Re: [patch 05/10] PCI/MSI: Switch to MSI descriptor locking to guard()

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Sun,  9 Mar 2025 09:41:49 +0100 (CET)
Thomas Gleixner <tglx@linutronix.de> wrote:

> Convert the code to use the new guard(msi_descs_lock).
> 
> No functional change intended.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Bjorn Helgaas <bhelgaas@google.com>
> Cc: linux-pci@vger.kernel.org

This one runs into some of the stuff that the docs say
to not do.  I don't there there are bugs in here as such
but it gets harder to reason about and fragile in the long
run.  Easy enough to avoid though - see inline.

Jonathan

> --- a/drivers/pci/msi/msi.c
> +++ b/drivers/pci/msi/msi.c
> @@ -351,7 +351,7 @@ static int msi_verify_entries(struct pci
>  static int msi_capability_init(struct pci_dev *dev, int nvec,
>  			       struct irq_affinity *affd)
>  {
> -	struct irq_affinity_desc *masks = NULL;
> +	struct irq_affinity_desc *masks __free(kfree) = NULL;

This is a pattern that the cleanup.h docs talk about that
Linus really didn't like in some of the early patches because
of wanting constructors and destructors together.

Maybe use an inline definition as

	struct irq_affinity_desc *masks =
		affd ? irq_create_affinity_masks(nvec, affd) : NULL;


>  	struct msi_desc *entry, desc;
>  	int ret;
>  
> @@ -369,7 +369,7 @@ static int msi_capability_init(struct pc
>  	if (affd)
>  		masks = irq_create_affinity_masks(nvec, affd);
>  
> -	msi_lock_descs(&dev->dev);
> +	guard(msi_descs_lock)(&dev->dev);
>  	ret = msi_setup_msi_desc(dev, nvec, masks);
>  	if (ret)
>  		goto fail;

This runs against the advice in cleanup.h. It is probably
correct and doesn't hit the undefined paths that motivated
that guidance, but still maybe worth avoiding the mix of
cleanup and goto handling.

Easiest way being to factor out the code after guard to a helper.


> @@ -399,16 +399,13 @@ static int msi_capability_init(struct pc
>  
>  	pcibios_free_irq(dev);
>  	dev->irq = entry->irq;
> -	goto unlock;
> +	return 0;
>  
>  err:
>  	pci_msi_unmask(&desc, msi_multi_mask(&desc));
>  	pci_free_msi_irqs(dev);
>  fail:
>  	dev->msi_enabled = 0;
> -unlock:
> -	msi_unlock_descs(&dev->dev);
> -	kfree(masks);
>  	return ret;
>  }
>  
> @@ -669,13 +666,13 @@ static void msix_mask_all(void __iomem *
>  static int msix_setup_interrupts(struct pci_dev *dev, struct msix_entry *entries,
>  				 int nvec, struct irq_affinity *affd)
>  {
> -	struct irq_affinity_desc *masks = NULL;
> +	struct irq_affinity_desc *masks __free(kfree) = NULL;
Similar to above.
>  	int ret;
>  
>  	if (affd)
>  		masks = irq_create_affinity_masks(nvec, affd);
>  
> -	msi_lock_descs(&dev->dev);
> +	guard(msi_descs_lock)(&dev->dev);
>  	ret = msix_setup_msi_descs(dev, entries, nvec, masks);
>  	if (ret)
>  		goto out_free;
> @@ -690,13 +687,10 @@ static int msix_setup_interrupts(struct
>  		goto out_free;
>  
>  	msix_update_entries(dev, entries);
> -	goto out_unlock;
> +	return 0;
>  
>  out_free:
Here as well.
>  	pci_free_msi_irqs(dev);
> -out_unlock:
> -	msi_unlock_descs(&dev->dev);
> -	kfree(masks);
>  	return ret;
>  }
>  
> @@ -871,13 +865,13 @@ void __pci_restore_msix_state(struct pci
>  
>  	write_msg = arch_restore_msi_irqs(dev);
>  
> -	msi_lock_descs(&dev->dev);
> -	msi_for_each_desc(entry, &dev->dev, MSI_DESC_ALL) {
> -		if (write_msg)
> -			__pci_write_msi_msg(entry, &entry->msg);
> -		pci_msix_write_vector_ctrl(entry, entry->pci.msix_ctrl);
> +	scoped_guard (msi_descs_lock, &dev->dev) {
> +		msi_for_each_desc(entry, &dev->dev, MSI_DESC_ALL) {
> +			if (write_msg)
> +				__pci_write_msi_msg(entry, &entry->msg);
> +			pci_msix_write_vector_ctrl(entry, entry->pci.msix_ctrl);
> +		}
>  	}
> -	msi_unlock_descs(&dev->dev);
>  
>  	pci_msix_clear_and_set_ctrl(dev, PCI_MSIX_FLAGS_MASKALL, 0);
>  }
> 
> 
>