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

From: "Arnd Bergmann" <arnd@arndb.de>
To: "Nadav Amit" <nadav.amit@gmail.com>,
	"Thomas Gleixner" <tglx@linutronix.de>
Cc: linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-ia64@vger.kernel.org,
	linux-um@lists.infradead.org,
	Linux-Arch <linux-arch@vger.kernel.org>,
	linux-mm@kvack.org, "Andy Lutomirski" <luto@kernel.org>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	x86@kernel.org, "Richard Weinberger" <richard@nod.at>,
	"Anton Ivanov" <anton.ivanov@cambridgegreys.com>,
	"Johannes Berg" <johannes@sipsolutions.net>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Nadav Amit" <namit@vmware.com>,
	"Steven Rostedt" <rostedt@goodmis.org>
Subject: Re: [PATCH 3/3] compiler: inline does not imply notrace
Date: Tue, 22 Nov 2022 21:09:08 +0100	[thread overview]
Message-ID: <de999ab8-78ff-44f7-aacc-68561897c6e2@app.fastmail.com> (raw)
In-Reply-To: <20221122195329.252654-4-namit@vmware.com>

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

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