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

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Message-ID: <9e20168a-0498-4d98-8852-b3342d46d693@linux.intel.com>
Date: Mon, 24 Feb 2025 13:37:13 +0800
From: Ethan Zhao <haifeng.zhao@...ux.intel.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>, Yunhui Cui
 <cuiyunhui@...edance.com>, dwmw2@...radead.org, joro@...tes.org,
 will@...nel.org, robin.murphy@....com, iommu@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/vt-d: fix system hang on reboot -f


在 2025/2/24 9:02, Baolu Lu 写道:
> On 2/20/25 18:15, Yunhui Cui wrote:
>> When entering intel_iommu_shutdown, system interrupts are disabled,
>> and the reboot process might be scheduled out by down_write(). If the
>> scheduled process does not yield (e.g., while(1)), the system will hang.
>>
>> Signed-off-by: Yunhui Cui<cuiyunhui@...edance.com>
>> ---
>>   drivers/iommu/intel/iommu.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
>> index cc46098f875b..76a1d83b46bf 100644
>> --- a/drivers/iommu/intel/iommu.c
>> +++ b/drivers/iommu/intel/iommu.c
>> @@ -2871,7 +2871,8 @@ void intel_iommu_shutdown(void)
>>       if (no_iommu || dmar_disabled)
>>           return;
>>   -    down_write(&dmar_global_lock);

Only BSP is running at this point, no DMAR concurrency access protection is needed
anyore, even there is interrupt (only legacy & NMI) coming in, it is impossible to for
scheduler to run any other iommu access code.**
Thanks,
Ethan**

>> +    if (!down_write_trylock(&dmar_global_lock))
>> +        return;
>
> If system interrupts are disabled here, locking is unnecessary. Hotplug
> operations depend on interrupt events, so it's better to remove the
> lock. The shutdown helper then appears like this:
>
> void intel_iommu_shutdown(void)
> {
>         struct dmar_drhd_unit *drhd;
>         struct intel_iommu *iommu = NULL;
>
>         if (no_iommu || dmar_disabled)
>                 return;
>
>         /*
>          * System interrupts are disabled when it reaches here. Locking
>          * is unnecessary when iterating the IOMMU list.
>          */
>         list_for_each_entry(drhd, &dmar_drhd_units, list) {
>                 if (drhd->ignored)
>                         continue;
>
>                 iommu = drhd->iommu;
>                 /* Disable PMRs explicitly here. */
>                 iommu_disable_protect_mem_regions(iommu);
>                 iommu_disable_translation(iommu);
>         }
> }
>
> Does it work for you?
>
> Thanks,
> baolu
>
-- 
"firm, enduring, strong, and long-lived"