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Message-ID: <86h5xtdj6m.wl-maz@kernel.org>
Date: Wed, 27 Aug 2025 13:48:33 +0100
From: Marc Zyngier <maz@...nel.org>
To: Koichiro Den <den@...inux.co.jp>
Cc: linux-arm-kernel@...ts.infradead.org,
tglx@...utronix.de,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] irqchip/gic-v3-its: Fix invalid wait context lockdep report
On Wed, 27 Aug 2025 08:38:48 +0100,
Koichiro Den <den@...inux.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.
> ...
>
> 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.
> Fixes: d011e4e654d7 ("irqchip/gic-v3-its: Add VLPI map/unmap operations")
No. This code predates RT being merged, and this problem cannot occur
before RT.
> Signed-off-by: Koichiro Den <den@...inux.co.jp>
> ---
> drivers/irqchip/irq-gic-v3-its.c | 36 ++++++++++++++++++--------------
> 1 file changed, 20 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> index 467cb78435a9..b933be8ddc51 100644
> --- a/drivers/irqchip/irq-gic-v3-its.c
> +++ b/drivers/irqchip/irq-gic-v3-its.c
> @@ -1923,19 +1923,10 @@ static int its_vlpi_map(struct irq_data *d, struct its_cmd_info *info)
> if (!info->map)
> return -EINVAL;
>
> - if (!its_dev->event_map.vm) {
> - struct its_vlpi_map *maps;
> -
> - maps = kcalloc(its_dev->event_map.nr_lpis, sizeof(*maps),
> - GFP_ATOMIC);
> - if (!maps)
> - return -ENOMEM;
> -
> + if (!its_dev->event_map.vm)
> its_dev->event_map.vm = info->map->vm;
> - its_dev->event_map.vlpi_maps = maps;
> - } else if (its_dev->event_map.vm != info->map->vm) {
> + else if (its_dev->event_map.vm != info->map->vm)
> return -EINVAL;
> - }
>
> /* Get our private copy of the mapping information */
> its_dev->event_map.vlpi_maps[event] = *info->map;
> @@ -2010,10 +2001,8 @@ static int its_vlpi_unmap(struct irq_data *d)
> * Drop the refcount and make the device available again if
> * this was the last VLPI.
> */
> - if (!--its_dev->event_map.nr_vlpis) {
> + if (!--its_dev->event_map.nr_vlpis)
> its_dev->event_map.vm = NULL;
> - kfree(its_dev->event_map.vlpi_maps);
> - }
>
> return 0;
> }
> @@ -3469,6 +3458,7 @@ static struct its_device *its_create_device(struct its_node *its, u32 dev_id,
> {
> struct its_device *dev;
> unsigned long *lpi_map = NULL;
> + struct its_vlpi_map *vlpi_maps;
> unsigned long flags;
> u16 *col_map = NULL;
> void *itt;
> @@ -3497,16 +3487,28 @@ static struct its_device *its_create_device(struct its_node *its, u32 dev_id,
>
> if (alloc_lpis) {
> lpi_map = its_lpi_alloc(nvecs, &lpi_base, &nr_lpis);
> - if (lpi_map)
> + if (lpi_map) {
> col_map = kcalloc(nr_lpis, sizeof(*col_map),
> GFP_KERNEL);
> +
> + /*
> + * Pre-allocate vlpi_maps to avoid slab allocation
> + * under the strict raw spinlock wait context of
> + * irq_set_vcpu_affinity. This could waste memory
> + * if no vlpi map is ever created.
> + */
> + if (is_v4(its) && nr_lpis > 0)
> + vlpi_maps = kcalloc(nr_lpis, sizeof(*vlpi_maps),
> + GFP_KERNEL);
> + }
> } else {
> col_map = kcalloc(nr_ites, sizeof(*col_map), GFP_KERNEL);
> nr_lpis = 0;
> lpi_base = 0;
> }
>
> - if (!dev || !itt || !col_map || (!lpi_map && alloc_lpis)) {
> + if (!dev || !itt || !col_map ||
> + (alloc_lpis && (!lpi_map || (is_v4(its) && !vlpi_maps)))) {
This needs to be collapsed into a single boolean evaluated with the
pointer being NULL.
> kfree(dev);
> itt_free_pool(itt, sz);
> bitmap_free(lpi_map);
Where are you freeing vlpi_maps if on the failure path??
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
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