Re: [PATCH] irqchip/gic-v3-its: Fix invalid wait context lockdep report

[Marc Zyngier <maz@kernel.org>]

On Thu, 28 Aug 2025 04:09:00 +0100,
Koichiro Den <den@valinux.co.jp> wrote:
> 
> On Wed, Aug 27, 2025 at 01:48:33PM +0100, Marc Zyngier wrote:
> > On Wed, 27 Aug 2025 08:38:48 +0100,
> > Koichiro Den <den@valinux.co.jp> wrote:
> > > 
> > > its_irq_set_vcpu_affinity() always runs under a raw_spin_lock wait
> > > context, so calling kcalloc there is not permitted and RT-unsafe since
> > > ___slab_alloc() may acquire a local lock. The below is the actual
> > > lockdep report observed:
> > > 
> > >   =============================
> > >   [ BUG: Invalid wait context ]
> > >   6.16.0-rc3-irqchip-next-7e28bba92c5c+ #1 Tainted: G S
> > >   -----------------------------
> > >   qemu-system-aar/2129 is trying to lock:
> > >   ffff0085b74f2178 (batched_entropy_u32.lock){..-.}-{3:3}, at: get_random_u32+0x9c/0x708
> > >   other info that might help us debug this:
> > >   context-{5:5}
> > >   6 locks held by qemu-system-aar/2129:
> > >    #0: ffff0000b84a0738 (&vdev->igate){+.+.}-{4:4}, at: vfio_pci_core_ioctl+0x40c/0x748 [vfio_pci_core]
> > >    #1: ffff8000883cef68 (lock#6){+.+.}-{4:4}, at: irq_bypass_register_producer+0x64/0x2f0
> > >    #2: ffff0000ac0df960 (&its->its_lock){+.+.}-{4:4}, at: kvm_vgic_v4_set_forwarding+0x224/0x6f0
> > >    #3: ffff000086dc4718 (&irq->irq_lock#3){....}-{2:2}, at: kvm_vgic_v4_set_forwarding+0x288/0x6f0
> > >    #4: ffff0001356200c8 (&irq_desc_lock_class){-.-.}-{2:2}, at: __irq_get_desc_lock+0xc8/0x158
> > >    #5: ffff00009eae4850 (&dev->event_map.vlpi_lock){....}-{2:2}, at: its_irq_set_vcpu_affinity+0x8c/0x528
> > >   ...
> > >   Call trace:
> > >    show_stack+0x30/0x98 (C)
> > >    dump_stack_lvl+0x9c/0xd0
> > >    dump_stack+0x1c/0x34
> > >    __lock_acquire+0x814/0xb40
> > >    lock_acquire.part.0+0x16c/0x2a8
> > >    lock_acquire+0x8c/0x178
> > >    get_random_u32+0xd4/0x708
> > >    __get_random_u32_below+0x20/0x80
> > >    shuffle_freelist+0x5c/0x1b0
> > >    allocate_slab+0x15c/0x348
> > >    new_slab+0x48/0x80
> > >    ___slab_alloc+0x590/0x8b8
> > >    __slab_alloc.isra.0+0x3c/0x80
> > >    __kmalloc_noprof+0x174/0x520
> > >    its_vlpi_map+0x834/0xce0
> > >    its_irq_set_vcpu_affinity+0x21c/0x528
> > >    irq_set_vcpu_affinity+0x160/0x1b0
> > >    its_map_vlpi+0x90/0x100
> > >    kvm_vgic_v4_set_forwarding+0x3c4/0x6f0
> > >    kvm_arch_irq_bypass_add_producer+0xac/0x108
> > >    __connect+0x138/0x1b0
> > >    irq_bypass_register_producer+0x16c/0x2f0
> > >    vfio_msi_set_vector_signal+0x2c0/0x5a8 [vfio_pci_core]
> > >    vfio_msi_set_block+0x8c/0x120 [vfio_pci_core]
> > >    vfio_pci_set_msi_trigger+0x120/0x3d8 [vfio_pci_core]
> > 
> > Huh. I guess this is due to RT not being completely compatible with
> > GFP_ATOMIC...  Why you'd want RT and KVM at the same time is beyond
> > me, but hey.
> 
> For the record, I didn't run KVM on RT, though I still believe it's better
> to conform to the wait context rule and avoid triggering the lockdep
> splat.

Then I don't understand how you get this, because I have not seen it
so far.

> 
> I don't know if there are any plans which make kmalloc with GFP_ATOMIC
> workable under a stricter wait context (getting rid of the local lock
> in some way?), but I think it would be nicer.

GFP_ATOMIC is documented as being compatible with raw spinlocks in the
absence of RT, making the above trace pretty odd.

> 
> > 
> > >   ...
> > > 
> > > To avoid this, simply pre-allocate vlpi_maps when creating an ITS v4
> > > device with LPIs allcation. The trade-off is some wasted memory
> > > depending on nr_lpis, if none of those LPIs are never upgraded to VLPIs.
> > >
> > > An alternative would be to move the vlpi_maps allocation out of
> > > its_map_vlpi() and introduce a two-stage prepare/commit flow, allowing a
> > > caller (KVM in the lockdep splat shown above) to do the allocation
> > > outside irq_set_vcpu_affinity(). However, this would unnecessarily add
> > > complexity.
> > 
> > That's debatable. It is probably fine for now, but if this was to
> > grow, we'd need to revisit this.
> 
> Just curious but do you have any plans to replace the current
> irq_set_vcpu_affinity() approach with something else?

Who knows. This is the Linux kernel, everything changes all the time
without the need for a good reason. More significantly, the amount of
*data* being associated with a VLPI could become much higher in the
future, and add more unnecessary allocation.

	M.

-- 
Without deviation from the norm, progress is not possible.