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

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

On Wed, Feb 28, 2024 at 12:18:31PM +0100, Jan Beulich wrote:
> On 28.02.2024 11:53, Roger Pau Monné wrote:
> > 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.
> 
> That's fine with me, but an ack from Tamas is still pending, unless I
> missed something somewhere.

No, just wanted to clarify that I wasn't expecting to do further
changes here, FTAOD.  Not sure if Tamas was expecting me to further
adjust the code.

Thanks, Roger.