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

[Steven Rostedt]

On Wed, 23 Mar 2022 11:23:23 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> 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