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

[Ethan Zhao]

在 2025/2/26 13:18, Baolu Lu 写道:
> On 2/26/25 11:50, Ethan Zhao wrote:
> > > > > >
> > > 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().
>
> I don't think trylock is a reasonable solution. intel_iommu_shutdown()
> should not become a no-op simply because it cannot acquire a lock
> immediately.

No other CPUs is holding lock after they were brought down by sync call to

functionnative_stop_other_cpus(1).

So actually it wouldn't fail to acquire a lock.  this is also the reason why we don't

need to down_write() thedmar_global_lock.

> The lock here is to protect the drhd (representation of iommu hardware)
> list. It needs protection because this driver supports iommu hot-add and
> remove, which is triggered by an ACPI event for I/O board hotplug.

Yup, the lock is used to protect the global listdmar_drhd_units.

but here all IOAPIC/LAPIC are brought down, hotplug interrupts couldn't 
happend either. (only legacy and NMI are alive).

> Provided the system does not respond to those events when this function
> is called, it's fine to remove the lock.
I agree.
> Thanks,
> baolu

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