Re: [PATCH] smp: prevent soft lockup in smp_call_function_many_cond

[Paul E. McKenney]

On Tue, Jun 02, 2026 at 09:18:39PM +0000, Chris Packham wrote:
> (adding others suggested by get_maintainer.pl)
> 
> On 27/05/2026 15:16, Mark Tomlinson wrote:
> > Using the PowerPC P2040 (e500mc) CPU, soft lockups can occasionally be
> > seen in smp_call_function_many_cond(). The conclusion is that this CPU
> > does not process the doorbell interrupt while in a data-storage (MMU)
> > exception. If more than one CPU in a multi core environment is calling
> > this function at the same time, it is possible for a deadlock to occur.
> >
> > The fix for this is to call flush_smp_call_function_queue() before
> > waiting for responses from other CPUs. If there is something in the
> > queue, this is a good time to process it before busy-waiting on other
> > CPUs. On other architectures this call will quickly do nothing, as the
> > queue will be empty.

OK, I will bite...

How do we know that another entry will not get added by some other CPU
just before this new call to flush_smp_call_function_queue()?

							Thanx, Paul

> > Signed-off-by: Mark Tomlinson <mark.tomlinson@xxxxxxxxxxxxxxxxxxx>
> > ---
> >   kernel/smp.c | 2 ++
> >   1 file changed, 2 insertions(+)
> >
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > index a0bb56bd8dda..3c4467654ab0 100644
> > --- a/kernel/smp.c
> > +++ b/kernel/smp.c
> > @@ -884,6 +884,8 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
> >   		local_irq_restore(flags);
> >   	}
> >   
> > +	flush_smp_call_function_queue();
> > +
> >   	if (run_remote && wait) {
> >   		for_each_cpu(cpu, cfd->cpumask) {
> >   			call_single_data_t *csd;