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

Ethan Zhao
2025-02-25T08:54:59Z

在 2025/2/25 15:01, Baolu Lu 写道:
> 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.
>>
>> 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.
>>
>> To resolve this issue, the intel_iommu_shutdown() function should not
>> execute down_write(), which can potentially cause the process to be
>> scheduled out. Furthermore, since only the BSP is running during the
>> later
>> stages of the reboot, there is no need for protection against parallel
>> access to the DMAR (DMA Remapping) unit. Therefore, the following lines
>> could be removed:
>
> Good summary! Thank you!
>
>>
>> down_write(&dmar_global_lock);
>> up_write(&dmar_global_lock);
>>
>> After testing, the issue has been resolved.
>>
>> Fixes: 6c3a44ed3c55 ("iommu/vt-d: Turn off translations at shutdown")
>> Co-developed-by: Ethan Zhao <haifeng.zhao@linux.intel.com>
>> Signed-off-by: Ethan Zhao <haifeng.zhao@linux.intel.com>
>> Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
>> ---
>>   drivers/iommu/intel/iommu.c | 4 ----
>>   1 file changed, 4 deletions(-)
>>
>> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
>> index cc46098f875b..6d9f2e56ce88 100644
>> --- a/drivers/iommu/intel/iommu.c
>> +++ b/drivers/iommu/intel/iommu.c
>> @@ -2871,16 +2871,12 @@ void intel_iommu_shutdown(void)
>>       if (no_iommu || dmar_disabled)
>>           return;
>>   -    down_write(&dmar_global_lock);
>> -
>>       /* Disable PMRs explicitly here. */
>>       for_each_iommu(iommu, drhd)
>
> Removing the locking for for_each_iommu() will trigger a suspicious RCU
> usage splat. You need to replace this helper with a raw
> list_for_each_entry() with some comments around it to explain why it is
> safe.
>
Oops,  RCU checking hids behind the for_each_iommu() macro.

How about

void intel_iommu_shutdown(void)

{

struct dmar_drhd_unit *drhd;

struct intel_iommu *iommu = NULL;

if (no_iommu || dmar_disabled)

return;

/* Here only BSP is running, no RCU cocurrent lock checking needed */

list_for_each_entry(drhd, &dmar_drhd_units, list) {

iommu = drhd->iommu;

/* Disable PMRs explicitly here. */

iommu_disable_protect_mem_regions(iommu);

iommu_disable_translation(iommu);

}

}

Thanks,

Ethan

>> iommu_disable_protect_mem_regions(iommu);
>>         /* Make sure the IOMMUs are switched off */
>>       intel_disable_iommus();
>> -
>> -    up_write(&dmar_global_lock);
>>   }
>>     static struct intel_iommu *dev_to_intel_iommu(struct device *dev)
>
> Thanks,
> baolu

--
"firm, enduring, strong, and long-lived"