Re: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage

On Mon, Nov 15, 2021 at 01:59:36PM +0100, Miroslav Benes wrote:

On Fri, 12 Nov 2021, Josh Poimboeuf wrote:

quoted

If the child schedules out, and then the parent gets patched, things can
go off-script if the child later jumps back to the unpatched version of
the parent, and then for example the old parent tries to call another
patched function with a since-changed ABI.

...
 

quoted

I don't know about other patch creation tooling, but I'd imagine they
also need to know about .cold functions, unless they have that
optimization disabled.  Because the func and its .cold counterpart
always need to be patched together.

We, at SUSE, solve the issue differently... the new patched parent would 
call that another patched function with a changed ABI statically in a live 
patch. So in that example, .cold child would jump back to the unpatched 
parent which would then call, also, the unpatched function.

So if I understand correctly, if a function's ABI changes then you don't
patch it directly, but only patch its callers to call the replacement?

Is there a reason for that?

-- 
Josh