linux-kernel - Re: [RFC 01/20] iommu/iommufd: Add /dev/iommu core

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Date:   Tue, 19 Oct 2021 10:11:34 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     "Liu, Yi L" <yi.l.liu@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "hch@....de" <hch@....de>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "lkml@...ux.net" <lkml@...ux.net>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "lushenming@...wei.com" <lushenming@...wei.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "david@...son.dropbear.id.au" <david@...son.dropbear.id.au>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [RFC 01/20] iommu/iommufd: Add /dev/iommu core

Hi Jason,

On Tue, 19 Oct 2021 13:57:47 -0300, Jason Gunthorpe <jgg@...dia.com> wrote:

> On Tue, Oct 19, 2021 at 09:57:34AM -0700, Jacob Pan wrote:
> > Hi Jason,
> > 
> > On Fri, 15 Oct 2021 08:18:07 -0300, Jason Gunthorpe <jgg@...dia.com>
> > wrote: 
> > > On Fri, Oct 15, 2021 at 09:18:06AM +0000, Liu, Yi L wrote:
> > >   
> > > > >   Acquire from the xarray is
> > > > >    rcu_lock()
> > > > >    ioas = xa_load()
> > > > >    if (ioas)
> > > > >       if (down_read_trylock(&ioas->destroying_lock))    
> > > > 
> > > > all good suggestions, will refine accordingly. Here destroying_lock
> > > > is a rw_semaphore. right? Since down_read_trylock() accepts a
> > > > rwsem.    
> > > 
> > > Yes, you probably need a sleeping lock
> > >   
> > I am not following why we want a sleeping lock inside RCU protected
> > section?  
> 
> trylock is not sleeping
Of course, thanks for clarifying.

> > For ioas, do we really care about the stale data to choose rw_lock vs
> > RCU? Destroying can be kfree_rcu?  
> 
> It needs a hard fence so things don't continue to use the IOS once it
> is destroyed.
I guess RCU can do that also perhaps can scale better?

> Jason


Thanks,

Jacob