
> From: Nadav Amit <namit@...are.com>
>
> Functions that are marked as "inline" are currently also not tracable.
> Apparently, this has been done to prevent differences between different
> configs that caused different functions to be tracable on different
> platforms.
>
> Anyhow, this consideration is not very strong, and tying "inline" and
> "notrace" does not seem very beneficial. The "inline" keyword is just a
> hint, and many functions are currently not tracable due to this reason.

The original reason was listed in 93b3cca1ccd3 ("ftrace: Make all
inline tags also include notrace"), which describes

    Commit 5963e317b1e9d2a ("ftrace/x86: Do not change stacks in DEBUG when
    calling lockdep") prevented lockdep calls from the int3 breakpoint handler
    from reseting the stack if a function that was called was in the process
    of being converted for tracing and had a breakpoint on it. The idea is,
    before calling the lockdep code, do a load_idt() to the special IDT that
    kept the breakpoint stack from reseting. This worked well as a quick fix
    for this kernel release, until a certain config caused a lockup in the
    function tracer start up tests.
    
    Investigating it, I found that the load_idt that was used to prevent
    the int3 from changing stacks was itself being traced!

and this sounds like a much stronger reason than what you describe,
and I would expect your change to cause regressions in similar places.

It's possible that the right answer is that the affected functions
should be marked as __always_inline. 

      Arnd