Re: [PATCH v2] iommu/vt-d: fix system hang on reboot -f

Ethan Zhao
2025-02-26T03:50:50Z

在 2025/2/25 22:26, Jason Gunthorpe 写道:
> On Tue, Feb 25, 2025 at 04:54:54PM +0800, Ethan Zhao wrote:
>>> On 2025/2/25 14:48, Yunhui Cui wrote:
>>>> We found that executing the command ./a.out &;reboot -f (where a.out
>>>> is a
>>>> program that only executes a while(1) infinite loop) can
>>>> probabilistically
>>>> cause the system to hang in the intel_iommu_shutdown() function,
>>>> rendering
>>>> it unresponsive. Through analysis, we identified that the factors
>>>> contributing to this issue are as follows:
>>>>
>>>> 1. The reboot -f command does not prompt the kernel to notify the
>>>> application layer to perform cleanup actions, allowing the
>>>> application to
>>>> continue running.
>>>>
>>>> 2. When the kernel reaches the intel_iommu_shutdown() function, only the
>>>> BSP (Bootstrap Processor) CPU is operational in the system.
>>>>
>>>> 3. During the execution of intel_iommu_shutdown(), the function
>>>> down_write
>>>> (&dmar_global_lock) causes the process to sleep and be scheduled out.
> Why does this happen? If the kernel has shutdown other CPUs then what
> thread is holding the other side of this lock and why?
>
>>>> 4. At this point, though the processor's interrupt flag is not cleared,
>>>>   allowing interrupts to be accepted. However, only legacy devices
>>>> and NMI
>>>> (Non-Maskable Interrupt) interrupts could come in, as other interrupts
>>>> routing have already been disabled. If no legacy or NMI interrupts occur
>>>> at this stage, the scheduler will not be able to run.
>>>> 5. If the application got scheduled at this time is executing a
>>>> while(1)-
>>>> type loop, it will be unable to be preempted, leading to an infinite
>>>> loop
>>>> and causing the system to become unresponsive.
> If the schedular doesn't run how did we get from 4 -> 5?
>
> Maybe the issue is the shutdown handler here is running in the wrong
> time and it should not be running after the scheduler has been shut
> down.
>
> I don't think removing the lock is a great idea without more
> explanation.

Seems it is not so simple job to explain why there is no race window between
this iommu_shutdown() and following dmar_global_lock holders.

1. PCIe hotplug dmar_pci_bus_notifier()

2. mm_core_init detect_intel_iommu()

3. late_initcall dmar_free_unused_resources()

4. acpi attach dmar_device_hotplug()

5. pci_iommu_init intel_iommu_init() init_dmars()

6. rootfs_initcall ir_dev_scope_init()

though here is the last stage of reboot. then how about we turn back to v1

Just repalce with own_write() with down_write_trylock().

Thanks,

Ethan

>
> Jason

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