From: Jason Gunthorpe <jgg@nvidia.com>
To: Vasant Hegde <vasant.hegde@amd.com>
Cc: iommu@lists.linux.dev, joro@8bytes.org,
suravee.suthikulpanit@amd.com,
Daniel Marcovitch <dmarcovitch@nvidia.com>
Subject: Re: [PATCH 1/3] iommu/amd/iommu_v2: Fix pasid_state refcount dec hit 0 warning on pasid unbind
Date: Tue, 24 Jan 2023 11:23:06 -0400 [thread overview]
Message-ID: <Y8/32rScnCHTacQT@nvidia.com> (raw)
In-Reply-To: <20230124104355.119166-2-vasant.hegde@amd.com>
On Tue, Jan 24, 2023 at 10:43:53AM +0000, Vasant Hegde wrote:
> From: Daniel Marcovitch <dmarcovitch@nvidia.com>
>
> When unbinding pasid - a race condition exists vs outstanding page faults.
>
> To prevent this, the pasid_state object contains a refcount.
> * set to 1 on pasid bind
> * incremented on each ppr notification start
> * decremented on each ppr notification done
> * decremented on pasid unbind
I've no objection to this quick fix
But the issue here is using a refcount for something that is not a
refcount. This usage model is should just be a normal atomic if it is
only counting the number of notifications in progress.
To be a refcount to have to follow one of the standard refcount
patterns, ie get rid of the event queue and consistently do:
if (refcount_dec_and_test())
free_pasid_state()
Which implies the ppr notification done can do a defered free of the
pasid_state.
Jason
next prev parent reply other threads:[~2023-01-24 15:23 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-24 10:43 [PATCH 0/3] iommu/amd/iommu_v2: Fix refcount related issues Vasant Hegde
2023-01-24 10:43 ` [PATCH 1/3] iommu/amd/iommu_v2: Fix pasid_state refcount dec hit 0 warning on pasid unbind Vasant Hegde
2023-01-24 15:23 ` Jason Gunthorpe [this message]
2023-01-27 4:44 ` Vasant Hegde
2023-01-27 12:58 ` Jason Gunthorpe
2023-01-24 10:43 ` [PATCH 2/3] iommu/amd/iommu_v2: Fix pasid_state->wq race Vasant Hegde
2023-01-24 15:30 ` Jason Gunthorpe
2023-01-24 10:43 ` [PATCH 3/3] iommu/amd/iommu_v2: Prevent scheduling new ppr notifier during unbind_pasid Vasant Hegde
2023-01-24 15:33 ` Jason Gunthorpe
2023-01-27 4:58 ` Vasant Hegde
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