iommu: Avoid introducing more races

Although the lock-juggling is only a temporary workaround, we don't want
it to make things avoidably worse. Jason was right to be nervous, since
bus_iommu_probe() doesn't care *which* IOMMU instance it's probing for,
so it probably is possible for one walk to finish a probe which a
different walk started, thus we do want to check for that.

Also there's no need to drop the lock just to have of_iommu_configure()
do nothing when a fwspec already exists; check that directly and avoid
opening a window at all in that (still somewhat likely) case.

Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/09d901ad11b3a410fbb6e27f7d04ad4609c3fe4a.1741706365.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 4f91a74..264383b 100644 (file)
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -422,13 +422,15 @@ static int iommu_init_device(struct device *dev)
         * is buried in the bus dma_configure path. Properly unpicking that is
         * still a big job, so for now just invoke the whole thing. The device
         * already having a driver bound means dma_configure has already run and
-        * either found no IOMMU to wait for, or we're in its replay call right
-        * now, so either way there's no point calling it again.
+        * found no IOMMU to wait for, so there's no point calling it again.
         */
-       if (!dev->driver && dev->bus->dma_configure) {
+       if (!dev->iommu->fwspec && !dev->driver && dev->bus->dma_configure) {
                mutex_unlock(&iommu_probe_device_lock);
                dev->bus->dma_configure(dev);
                mutex_lock(&iommu_probe_device_lock);
+               /* If another instance finished the job for us, skip it */
+               if (!dev->iommu || dev->iommu_group)
+                       return -ENODEV;
        }
        /*
         * At this point, relevant devices either now have a fwspec which will