On Thu, Feb 22, 2024 at 5:06 AM Jan Beulich <jbeul...@suse.com> 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.coop...@citrix.com>
> > Signed-of-by: Roger Pau Monné <roger....@citrix.com>
>
> Reviewed-by: Jan Beulich <jbeul...@suse.com>
> 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?
Tamas
