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Date:   Thu, 25 Aug 2022 19:05:44 +0100
Message-ID: <875yigw64n.wl-maz@kernel.org>
From:   Marc Zyngier <maz@kernel.org>
To:     Pierre Gondois <pierre.gondois@arm.com>
Cc:     linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] irqchip/gic-v3-its: Remove cpumask_var_t allocation
In-Reply-To: <20220825152348.1634133-1-pierre.gondois@arm.com>
References: <20220825152348.1634133-1-pierre.gondois@arm.com>
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On Thu, 25 Aug 2022 16:23:48 +0100,
Pierre Gondois <pierre.gondois@arm.com> wrote:
> 
> In KvmTool, on a ThunderX2, when running a preemp_rt kernel based on

How is this specific to kvmtool? Or to running as a guest?

> v5.19-rc3-rt4, the following happens:
> [    4.070739] [ BUG: Invalid wait context ]
> [    4.070740] 5.19.0-rc3-rt4-00001-g1a6597c0bdcf #153 Not tainted
> [    4.070742] -----------------------------
> [    4.070743] swapper/0/1 is trying to lock:
> [    4.070744] ffff0000ab7405d8 ((&c->lock)){+.+.}-{3:3}, at: ___slab_alloc (mm/slub.c:2954)
> [    4.070757] other info that might help us debug this:
> [    4.070758] context-{5:5}
> [    4.070759] 5 locks held by swapper/0/1:
> [    4.070760] #0: ffff0000811491e0 (&dev->mutex){....}-{4:4}, at: __device_driver_lock (drivers/base/dd.c:1055)
> [    4.070769] #1: ffff0000846c5670 (&desc->request_mutex){+.+.}-{4:4}, at: __setup_irq (kernel/irq/internals.h:147)
> [    4.070778] #2: ffff0000846c54c8 (&irq_desc_lock_class){....}-{2:2}, at: __setup_irq (kernel/irq/manage.c:1612)
> [    4.070784] #3: ffff80000b23ea78 (mask_lock){....}-{2:2}, at: irq_setup_affinity (./include/linux/irq.h:381)
> [    4.070791] #4: ffff80000b23ea38 (tmp_mask_lock){....}-{2:2}, at: irq_do_set_affinity (./include/linux/irq.h:381)
> [    4.070797] stack backtrace:
> [    4.070801] CPU: 2 PID: 1 Comm: swapper/0 Not tainted 5.19.0-rc3-rt4-00001-g1a6597c0bdcf #153
> [    4.070805] Call trace:
> [    4.070806] dump_backtrace (arch/arm64/kernel/stacktrace.c:200)
> [    4.070811] show_stack (arch/arm64/kernel/stacktrace.c:207)
> [    4.070813] dump_stack_lvl (lib/dump_stack.c:107)
> [    4.070818] dump_stack (lib/dump_stack.c:114)
> [    4.070820] __lock_acquire (kernel/locking/lockdep.c:4707)
> [    4.070823] lock_acquire (kernel/locking/lockdep.c:466)
> [    4.070825] rt_spin_lock (./arch/arm64/include/asm/current.h:19 (discriminator 4))
> [    4.070830] ___slab_alloc (mm/slub.c:2954)
> [    4.070832] __slab_alloc.isra.0 (mm/slub.c:3116)
> [    4.070835] __kmalloc_node (mm/slub.c:3207)
> [    4.070837] alloc_cpumask_var_node (lib/cpumask.c:115)
> [    4.070843] alloc_cpumask_var (lib/cpumask.c:147)
> [    4.070846] its_select_cpu (drivers/irqchip/irq-gic-v3-its.c:1580)
> [    4.070850] its_set_affinity (drivers/irqchip/irq-gic-v3-its.c:1659)
> [    4.070853] msi_domain_set_affinity (kernel/irq/msi.c:501)
> [    4.070857] irq_do_set_affinity (kernel/irq/manage.c:276)
> [    4.070860] irq_setup_affinity (kernel/irq/manage.c:633)
> [    4.070863] irq_startup (kernel/irq/chip.c:280)
> [    4.070865] __setup_irq (kernel/irq/manage.c:1777)
> [    4.070869] request_threaded_irq (kernel/irq/manage.c:2206)
> [    4.070872] vp_find_vqs_msix (./include/linux/interrupt.h:168)
> [    4.070876] vp_find_vqs (drivers/virtio/virtio_pci_common.c:400)
> [    4.070878] vp_modern_find_vqs (drivers/virtio/virtio_pci_modern.c:259)
> [    4.070880] init_vq (./include/linux/virtio_config.h:213)
> [    4.070885] virtblk_probe (drivers/block/virtio_blk.c:936)
> [    4.070887] virtio_dev_probe (drivers/virtio/virtio.c:303)
> [    4.070892] really_probe (drivers/base/dd.c:555)
> [    4.070895] __driver_probe_device (drivers/base/dd.c:764)
> [    4.070897] driver_probe_device (drivers/base/dd.c:794)
> [    4.070899] __driver_attach (drivers/base/dd.c:1164)
> [    4.070901] bus_for_each_dev (drivers/base/bus.c:301)
> [    4.070904] driver_attach (drivers/base/dd.c:1181)
> [    4.070906] bus_add_driver (drivers/base/bus.c:618)
> [    4.070908] driver_register (drivers/base/driver.c:240)
> [    4.070910] register_virtio_driver (drivers/virtio/virtio.c:356 (discriminator 4))
> [    4.070913] virtio_blk_init (drivers/block/virtio_blk.c:1218)
> [    4.070918] do_one_initcall (init/main.c:1295)
> [    4.070921] kernel_init_freeable (init/main.c:1367)
> [    4.070924] kernel_init (init/main.c:1503)
> [    4.070927] ret_from_fork (arch/arm64/kernel/entry.S:868)

Consider trimming the trace to the useful part.

> 
> commit cba4235e6031e ("genirq: Remove mask argument from
> setup_affinity()")
> and
> commit 11ea68f553e24 ("genirq, sched/isolation: Isolate from handling
> managed interrupts")
> overcome this issue by defining a static struct cpumask and protecting
> it by a raw spinlock. The code in these commits is executed with IRQs
> disabled.
> its_select_cpu() can be executed with IRQs enabled or disabled. Thus
> disabling IRQs is necesserary to avoid deadlocking.

Only if this code can be preempted by an interrupt and subsequently
called from interrupt context. Can you describe this code path?

> 
> This patch:

Please refer to the documentation and the section about avoiding
things like "this patch" in a commit message.

> - makes tmpmask a 'static struct cpumask'. This prevents storing it on
>   the stack and having to dynamically allocate it
> - protects tmpmask with a raw spinlock
> - disables IRQs around the spinlock for the case its_select_cpu() is
>   called with IRQs enabled
> 
> Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
> ---
>  drivers/irqchip/irq-gic-v3-its.c | 34 +++++++++++++++++---------------
>  1 file changed, 18 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> index 5ff09de6c48f..3cf89e59b036 100644
> --- a/drivers/irqchip/irq-gic-v3-its.c
> +++ b/drivers/irqchip/irq-gic-v3-its.c
> @@ -1574,14 +1574,15 @@ static int its_select_cpu(struct irq_data *d,
>  			  const struct cpumask *aff_mask)
>  {
>  	struct its_device *its_dev = irq_data_get_irq_chip_data(d);
> -	cpumask_var_t tmpmask;
> +	static DEFINE_RAW_SPINLOCK(tmpmask_lock);
> +	static struct cpumask tmpmask;
> +	unsigned long flags;
>  	int cpu, node;
> -
> -	if (!alloc_cpumask_var(&tmpmask, GFP_ATOMIC))
> -		return -ENOMEM;
> -
>  	node = its_dev->its->numa_node;
>  
> +	local_irq_save(flags);
> +	raw_spin_lock(&tmpmask_lock);

Why is this done in two steps? What is wrong with raw_spin_lock_irqsave()?

More importantly, a number of call paths reaching its_set_affinity()
already execute under a raw spinlock, with interrupts disabled. Is it
worth not hitting this lock at all times?

> +
>  	if (!irqd_affinity_is_managed(d)) {
>  		/* First try the NUMA node */
>  		if (node != NUMA_NO_NODE) {
> @@ -1589,8 +1590,8 @@ static int its_select_cpu(struct irq_data *d,
>  			 * Try the intersection of the affinity mask and the
>  			 * node mask (and the online mask, just to be safe).
>  			 */
> -			cpumask_and(tmpmask, cpumask_of_node(node), aff_mask);
> -			cpumask_and(tmpmask, tmpmask, cpu_online_mask);
> +			cpumask_and(&tmpmask, cpumask_of_node(node), aff_mask);
> +			cpumask_and(&tmpmask, &tmpmask, cpu_online_mask);

Consider reducing the churn by writing something like:

	static struct cpumask __tmpmask;
	struct cpumask *tmpmask = &__tmpmask;

which is strictly equivalent, and makes the patch much smaller.

Thanks,

	M.

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