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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Feb 23, 2024 at 08:43:24AM +0100, Jan Beulich wrote:
> 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).

I'm afraid that whatever is the outcome here, I will defer it to a
further commit, since the purpose here is to be a non-functional
change.

Thanks, Roger.