Re: [PATCH v2 6/7] s390/pci: Set MSI_FLAG_NO_AFFINITY at IRQ init time

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Mon, 2026-10-05 at 14:03 +0200, Tobias Schumacher wrote:
> MSI_FLAG_NO_AFFINITY is added to zpci_msi_parent_ops.required_flags from
> zpci_create_parent_msi_domain(), which runs for every new PCI bus,
> including buses created at runtime from a hotplug availability event.
> 
> That is a non-atomic read-modify-write on a field which
> msi_lib_init_dev_msi_info() reads without a common lock while setting up
> MSI for a device on an already existing bus:
> 
>       required_flags = pops->required_flags;
> 
> The stored value is always the same, so no caller observes a change, but
> the race need not exist: irq_delivery is decided once in zpci_irq_init()
> and never changes afterwards.
> 
> Set the flag there instead, before any parent domain exists.
> 
> Signed-off-by: Tobias Schumacher <ts@linux.ibm.com>
> ---
>  arch/s390/pci/pci_irq.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/s390/pci/pci_irq.c b/arch/s390/pci/pci_irq.c
> index 682bceb6525e..661867a1ffad 100644
> --- a/arch/s390/pci/pci_irq.c
> +++ b/arch/s390/pci/pci_irq.c
> @@ -523,9 +523,6 @@ int zpci_create_parent_msi_domain(struct zpci_bus *zbus)
>  		return -ENOMEM;
>  	}
>  
> -	if (irq_delivery == FLOATING)
> -		zpci_msi_parent_ops.required_flags |= MSI_FLAG_NO_AFFINITY;
> -
>  	zbus->msi_parent_domain = msi_create_parent_irq_domain(&info, &zpci_msi_parent_ops);
>  	if (!zbus->msi_parent_domain) {
>  		irq_domain_free_fwnode(info.fwnode);
> @@ -636,6 +633,9 @@ int __init zpci_irq_init(void)
>  	if (s390_pci_force_floating)
>  		irq_delivery = FLOATING;
>  
> +	if (irq_delivery == FLOATING)
> +		zpci_msi_parent_ops.required_flags |= MSI_FLAG_NO_AFFINITY;
> +
>  	if (irq_delivery == DIRECTED)
>  		zpci_airq.handler = zpci_directed_irq_handler;
>  

Looks good to me and I agree the race is benign as everyone races to
write the same value but this is certainly cleaner. Thanks.

Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>