linux-kernel - Re: linux-next: build warnings after merge of the tip tree

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Message-ID: <20220322224236.46f8c2f1@gandalf.local.home>
Date:   Tue, 22 Mar 2022 22:42:36 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        ast@...nel.org, hjl.tools@...il.com, rick.p.edgecombe@...el.com,
        rppt@...nel.org, linux-toolchains@...r.kernel.org,
        Andrew.Cooper3@...rix.com, ndesaulniers@...gle.com
Subject: Re: linux-next: build warnings after merge of the tip tree

On Wed, 23 Mar 2022 11:23:23 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> I see the __fexit__ is needed, but why __ftail__ is needed? I guess because
> func_B is notrace, in that case the __fexit__ will not be in the func_B.
> Am I correct?

I believe Peter and I agreed that the "best" solution so far, that has the
least amount of regressions (doesn't remove anything currently being
function graph traced, nor removes current tail calls) is:

> At that point giving us something like:
> 
> 1:
> 	pushsection __ftail_loc
> 	.long	1b - .
> 	popsection
> 
> 	jmp.d32	func_B
> 	call	__fexit__
> 	ret


Functions with a tail call will not have a __fexit__ and we can not rely on
the function that is the tail call to do the __fexit__ for the parent
function. Thus, the compromise is to add a label where the jmp to the
tail-call function is, and when we want to trace the return of that
function, we first have to patch the jmp into a call, which will then
return back to the call to __fexit__.

-- Steve