On 22/06/16 09:18, Andre Przywara wrote: > Hi Marc, > > On 17/06/16 13:08, Andre Przywara wrote: >> In the moment our struct vgic_irq's are statically allocated at guest >> creation time. So getting a pointer to an IRQ structure is trivial and >> safe. LPIs are more dynamic, they can be mapped and unmapped at any time >> during the guest's _runtime_. >> In preparation for supporting LPIs we introduce reference counting to >> those structures. Since private IRQs and SPIs are statically allocated, >> the reqcount never drops to 0 at the moment, but we increase it when an >> IRQ gets onto a VCPU list and decrease it when it gets removed. >> Also vgic_get_irq() gets changed to take the irq_lock already, this is >> later needed to avoid a race between a potential LPI unmap and the >> window between us getting the pointer and locking the IRQ. >> This introduces vgic_put_irq(), which just does the unlock and makes >> the new locking sequence look more symmetrical. >> This approach deviates from the classical Linux refcounting with using >> atomic_* types and functions, because the users of vgic_get_irq() take >> the irq_lock anyway, so we just use an int and adjust the refcount while >> holding the lock. > > As discussed verbally, I am almost done on an implementation that uses > the kernel's kref_* infrastructure to implement the ref-counting, on top > of the existing vgic_irq locking. > Since we consider the atomic inc/dec operations comparably cheap, that > shouldn't sacrifice performance, but will increase readability and > avoids nasty bugs in our own refcounting implementation by using well > tested and reviewed code. Indeed. And for further reference, here's the reason why I'm pushing for kref being used: https://lwn.net/Articles/75920/ Thanks, M. -- Jazz is not dead. It just smells funny... -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
