Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <69b7b496c3658b385f2404d6e3209970b3677c08.camel@linux.ibm.com>
Subject: Re: [PATCH] iommu/s390: Fix race with release_device ops
From:   Niklas Schnelle <schnelle@linux.ibm.com>
To:     Alexander Gordeev <agordeev@linux.ibm.com>,
        Matthew Rosato <mjrosato@linux.ibm.com>
Cc:     Pierre Morel <pmorel@linux.ibm.com>, iommu@lists.linux.dev,
        linux-s390@vger.kernel.org, borntraeger@linux.ibm.com,
        hca@linux.ibm.com, gor@linux.ibm.com,
        gerald.schaefer@linux.ibm.com, svens@linux.ibm.com,
        joro@8bytes.org, will@kernel.org, robin.murphy@arm.com,
        jgg@nvidia.com, linux-kernel@vger.kernel.org
Date:   Thu, 25 Aug 2022 13:11:47 +0200
In-Reply-To: <YwcjIPcT1b0uyyFn@li-4a3a4a4c-28e5-11b2-a85c-a8d192c6f089.ibm.com>
References: <20220823203059.81919-1-mjrosato@linux.ibm.com>
         <a6e42442-d9cb-0d63-bb71-da78a5669a51@linux.ibm.com>
         <04644ee5-2386-1f3d-c1a3-fc4227570cf7@linux.ibm.com>
         <YwcjIPcT1b0uyyFn@li-4a3a4a4c-28e5-11b2-a85c-a8d192c6f089.ibm.com>
Content-Type: text/plain; charset="UTF-8"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Precedence: bulk

On Thu, 2022-08-25 at 09:22 +0200, Alexander Gordeev wrote:
> On Wed, Aug 24, 2022 at 04:25:19PM -0400, Matthew Rosato wrote:
> > > > @@ -90,15 +90,39 @@ static int s390_iommu_attach_device(struct iommu_domain *domain,
> > > >       struct zpci_dev *zdev = to_zpci_dev(dev);
> > > >       struct s390_domain_device *domain_device;
> > > >       unsigned long flags;
> > > > -    int cc, rc;
> > > > +    int cc, rc = 0;
> > > >         if (!zdev)
> > > >           return -ENODEV;
> > > >   +    /* First check compatibility */
> > > > +    spin_lock_irqsave(&s390_domain->list_lock, flags);
> > > > +    /* First device defines the DMA range limits */
> > > > +    if (list_empty(&s390_domain->devices)) {
> > > > +        domain->geometry.aperture_start = zdev->start_dma;
> > > > +        domain->geometry.aperture_end = zdev->end_dma;
> > > > +        domain->geometry.force_aperture = true;
> > > > +    /* Allow only devices with identical DMA range limits */
> > > > +    } else if (domain->geometry.aperture_start != zdev->start_dma ||
> > > > +           domain->geometry.aperture_end != zdev->end_dma) {
> > > > +        rc = -EINVAL;
> > > > +    }
> > > > +    spin_unlock_irqrestore(&s390_domain->list_lock, flags);
> > > > +    if (rc)
> > > > +        return rc;
> > > > +
> > > >       domain_device = kzalloc(sizeof(*domain_device), GFP_KERNEL);
> > > >       if (!domain_device)
> > > >           return -ENOMEM;
> > > >   +    /* Leave now if the device has already been released */
> > > > +    spin_lock_irqsave(&zdev->dma_domain_lock, flags);
> > > > +    if (!dev_iommu_priv_get(dev)) {
> > > > +        spin_unlock_irqrestore(&zdev->dma_domain_lock, flags);
> > > > +        kfree(domain_device);
> > > > +        return 0;
> > > > +    }
> > > > +
> > > >       if (zdev->dma_table && !zdev->s390_domain) {
> > > >           cc = zpci_dma_exit_device(zdev);
> > > >           if (cc) {
> > > 
> > > Am I wrong? It seems to me that zpci_dma_exit_device here is called with the spin_lock locked but this function zpci_dma_exit_device calls vfree which may sleep.
> > > 
> > 
> > Oh, good point, I just enabled lockdep to verify that.
> > 
> > I think we could just replace this with a mutex instead, it's not a performance path.  I've been running tests successfully today with this patch modified to instead use a mutex for dma_domain_lock.
> 
> But your original version uses irq-savvy spinlocks.
> Are there data that need to be protected against interrupts?
> 
> Thanks!

I think that was a carry over from my original attempt that used the
zdev->dma_domain_lock in some more places including in interrupt
context. I think these are gone now so I think Matt is right in his
version this can be a mutex.