Re: [tip: sched/core] sched/fair: Skip sched_balance_running cmpxchg when balance is not due

[Peter Zijlstra]

On Mon, Nov 17, 2025 at 10:55:07AM -0800, Tim Chen wrote:

> >          if (!need_unlock && (sd->flags & SD_SERIALIZE)) {
> > -               if (!atomic_try_cmpxchg_acquire(&sched_balance_running, 0, 1))
> 
> The second argument of atomic_try_cmpxchg_acquire is "int *old" while that of atomic_cmpxchg_acquire
> is "int old". So the above check would result in NULL pointer access.  Probably have
> to do something like the following to use atomic_try_cmpxchg_acquire()
> 
> 		int zero = 0;
> 		if (!atomic_try_cmpxchg_acquire(&sched_balance_running, &zero, 1))
> 		
> Otherwise we should do atomic_cmpxchg_acquire() as below

Yes, and I'm all mightily miffed all the compilers accept 0 (which is
int) for an 'int *' argument without so much as a warning :/

Nathan, you looked into this a bit yesterday, afaict there is:

  -Wzero-as-null-pointer-constant

which is supposed to issue a warn here, but I can't get clang-22 to
object :/ (GCC doesn't take that warning for C mode, only C++, perhaps
that's the problem?).

And then there is modernize-use-nullptr, but that objects to using NULL,
although I suppose we could do:

  #define NULL nullptr

to get around that. Except I also cannot get clangd to report on the
issue.

Help?