Subject: Re: [PATCH net-next v4] net: skb: prevent the split of kfree_skb_reason() by gcc

From: Segher Boessenkool <[email-masked]>
Date: Tue, 6 Sep 2022 10:30:47 -0500

On Tue, Sep 06, 2022 at 02:37:47PM +0200, Florian Weimer wrote:
> > On Mon, Aug 22, 2022 at 4:01 PM Florian Weimer <[email-masked]> wrote:
> > I did some research on the 'sibcalls' you mentioned above. Feel like
> > It's a little similar to 'inline', and makes the callee use the same stack
> > frame with the caller, which obviously will influence the result of
> > '__builtin_return_address'.

Sibling calls are essentially calls that can be replaced by jumps (aka
"tail call"), without needing a separate entry point to the callee.

Different targets can have a slightly different implementation and
definition of what exactly is a sibling call, but that's the gist.

> > Hmm......but I'm not able to find any attribute to disable this optimization.
> > Do you have any ideas?
> 
> Unless something changed quite recently, GCC does not allow disabling
> the optimization with a simple attribute (which would have to apply to
> function pointers as well, not functions).

It isn't specified what a sibling call exactly *is*, certainly not on C
level (only in the generated machine code), and the details differs per
target.

> asm ("") barriers that move
> out a call out of the tail position are supposed to prevent the
> optimization.

Not just "supposed": they work 100%.  The asm has to stay after the
function call by the fundamental rules of C (the function call having a
sequence point, and the asm a side effect).

void g(void);
void f(void)
{
	g();		// This can not be optimised to a jump...
	asm("");	// ... because it has to stay before this.
}

Segher