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

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Wed, Feb 28, 2024 at 8:28 AM Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
>
> 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.

Acked-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>