linux-kernel - Re: [PATCH] smp: simplify smp_call_function

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Message-ID: <aOfNpZF5jlkVgN0k@yury>
Date: Thu, 9 Oct 2025 10:58:45 -0400
From: Yury Norov <yury.norov@...il.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Rik van Riel <riel@...riel.com>, Ingo Molnar <mingo@...nel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Roman Kisel <romank@...ux.microsoft.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] smp: simplify smp_call_function_any()

On Wed, Oct 08, 2025 at 01:06:18PM -0400, Mathieu Desnoyers wrote:
> On 2025-10-08 12:57, Yury Norov (NVIDIA) wrote:
> > The functions calls get_cpu()/put_cpu() meaningless because the actual
> > CPU that would execute the caller's function is not necessarily the
> > current one.
> > 
> > The smp_call_function_single() which is called by
> > smp_call_function_any() does the right get/put protection.
> > 
> > Signed-off-by: Yury Norov (NVIDIA) <yury.norov@...il.com>
> > ---
> >   kernel/smp.c | 8 ++------
> >   1 file changed, 2 insertions(+), 6 deletions(-)
> > 
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > index 02f52291fae4..fa50ed459703 100644
> > --- a/kernel/smp.c
> > +++ b/kernel/smp.c
> > @@ -754,17 +754,13 @@ EXPORT_SYMBOL_GPL(smp_call_function_single_async);
> >   int smp_call_function_any(const struct cpumask *mask,
> >   			  smp_call_func_t func, void *info, int wait)
> >   {
> > -	unsigned int cpu;
> > -	int ret;
> > +	unsigned int cpu = smp_processor_id();
> 
> I wonder whether this passes any moderate testing with kernel debug
> options enabled. I would at the very least expect a
> raw_smp_processor_id() call here not to trip debug warnings.
> 
> AFAIU smp_call_function_any call be called from preemptible context,
> right ?

You're right, we need to retain current CPU unless the work is
scheduled. I need to test better. Sorry for the noise.