Messages in this thread |  | | Date | Wed, 09 Mar 2022 10:45:49 +0000 | From | Marc Zyngier <> | Subject | Re: [PATCH V3 05/10] virtio-pci: harden INTX interrupts |
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[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.
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