Re: [PATCH 1/2] x86/memsharing: use an atomic add instead of a cmpxchg loop

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.02.2024 19:03, Tamas K Lengyel wrote:
> On Thu, Feb 22, 2024 at 5:06 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>> On 22.02.2024 10:05, Roger Pau Monne wrote:
>>> The usage of a cmpxchg loop in get_next_handle() is unnecessary, as the same
>>> can be achieved with an atomic increment, which is both simpler to read, and
>>> avoid any need for a loop.
>>>
>>> The cmpxchg usage is likely a remnant of 32bit support, which didn't have an
>>> instruction to do an atomic 64bit add, and instead a cmpxchg had to be used.
>>>
>>> Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>> Signed-of-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>
>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>> albeit ...
>>
>>> --- a/xen/arch/x86/mm/mem_sharing.c
>>> +++ b/xen/arch/x86/mm/mem_sharing.c
>>> @@ -179,13 +179,7 @@ static void mem_sharing_page_unlock(struct page_info 
>>> *pg)
>>>
>>>  static shr_handle_t get_next_handle(void)
>>>  {
>>> -    /* Get the next handle get_page style */
>>> -    uint64_t x, y = next_handle;
>>> -    do {
>>> -        x = y;
>>> -    }
>>> -    while ( (y = cmpxchg(&next_handle, x, x + 1)) != x );
>>> -    return x + 1;
>>> +    return arch_fetch_and_add(&next_handle, 1) + 1;
>>>  }
>>
>> ... the adding of 1 here is a little odd when taken together with
>> next_handle's initializer. Tamas, you've not written that code, but do
>> you have any thoughts towards the possible removal of either the
>> initializer or the adding here? Plus that variable of course could
>> very well do with moving into this function.
> 
> I have to say I find the existing logic here hard to parse but by the
> looks I don't think we need the + 1 once we switch to
> arch_fetch_and_add. Also could go without initializing next_handle to
> 1. Moving it into the function would not really accomplish anything
> other than style AFAICT?

Well, limiting scope of things can be viewed as purely style, but I
think it's more than that: It makes intentions more clear and reduces
the chance of abuse (deliberate or unintentional).

Jan