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Date:   Mon, 21 Mar 2022 17:40:32 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Stephen Rothwell <sfr@canb.auug.org.au>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Next Mailing List <linux-next@vger.kernel.org>,
        mhiramat@kernel.org, ast@kernel.org, hjl.tools@gmail.com,
        rick.p.edgecombe@intel.com, rppt@kernel.org,
        linux-toolchains@vger.kernel.org, Andrew.Cooper3@citrix.com,
        ndesaulniers@google.com
Subject: Re: linux-next: build warnings after merge of the tip tree
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On Mon, Mar 21, 2022 at 12:15:49PM -0400, Steven Rostedt wrote:
> On Mon, 21 Mar 2022 12:12:09 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > > > funcB:
> > > > 	call __fentry__    
> > > 			push funcB on trace-stack  
> > > > 
> > > > 	[..]    
> > > 	call __fexit__
> > > 			pop trace-stack until empty
> > > 			  'exit funcB'
> > > 			  'exit funcA'  
> > 
> > And what happens if funcC called funcA and it too was on the stack. We pop
> > that too? But it's not done yet, because calling of funcA was not a tail
> > call.

Hmm, yeah, how about we have __ftail__ mark the left function.

func_B()
{
	...
}

func_A()
{
	...
	return func_B();
}

func_C()
{
	func_A();
	...
	return;
}

func_B:
	call __fentry__	/* push func_B */
	...
	call __fexit__	/* pop 1 + tails */
	ret

func_A:
	call __fentry__ /* push func_A */
	...
	call __ftail__  /* mark func_A tail */
	jmp func_B

func_C:
	call __fentry__ /* push func_C */
	call func_A;
	...
	call __fexit__  /* pop 1 + tails */
	ret;


Then the stack at the end of func_B looks something like:

	func_C
	func_A (tail)
	func_B

And it will pop func_B plus all tails (func_A).

> And I just thought of another issue, where even my solution wont fix it.
> What happens if we trace funcA but not funcB? How do we get to trace the
> end of funcA?

Disallow tail calls to notrace?