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

[Arnd Bergmann]

On Tue, Nov 22, 2022, at 20:53, Nadav Amit wrote:
> From: Nadav Amit <namit@xxxxxxxxxx>
>
> 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