[PATCH] iommu: Avoid introducing more races

[Robin Murphy <robin.murphy@arm.com>]

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>
---
I was rather expecting to send a v3 of "iommu: Get DT/ACPI parsing into
the proper probe path", so I'm grateful that v2 was picked up, thanks!
This is the difference as a fix/squash commit instead.
---
 drivers/iommu/iommu.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index d824fa5166e3..c6d40abe577e 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -415,13 +415,15 @@ 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
-- 
2.39.2.101.g768bb238c484.dirty