Re: [PATCH v2] stop_machine: Make stop_one_cpu_nowait() return void

[Yury Norov <ynorov@nvidia.com>]

On Wed, Jul 29, 2026 at 02:51:33PM +0200, Peter Zijlstra wrote:
> On Tue, Jul 28, 2026 at 10:23:54PM -0400, Yury Norov wrote:
> > No caller checks the return value from stop_one_cpu_nowait(). All
> > callers require the callback to run and arrange for the target CPU's
> > stopper to remain enabled while queuing the work. In particular, commit
> > f0498d2a54e7 ("sched: Fix stop_one_cpu_nowait() vs hotplug") added
> > preemption protection to the scheduler callers so that queuing must
> > succeed once the target CPU has been observed online.
> > 
> > Therefore, a failure is an unrecoverable violation rather than a condition
> > individual callers can recover from. Diagnose it with WARN_ON_ONCE() in
> > stop_one_cpu_nowait(). A check in the common helper covers current and
> > future callers consistently, while individual checks would duplicate
> > the same non-recoverable handling at every call site.
> > 
> > Make the function return void because there is no longer a meaningful
> > result for callers to consume.
> > 
> > On UP, warn if the supplied CPU is not the current CPU because the work
> > cannot be scheduled in that case.
> > 
> > CC: Bradley Morgan <include@grrlz.net>
> > Signed-off-by: Yury Norov <ynorov@nvidia.com>
> 
> I suppose; you want me to take this through the scheduler tree?

Sure, thanks!