[PATCH 1/2] powernv/npu: Add lock to prevent race in concurrent context init/destroy

[Alistair Popple <alistair@popple.id.au>]

The pnv_npu2_init_context() and pnv_npu2_destroy_context() functions are
used to allocate/free contexts to allow address translation and shootdown
by the NPU on a particular GPU. Context initialisation is implicitly safe
as it is protected by the requirement mmap_sem be held in write mode,
however pnv_npu2_destroy_context() does not require mmap_sem to be held and
it is not safe to call with a concurrent initialisation for a different
GPU.

It was assumed the driver would ensure destruction was not called
concurrently with initialisation. However the driver may be simplified by
allowing concurrent initialisation and destruction for different GPUs. As
npu context creation/destruction is not a performance critical path and the
critical section is not large a single spinlock is used for simplicity.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
---
 arch/powerpc/platforms/powernv/npu-dma.c | 51 ++++++++++++++++++++++++++------
 1 file changed, 42 insertions(+), 9 deletions(-)

diff --git a/arch/powerpc/platforms/powernv/npu-dma.c b/arch/powerpc/platforms/powernv/npu-dma.c
index 1cbef1f9cd37..cb77162f4e7a 100644
--- a/arch/powerpc/platforms/powernv/npu-dma.c
+++ b/arch/powerpc/platforms/powernv/npu-dma.c
@@ -34,6 +34,12 @@
 #define npu_to_phb(x) container_of(x, struct pnv_phb, npu)
 
 /*
+ * spinlock to protect initialisation of an npu_context for a particular
+ * mm_struct.
+ */
+DEFINE_SPINLOCK(npu_context_lock);
+
+/*
  * Other types of TCE cache invalidation are not functional in the
  * hardware.
  */
@@ -694,7 +700,8 @@ static const struct mmu_notifier_ops nv_nmmu_notifier_ops = {
  * Returns an error if there no contexts are currently available or a
  * npu_context which should be passed to pnv_npu2_handle_fault().
  *
- * mmap_sem must be held in write mode.
+ * mmap_sem must be held in write mode and must not be called from interrupt
+ * context.
  */
 struct npu_context *pnv_npu2_init_context(struct pci_dev *gpdev,
 			unsigned long flags,
@@ -741,7 +748,9 @@ struct npu_context *pnv_npu2_init_context(struct pci_dev *gpdev,
 	/*
 	 * Setup the NPU context table for a particular GPU. These need to be
 	 * per-GPU as we need the tables to filter ATSDs when there are no
-	 * active contexts on a particular GPU.
+	 * active contexts on a particular GPU. It is safe for these to be
+	 * called concurrently with destroy as the OPAL call takes appropriate
+	 * locks and refcounts on init/destroy.
 	 */
 	rc = opal_npu_init_context(nphb->opal_id, mm->context.id, flags,
 				PCI_DEVID(gpdev->bus->number, gpdev->devfn));
@@ -752,8 +761,19 @@ struct npu_context *pnv_npu2_init_context(struct pci_dev *gpdev,
 	 * We store the npu pci device so we can more easily get at the
 	 * associated npus.
 	 */
+	spin_lock(&npu_context_lock);
 	npu_context = mm->context.npu_context;
+	if (npu_context)
+		WARN_ON(!kref_get_unless_zero(&npu_context->kref));
+	spin_unlock(&npu_context_lock);
+
 	if (!npu_context) {
+		/*
+		 * We can set up these fields without holding the
+		 * npu_context_lock as the npu_context hasn't been returned to
+		 * the caller meaning it can't be destroyed. Parallel allocation
+		 * is protected against by mmap_sem.
+		 */
 		rc = -ENOMEM;
 		npu_context = kzalloc(sizeof(struct npu_context), GFP_KERNEL);
 		if (npu_context) {
@@ -772,8 +792,6 @@ struct npu_context *pnv_npu2_init_context(struct pci_dev *gpdev,
 		}
 
 		mm->context.npu_context = npu_context;
-	} else {
-		WARN_ON(!kref_get_unless_zero(&npu_context->kref));
 	}
 
 	npu_context->release_cb = cb;
@@ -811,15 +829,16 @@ static void pnv_npu2_release_context(struct kref *kref)
 		mm_context_remove_copro(npu_context->mm);
 
 	npu_context->mm->context.npu_context = NULL;
-	mmu_notifier_unregister(&npu_context->mn,
-				npu_context->mm);
-
-	kfree(npu_context);
 }
 
+/*
+ * Destroy a context on the given GPU. May free the npu_context if it is no
+ * longer active on any GPUs. Must not be called from interrupt context.
+ */
 void pnv_npu2_destroy_context(struct npu_context *npu_context,
 			struct pci_dev *gpdev)
 {
+	int removed;
 	struct pnv_phb *nphb;
 	struct npu *npu;
 	struct pci_dev *npdev = pnv_pci_get_npu_dev(gpdev, 0);
@@ -841,7 +860,21 @@ void pnv_npu2_destroy_context(struct npu_context *npu_context,
 	WRITE_ONCE(npu_context->npdev[npu->index][nvlink_index], NULL);
 	opal_npu_destroy_context(nphb->opal_id, npu_context->mm->context.id,
 				PCI_DEVID(gpdev->bus->number, gpdev->devfn));
-	kref_put(&npu_context->kref, pnv_npu2_release_context);
+	spin_lock(&npu_context_lock);
+	removed = kref_put(&npu_context->kref, pnv_npu2_release_context);
+	spin_unlock(&npu_context_lock);
+
+	/*
+	 * We need to do this outside of pnv_npu2_release_context so that it is
+	 * outside the spinlock as mmu_notifier_destroy uses SRCU.
+	 */
+	if (removed) {
+		mmu_notifier_unregister(&npu_context->mn,
+					npu_context->mm);
+
+		kfree(npu_context);
+	}
+
 }
 EXPORT_SYMBOL(pnv_npu2_destroy_context);
 
-- 
2.11.0