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

[Chris Packham]

(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.
>
> 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;