linux-kernel - Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

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Message-ID: <20210504151154.02908c63@jacob-builder>
Date:   Tue, 4 May 2021 15:11:54 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        Auger Eric <eric.auger@...hat.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Jonathan Corbet <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>, jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs

Hi Jason,

On Tue, 4 May 2021 15:00:50 -0300, Jason Gunthorpe <jgg@...dia.com> wrote:

> On Tue, May 04, 2021 at 08:41:48AM -0700, Jacob Pan wrote:
> > > > 
> > > > (also looking at ioasid.c, why do we need such a thin and odd
> > > > wrapper around xarray?)
> > > >     
> > > 
> > > I'll leave it to Jean and Jacob.  
> 
> > Could you elaborate?  
> 
> I mean stuff like this:
> 
> int ioasid_set_data(ioasid_t ioasid, void *data)
> {
>         struct ioasid_data *ioasid_data;
>         int ret = 0;
> 
>         spin_lock(&ioasid_allocator_lock);
>         ioasid_data = xa_load(&active_allocator->xa, ioasid);
>         if (ioasid_data)
>                 rcu_assign_pointer(ioasid_data->private, data);
>         else
>                 ret = -ENOENT;
>         spin_unlock(&ioasid_allocator_lock);
> 
>         /*
>          * Wait for readers to stop accessing the old private data, so the
>          * caller can free it.
>          */
>         if (!ret)
>                 synchronize_rcu();
> 
>         return ret;
> }
> EXPORT_SYMBOL_GPL(ioasid_set_data);
> 
> It is a weird way to use xarray to have a structure which
> itself is just a wrapper around another RCU protected structure.
> 
> Make the caller supply the ioasid_data memory, embedded in its own
> element, get rid of the void * and rely on XA_ZERO_ENTRY to hold
> allocated but not active entries.
> 
Let me try to paraphrase to make sure I understand. Currently
struct ioasid_data is private to the iasid core, its memory is allocated by
the ioasid core.

You are suggesting the following:
1. make struct ioasid_data public
2. caller allocates memory for ioasid_data, initialize it then pass it to
ioasid_alloc to store in the xarray
3. caller will be responsible for setting private data inside ioasid_data
and do call_rcu after update if needed.

Correct?

> Make the synchronize_rcu() the caller responsiblity, and callers
> should really be able to use call_rcu()
> 
> Jason


Thanks,

Jacob