Re: [PATCH v2 17/18] kvm/eventfd: Use sequence counter with associated spinlock

Paolo Bonzini
2020-06-08T12:57:28Z

On 08/06/20 02:57, Ahmed S. Darwish wrote:
> A sequence counter write side critical section must be protected by some
> form of locking to serialize writers. A plain seqcount_t does not
> contain the information of which lock must be held when entering a write
> side critical section.
>
> Use the new seqcount_spinlock_t data type, which allows to associate a
> spinlock with the sequence counter. This enables lockdep to verify that
> the spinlock used for writer serialization is held when the write side
> critical section is entered.
>
> If lockdep is disabled this lock association is compiled out and has
> neither storage size nor runtime overhead.
>
> Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
> ---
> include/linux/kvm_irqfd.h | 2 +-
> virt/kvm/eventfd.c | 2 +-
> 2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h
> index dc1da020305b..dac047abdba7 100644
> --- a/include/linux/kvm_irqfd.h
> +++ b/include/linux/kvm_irqfd.h
> @@ -42,7 +42,7 @@ struct kvm_kernel_irqfd {
> wait_queue_entry_t wait;
> /* Update side is protected by irqfds.lock */
> struct kvm_kernel_irq_routing_entry irq_entry;
> - seqcount_t irq_entry_sc;
> + seqcount_spinlock_t irq_entry_sc;
> /* Used for level IRQ fast-path */
> int gsi;
> struct work_struct inject;
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index 67b6fc153e9c..8694a2920ea9 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -303,7 +303,7 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
> INIT_LIST_HEAD(&irqfd->list);
> INIT_WORK(&irqfd->inject, irqfd_inject);
> INIT_WORK(&irqfd->shutdown, irqfd_shutdown);
> - seqcount_init(&irqfd->irq_entry_sc);
> + seqcount_spinlock_init(&irqfd->irq_entry_sc, &kvm->irqfds.lock);
>
> f = fdget(args->fd);
> if (!f.file) {
>

Acked-by: Paolo Bonzini <pbonzini@redhat.com>