linux-kernel - Re: [PATCH 3/8] iommufd: Support attach/replace hwpt per pasid

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Message-ID: <20240117125625.GF734935@nvidia.com>
Date: Wed, 17 Jan 2024 08:56:25 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Yi Liu <yi.l.liu@...el.com>
Cc: "Tian, Kevin" <kevin.tian@...el.com>,
	"joro@...tes.org" <joro@...tes.org>,
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	"Zeng, Xin" <xin.zeng@...el.com>,
	"Zhao, Yan Y" <yan.y.zhao@...el.com>
Subject: Re: [PATCH 3/8] iommufd: Support attach/replace hwpt per pasid

On Wed, Jan 17, 2024 at 04:24:24PM +0800, Yi Liu wrote:
> Above indeed makes more sense if there can be concurrent attach/replace/detach
> on a single pasid. Just have one doubt should we add lock to protect the
> whole attach/replace/detach paths. In the attach/replace path[1] [2], the
> xarray entry is verified firstly, and then updated after returning from
> iommu attach/replace API. It is uneasy to protect the xarray operations only
> with xa_lock as a detach path can acquire xa_lock right after attach/replace
> path checks the xarray. To avoid it, may need a mutex to protect the whole
> attach/replace/detach path to avoid race. Or maybe the attach/replace path
> should mark the corresponding entry as a special state that can block the
> other path like detach until the attach/replace path update the final hwpt to
> the xarray. Is there such state in xarray?

If the caller is not allowed to make concurrent attaches/detaches to
the same pasid then you can document that in a comment, but it is
still better to use xarray in a self-consistent way.

Jason