> On 11 Feb 2022, at 15:26, Julien Grall <jul...@xen.org> wrote:
>
> Hi Luca,
>
> On 11/02/2022 15:00, Luca Fancellu wrote:
>> pend_lpi_tree is a radix tree used to store pending irqs, the tree is
>> protected by a lock for read/write operations.
>> Currently the radix tree default function to free items uses the
>> RCU mechanism, calling call_rcu and deferring the operation.
>> However every access to the structure is protected by the lock so we
>> can avoid using the default free function that, by using RCU,
>> increases memory usage and impacts the predictability of the system.
>
> I understand goal but looking at the implementation of
> vgic_v3_lpi_to_pending() (Copied below for convenience). We would release the
> lock as soon as the look-up finish, yet the element is returned.
>
> static struct pending_irq *vgic_v3_lpi_to_pending(struct domain *d,
> unsigned int lpi)
> {
> struct pending_irq *pirq;
>
> read_lock(&d->arch.vgic.pend_lpi_tree_lock);
> pirq = radix_tree_lookup(&d->arch.vgic.pend_lpi_tree, lpi);
> read_unlock(&d->arch.vgic.pend_lpi_tree_lock);
>
> return pirq;
> }
>
> So the lock will not protect us against removal. If you want to drop the RCU,
> you will need to ensure the structure pending_irq is suitably protected. I
> haven't check whether there are other locks that may suit us here.
>
Hi Julien,
Yes you are right! I missed that, sorry for the noise.
Cheers,
Luca
> Cheers,
>
> --
> Julien Grall
