Re: [PATCH V3 05/10] virtio-pci: harden INTX interrupts

[Marc Zyngier <maz@kernel.org>]

[Adding Will to check on my understanding of the interactions between
 spinlocks and WRITE_ONCE()]

On Tue, 19 Oct 2021 08:01:47 +0100,
Jason Wang <jasowang@redhat.com> wrote:
> 
> This patch tries to make sure the virtio interrupt handler for INTX
> won't be called after a reset and before virtio_device_ready(). We
> can't use IRQF_NO_AUTOEN since we're using shared interrupt
> (IRQF_SHARED). So this patch tracks the INTX enabling status in a new
> intx_soft_enabled variable and toggle it during in
> vp_disable/enable_vectors(). The INTX interrupt handler will check
> intx_soft_enabled before processing the actual interrupt.
> 
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/virtio/virtio_pci_common.c | 23 +++++++++++++++++++++--
>  drivers/virtio/virtio_pci_common.h |  1 +
>  2 files changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
> index 8d8f83aca721..1bce254a462a 100644
> --- a/drivers/virtio/virtio_pci_common.c
> +++ b/drivers/virtio/virtio_pci_common.c
> @@ -30,8 +30,16 @@ void vp_disable_cbs(struct virtio_device *vdev)
>  	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
>  	int i;
>  
> -	if (vp_dev->intx_enabled)
> +	if (vp_dev->intx_enabled) {
> +		/*
> +		 * The below synchronize() guarantees that any
> +		 * interrupt for this line arriving after
> +		 * synchronize_irq() has completed is guaranteed to see
> +		 * intx_soft_enabled == false.
> +		 */
> +		WRITE_ONCE(vp_dev->intx_soft_enabled, false);
>  		synchronize_irq(vp_dev->pci_dev->irq);
> +	}
>  
>  	for (i = 0; i < vp_dev->msix_vectors; ++i)
>  		disable_irq(pci_irq_vector(vp_dev->pci_dev, i));
> @@ -43,8 +51,16 @@ void vp_enable_cbs(struct virtio_device *vdev)
>  	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
>  	int i;
>  
> -	if (vp_dev->intx_enabled)
> +	if (vp_dev->intx_enabled) {
> +		disable_irq(vp_dev->pci_dev->irq);
> +		/*
> +		 * The above disable_irq() provides TSO ordering and
> +		 * as such promotes the below store to store-release.
> +		 */
> +		WRITE_ONCE(vp_dev->intx_soft_enabled, true);

What do you mean by TSO here? AFAICT, the CPU is allowed hoist this
write up into the lock used by disable_irq(), as the unlock only has
release semantics. Is that what you are relying on? I don't see how
this upgrades WRITE_ONCE() to have release semantics.

> +		enable_irq(vp_dev->pci_dev->irq);

Same thing does here: my understanding is that the write can be pushed
down into the lock, which has acquire semantics only.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.