Re: [Intel-gfx] [RFC DO NOT MERGE] treewide: use __xchg in most obvious places

[Andrzej Hajda]

On 10.01.2023 12:07, Andy Shevchenko wrote:
> On Tue, Jan 10, 2023 at 11:53:06AM +0100, Andrzej Hajda wrote:
> > This patch tries to show usability of __xchg helper.
> > It is not intended to be merged, but I can convert
> > it to proper patchset if necessary.
> >
> > There are many more places where __xchg can be used.
> > This demo shows the most spectacular cases IMHO:
> > - previous value is returned from function,
> > - temporary variables are in use.
> >
> > As a result readability is much better and diffstat is quite
> > nice, less local vars to look at.
> > In many cases whole body of functions is replaced
> > with __xchg(ptr, val), so as further refactoring the whole
> > function can be removed and __xchg can be called directly.
>
> ...
>
> >   arch_uretprobe_hijack_return_addr(unsigned long trampoline_vaddr,
> >   				  struct pt_regs *regs)
> >   {
> > -	unsigned long orig_ret_vaddr;
> > -
> > -	orig_ret_vaddr = regs->ARM_lr;
> > -	/* Replace the return addr with trampoline addr */
> > -	regs->ARM_lr = trampoline_vaddr;
> > -	return orig_ret_vaddr;
> > +	return __xchg(&regs->ARM_lr, trampoline_vaddr);
> >   }
>
> If it's not a callback, the entire function can be killed.
> And this is a good example of the function usage.
> OTOH, these places might have a side effect (if it's in deep CPU
> handlers), means we need to do this carefully.
>
> ...
>
> >   static inline void *qed_chain_produce(struct qed_chain *p_chain)
> >   {
> > -	void *p_ret = NULL, *p_prod_idx, *p_prod_page_idx;
> > +	void *p_prod_idx, *p_prod_page_idx;
> >   
> >   	if (is_chain_u16(p_chain)) {
> >   		if ((p_chain->u.chain16.prod_idx &
> > @@ -390,11 +391,8 @@ static inline void *qed_chain_produce(struct qed_chain *p_chain)
> >   		p_chain->u.chain32.prod_idx++;
> >   	}
> >   
> > -	p_ret = p_chain->p_prod_elem;
> > -	p_chain->p_prod_elem = (void *)(((u8 *)p_chain->p_prod_elem) +
> > -					p_chain->elem_size);
> > -
> > -	return p_ret;
> > +	return __xchg(&p_chain->p_prod_elem,
> > +		      (void *)(((u8 *)p_chain->p_prod_elem) + p_chain->elem_size));
>
> Wondering if you still need a (void *) casting after the change. Ditto for the
> rest of similar cases.

IMHO it is not needed also before the change and IIRC gcc has an 
extension which allows to drop (u8 *) cast as well [1].

[1]: https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html

> >   }
>
> ...
>
> Btw, is it done by coccinelle? If no, why not providing the script?

Yes I have used cocci. My cocci skills are far from perfect, so I did 
not want to share my dirty code, but this is nothing secret:

@r1@
expression x, v;
local idexpression p;
@@
-       p = x;
-       x = v;
-       return p;
+       return __xchg(&x, v);

@depends on r1@
expression e;
@@
        __xchg(
-       &*e,
+       e,
        ...)

@depends on r1@
expression t;
@@
-       if (t) {
+       if (t)
                return __xchg(...);
-       }

@depends on r1@
type t;
identifier p;
expression e;
@@
(
-       t p;
|
-       t p = e;
)
        ... when != p

Regards
Andrzej