linux-kernel - Re: [patch V2 02/31] genirq/msi: Add mutex for MSI list protection

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Date:   Wed, 8 Dec 2021 20:47:02 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Marc Zygnier <maz@...nel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Megha Dey <megha.dey@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-pci@...r.kernel.org,
        Cedric Le Goater <clg@...d.org>,
        xen-devel@...ts.xenproject.org, Juergen Gross <jgross@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Niklas Schnelle <schnelle@...ux.ibm.com>,
        linux-s390@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Jon Mason <jdmason@...zu.us>,
        Dave Jiang <dave.jiang@...el.com>,
        Allen Hubbe <allenbh@...il.com>, linux-ntb@...glegroups.com
Subject: Re: [patch V2 02/31] genirq/msi: Add mutex for MSI list protection

On Mon, Dec 06, 2021 at 11:51:05PM +0100, Thomas Gleixner wrote:
> +++ b/kernel/irq/msi.c
> @@ -127,12 +127,37 @@ int msi_setup_device_data(struct device
>  		return -ENOMEM;
>  
>  	INIT_LIST_HEAD(&md->list);
> +	mutex_init(&md->mutex);
>  	dev->msi.data = md;
>  	devres_add(dev, md);
>  	return 0;
>  }
>  
>  /**
> + * msi_lock_descs - Lock the MSI descriptor storage of a device
> + * @dev:	Device to operate on
> + */
> +void msi_lock_descs(struct device *dev)
> +{
> +	if (WARN_ON_ONCE(!dev->msi.data))
> +		return;

Is this useful? Other places that call msi_lock_descs will continue on and deref
null dev->msi anyhow - is the dump from the WARN_ON that much better
than the oops from the null deref here:

> +	mutex_lock(&dev->msi.data->mutex);

?

Honestly, still a bit unclear on what the community consensus is for
using WARN_ON.

Jason