linux-kernel - RE: [PATCH v4 09/10] iommu: Make iommu_queue

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Date:   Wed, 30 Aug 2023 07:55:39 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        "Will Deacon" <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        "Jason Gunthorpe" <jgg@...pe.ca>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Nicolin Chen <nicolinc@...dia.com>
CC:     "Liu, Yi L" <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v4 09/10] iommu: Make iommu_queue_iopf() more generic

> From: Baolu Lu <baolu.lu@...ux.intel.com>
> Sent: Saturday, August 26, 2023 4:04 PM
> 
> On 8/25/23 4:17 PM, Tian, Kevin wrote:
> >> +static void assert_no_pending_iopf(struct device *dev, ioasid_t pasid)
> >> +{
> >> +	struct iommu_fault_param *iopf_param = dev->iommu-
> >>> fault_param;
> >> +	struct iopf_fault *iopf;
> >> +
> >> +	if (!iopf_param)
> >> +		return;
> >> +
> >> +	mutex_lock(&iopf_param->lock);
> >> +	list_for_each_entry(iopf, &iopf_param->partial, list) {
> >> +		if (WARN_ON(iopf->fault.prm.pasid == pasid))
> >> +			break;
> >> +	}
> > partial list is protected by dev_iommu lock.
> >
> 
> Ah, do you mind elaborating a bit more? In my mind, partial list is
> protected by dev_iommu->fault_param->lock.
> 

well, it's not how the code is currently written. iommu_queue_iopf()
doesn't hold dev_iommu->fault_param->lock to update the partial
list. 

while at it looks there is also a mislocking in iopf_queue_discard_partial()
which only acquires queue->lock.

So we have three places touching the partial list all with different locks:

- iommu_queue_iopf() relies on dev_iommu->lock
- iopf_queue_discard_partial() relies on queue->lock
- this new assert function uses dev_iommu->fault_param->lock