Re: [PATCH 03/17] vfio/pci: Consistently acquire mutex for interrupt management

[Reinette Chatre]

Hi Alex,

On 2/5/2024 2:34 PM, Alex Williamson wrote:
> On Thu,  1 Feb 2024 20:56:57 -0800
> Reinette Chatre <reinette.chatre@xxxxxxxxx> wrote:
> 
>> vfio_pci_set_irqs_ioctl() is the entrypoint for interrupt management
>> via the VFIO_DEVICE_SET_IRQS ioctl(). The igate mutex is obtained
>> before calling vfio_pci_set_irqs_ioctl() for management of all interrupt
>> types to protect against concurrent changes to the eventfds associated
>> with device request notification and error interrupts.
>>
>> The igate mutex is not acquired consistently. The mutex is always
>> (for all interrupt types) acquired from within vfio_pci_ioctl_set_irqs()
>> before calling vfio_pci_set_irqs_ioctl(), but vfio_pci_set_irqs_ioctl() is
>> called via vfio_pci_core_disable() without the mutex held. The latter
>> is expected to be correct if the code flow can be guaranteed that
>> the provided interrupt type is not a device request notification or error
>> interrupt.
> 
> The latter is correct because it's always a physical interrupt type
> (INTx/MSI/MSIX), vdev->irq_type dictates this, and the interrupt code
> prevents the handler from being called after the interrupt is disabled.

Thank you for confirming. 

> It's intentional that we don't acquire igate here since we only need to
> prevent a race with concurrent user access, which cannot occur in the
> fd release path.  The igate mutex is acquired consistently, where it's
> required. 

Thank you. I do think it will be helpful to document some of this
in the code to help newcomers distinguish the scenarios (more below).

> It would be more forthcoming to describe that potential future emulated
> device interrupts don't make the same guarantees, but if that's true,
> why can't they?

As I understand an emulated interrupt will be triggered by VFIO PCI driver
as a result from, for example, a mmio write from user space. I thus expect
similar locking to existing device request notification and error interrupts.
I would like to focus this series on existing flows though.

>> Move igate mutex acquire and release into vfio_pci_set_irqs_ioctl()
>> to make the locking consistent irrespective of interrupt type.
>> This is one step closer to contain the interrupt management locking
>> internals within the interrupt management code so that the VFIO PCI
>> core can trigger management of the eventfds associated with device
>> request notification and error interrupts without needing to access
>> and manipulate VFIO interrupt management locks and data.
> 
> If all we want to do is move the mutex into vfio_pci_intr.c then we
> could rename to __vfio_pci_set_irqs_ioctl() and create a wrapper around
> it that grabs the mutex.  The disable path could use the lockless
> version and we wouldn't need to clutter the exit path unlocking the
> mutex as done below.  Thanks,

Will do. This creates an opportunity to document the flows involving
the mutex (essentially adding comments that includes your description
above).

Reinette