linux-kernel - Re: [PATCH v6 02/11] driver core: Add dma_cleanup callback in bus

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Message-ID: <20220223143011.GQ10061@nvidia.com>
Date:   Wed, 23 Feb 2022 10:30:11 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Robin Murphy <robin.murphy@....com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Christoph Hellwig <hch@...radead.org>,
        Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, kvm@...r.kernel.org,
        rafael@...nel.org, David Airlie <airlied@...ux.ie>,
        linux-pci@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Diana Craciun <diana.craciun@....nxp.com>,
        Dmitry Osipenko <digetx@...il.com>,
        Will Deacon <will@...nel.org>,
        Stuart Yoder <stuyoder@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Cornelia Huck <cohuck@...hat.com>,
        linux-kernel@...r.kernel.org, Li Yang <leoyang.li@....com>,
        iommu@...ts.linux-foundation.org,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v6 02/11] driver core: Add dma_cleanup callback in
 bus_type

On Wed, Feb 23, 2022 at 10:09:01AM -0400, Jason Gunthorpe wrote:
> On Wed, Feb 23, 2022 at 03:06:35PM +0100, Greg Kroah-Hartman wrote:
> > On Wed, Feb 23, 2022 at 09:46:27AM -0400, Jason Gunthorpe wrote:
> > > On Wed, Feb 23, 2022 at 01:04:00PM +0000, Robin Murphy wrote:
> > > 
> > > > 1 - tmp->driver is non-NULL because tmp is already bound.
> > > >   1.a - If tmp->driver->driver_managed_dma == 0, the group must currently be
> > > > DMA-API-owned as a whole. Regardless of what driver dev has unbound from,
> > > > its removal does not release someone else's DMA API (co-)ownership.
> > > 
> > > This is an uncommon locking pattern, but it does work. It relies on
> > > the mutex being an effective synchronization barrier for an unlocked
> > > store:
> > > 
> > > 				      WRITE_ONCE(dev->driver, NULL)
> > 
> > Only the driver core should be messing with the dev->driver pointer as
> > when it does so, it already has the proper locks held.  Do I need to
> > move that to a "private" location so that nothing outside of the driver
> > core can mess with it?
> 
> It would be nice, I've seen a abuse and mislocking of it in drivers

Though to be clear, what Robin is describing is still keeping the
dev->driver stores in dd.c, just reading it in a lockless way from
other modules.

Jason