Re: [PATCH] Fix race bug in nvme_poll_irqdisable()

[Bjorn Helgaas]

On Sat, Mar 07, 2026 at 02:46:36PM -0500, Sungwoo Kim wrote:
> In the following scenario, pdev can be disabled between (1) and (3) by
> (2). This sets pdev->msix_enabled = 0. Then, pci_irq_vector() will
> return MSI-X IRQ(>15) for (1) whereas return INTx IRQ(<=15) for (2).
> This causes IRQ warning because it tries to enable INTx IRQ that has
> never been disabled before.
> 
> To fix this, save IRQ number into a local variable and ensure
> disable_irq() and enable_irq() operate on the same IRQ number.
> Even if pci_free_irq_vectors() frees the IRQ concurrently, disable_irq()
> and enable_irq() on a stale IRQ number is still valid and safe, and the
> depth accounting reamins balanced.
> 
> task 1:
> nvme_poll_irqdisable()
>   disable_irq(pci_irq_vector(pdev, nvmeq->cq_vector)) ...(1)
>   enable_irq(pci_irq_vector(pdev, nvmeq->cq_vector))  ...(3)
> 
> task 2:
> nvme_reset_work()
>   nvme_dev_disable()
>     pdev->msix_enable = 0;  ...(2)
> ...

>  static void nvme_poll_irqdisable(struct nvme_queue *nvmeq)
>  {
>  	struct pci_dev *pdev = to_pci_dev(nvmeq->dev->dev);
> +	int irq;
>  
>  	WARN_ON_ONCE(test_bit(NVMEQ_POLLED, &nvmeq->flags));
>  
> -	disable_irq(pci_irq_vector(pdev, nvmeq->cq_vector));
> +	irq = pci_irq_vector(pdev, nvmeq->cq_vector);
> +	disable_irq(irq);
>  	spin_lock(&nvmeq->cq_poll_lock);
>  	nvme_poll_cq(nvmeq, NULL);
>  	spin_unlock(&nvmeq->cq_poll_lock);
> -	enable_irq(pci_irq_vector(pdev, nvmeq->cq_vector));
> +	enable_irq(irq);

An internal run of sashiko complained about this, and I think it's
right.  As the commit log mentions, the cached IRQ number is stale if
nvme_reset_work() frees all the vectors between (1) and (3).  It's
likely the pci_alloc_irq_vectors() in nvme_pci_enable() will get the
same IRQ number, but it would be a coincidence, and it doesn't feel
like a good idea to rely on it.

First sashiko review:

  This commit caches the IRQ number in nvme_poll_irqdisable() to
  ensure disable_irq() and enable_irq() operate on the same IRQ
  number, preventing an unbalanced enable warning if the device is
  concurrently disabled.

  If pci_free_irq_vectors() frees the IRQ concurrently, is it possible
  for the IRQ number to be reallocated to a completely different
  device before enable_irq() is called? Could this cause an unbalanced
  enable or incorrectly unmask the interrupt for the new device?

Second sashiko review:

  The commit caches the return value of pci_irq_vector() into a local
  variable to ensure disable_irq() and enable_irq() operate on the
  same IRQ number, preventing an unbalanced enable warning when
  pdev->msix_enabled is cleared concurrently.

  If pdev->msix_enabled is cleared concurrently and nvmeq->cq_vector >
  0, pci_irq_vector() will return -EINVAL. Since disable_irq() takes
  an unsigned int, does this result in passing a very large unsigned
  number (like 0xffffffea) to the IRQ subsystem?

  The commit message notes that operating on a stale IRQ number is
  valid and safe. However, if pci_free_irq_vectors() frees the MSI-X
  vector concurrently, couldn't this cached irq number be recycled and
  reallocated to a completely different device before enable_irq(irq)
  executes? If the new device had explicitly disabled its IRQ, does
  this code inadvertently enable it, breaking the other device's
  synchronization?