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

Andrzej Hajda
2023-01-10T12:46:37Z

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