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Date:   Tue, 25 Aug 2020 12:20:48 +0200
From:   Jean-Philippe Brucker <jean-philippe@linaro.org>
To:     Lu Baolu <baolu.lu@linux.intel.com>
Cc:     Jacob Pan <jacob.pan.linux@gmail.com>,
        iommu@lists.linux-foundation.org,
        LKML <linux-kernel@vger.kernel.org>,
        Jean-Philippe Brucker <jean-philippe@linaro.com>,
        Joerg Roedel <joro@8bytes.org>,
        David Woodhouse <dwmw2@infradead.org>,
        Yi Liu <yi.l.liu@intel.com>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        Raj Ashok <ashok.raj@intel.com>,
        Eric Auger <eric.auger@redhat.com>, Wu Hao <hao.wu@intel.com>
Subject: Re: [PATCH v2 4/9] iommu/ioasid: Add reference couting functions
Message-ID: <20200825102048.GB3252704@myrica>
References: <1598070918-21321-1-git-send-email-jacob.jun.pan@linux.intel.com>
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On Mon, Aug 24, 2020 at 10:26:55AM +0800, Lu Baolu wrote:
> Hi Jacob,
> 
> On 8/22/20 12:35 PM, Jacob Pan wrote:
> > There can be multiple users of an IOASID, each user could have hardware
> > contexts associated with the IOASID. In order to align lifecycles,
> > reference counting is introduced in this patch. It is expected that when
> > an IOASID is being freed, each user will drop a reference only after its
> > context is cleared.
> > 
> > Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
[...]
> > +/**
> >    * ioasid_find - Find IOASID data
> >    * @set: the IOASID set
> >    * @ioasid: the IOASID to find
> 
> Do you need to increase the refcount of the found ioasid and ask the
> caller to drop it after use? Otherwise, the ioasid might be freed
> elsewhere.

ioasid_find() takes a getter function as parameter, which ensures that the
returned data is valid. It fetches the IOASID data under rcu_read_lock()
and calls the getter on the private data (for example mmget_not_zero() for
bare-metal SVA). Given that, I don't think returning with a reference to
the IOASID is necessary. The IOASID may be freed once ioasid_find()
returns but not the returned data.

Thanks,
Jean