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

[Nathan Chancellor <nathan@kernel.org>]

On Tue, Nov 18, 2025 at 10:54:32AM +0100, Peter Zijlstra wrote:
> 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 :/

The C11 standard says in 6.3.2.3p3

  An integer constant expression with the value 0, or such an expression
  cast to type void *, is called a null pointer constant.

which seems to indicate to me that

  int *foo = 0;

and

  #define NULL (void *)0
  int *foo = NULL;

have to be treated the same way :/ I think that is a big part of the
motivation to bring nullptr into C in C23:

  https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3042.htm

> 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?).

Right, it appears to be the same case for clang, notice the comment in
diagnoseZeroToNullptrConversion():

  https://github.com/llvm/llvm-project/commit/d7ba86b6bf54740dd4007e65a927151cb9f510b4

That warning should probably be updated to work for C23 but that does
not really help us now because nullptr is not available in older
standards (and I think the support for C23 is only solid in really
recent compilers IIUC).

> Help?

Maybe we could have something like -Wnon-literal-null-conversion-strict
in clang that would behave like -Wnon-literal-null-conversion but warn
even in the literal zero conversion case (i.e., require a 'void *'
cast)... That does not really help GCC though since it does not warn on
any case of implicit conversion to NULL:

https://godbolt.org/z/M5WE5covz

Cheers,
Nathan