Re: [RFC PATCH] arm/vgic-v3: provide custom callbacks for pend_lpi_tree radix tree

[Julien Grall <julien@xxxxxxx>]

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.
Cheers,

--
Julien Grall