On Tue, 20 Feb 2024 17:43:03 +0000, Oliver Upton <oliver.upton@xxxxxxxxx> wrote: > > On Tue, Feb 20, 2024 at 05:24:50PM +0000, Marc Zyngier wrote: > > On Tue, 20 Feb 2024 16:30:24 +0000, > > Zenghui Yu <zenghui.yu@xxxxxxxxx> wrote: > > > > > > On 2024/2/17 02:41, Oliver Upton wrote: > > > > Using a linked-list for LPIs is less than ideal as it of course requires > > > > iterative searches to find a particular entry. An xarray is a better > > > > data structure for this use case, as it provides faster searches and can > > > > still handle a potentially sparse range of INTID allocations. > > > > > > > > Start by storing LPIs in an xarray, punting usage of the xarray to a > > > > subsequent change. > > > > > > > > Signed-off-by: Oliver Upton <oliver.upton@xxxxxxxxx> > > > > > > [..] > > > > > > > diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c > > > > index db2a95762b1b..c126014f8395 100644 > > > > --- a/arch/arm64/kvm/vgic/vgic.c > > > > +++ b/arch/arm64/kvm/vgic/vgic.c > > > > @@ -131,6 +131,7 @@ void __vgic_put_lpi_locked(struct kvm *kvm, struct vgic_irq *irq) > > > > return; > > > > list_del(&irq->lpi_list); > > > > + xa_erase(&dist->lpi_xa, irq->intid); > > > > > > We can get here *after* grabbing the vgic_cpu->ap_list_lock (e.g., > > > vgic_flush_pending_lpis()/vgic_put_irq()). And as according to vGIC's > > > "Locking order", we should disable interrupts before taking the xa_lock > > > in xa_erase() and we would otherwise see bad things like deadlock.. > > > > > > It's not a problem before patch #10, where we drop the lpi_list_lock and > > > start taking the xa_lock with interrupts enabled. Consider switching to > > > use xa_erase_irq() instead? > > > > But does it actually work? xa_erase_irq() uses spin_lock_irq(), > > followed by spin_unlock_irq(). So if we were already in interrupt > > context, we would end-up reenabling interrupts. At least, this should > > be the irqsave version. > > This is what I was planning to do, although I may kick it out to patch > 10 to avoid churn. > > > The question is whether we manipulate LPIs (in the get/put sense) on > > the back of an interrupt handler (like we do for the timer). It isn't > > obvious to me that it is the case, but I haven't spent much time > > staring at this code recently. > > I think we can get into here both from contexts w/ interrupts disabled > or enabled. irqfd_wakeup() expects to be called w/ interrupts disabled. > > All the more reason to use irqsave() / irqrestore() flavors of all of > this, and a reminder to go check all callsites that implicitly take the > xa_lock. Sounds good. Maybe you can also update the locking order "documentation" to include the xa_lock? I expect that it will ultimately replace lpi_list_lock. Thanks, M. -- Without deviation from the norm, progress is not possible.
