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

[Luca Fancellu <luca.fancellu@xxxxxxx>]

> On 11 Feb 2022, at 15:26, Julien Grall <julien@xxxxxxx> 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