[PATCH v2 1/2] iommu/amd/iommu_v2: Fix pasid_state refcount dec hit 0 warning on pasid unbind

[Vasant Hegde <vasant.hegde@amd.com>]

From: Daniel Marcovitch <dmarcovitch@nvidia.com>

When unbinding pasid - a race condition exists vs outstanding page faults.

To prevent this, the pasid_state object contains a refcount.
    * set to 1 on pasid bind
    * incremented on each ppr notification start
    * decremented on each ppr notification done
    * decremented on pasid unbind

Since refcount_dec assumes that refcount will never reach 0:
  the current implementation causes the following to be invoked on
  pasid unbind:
        REFCOUNT_WARN("decrement hit 0; leaking memory")

Fix this issue by changing refcount_dec to refcount_dec_and_test
to explicitly handle refcount=1.

Fixes: 8bc54824da4e ("iommu/amd: Convert from atomic_t to refcount_t on pasid_state->count")
Signed-off-by: Daniel Marcovitch <dmarcovitch@nvidia.com>
Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
---
 drivers/iommu/amd/iommu_v2.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/iommu/amd/iommu_v2.c b/drivers/iommu/amd/iommu_v2.c
index 864e4ffb6aa9..858748baae0b 100644
--- a/drivers/iommu/amd/iommu_v2.c
+++ b/drivers/iommu/amd/iommu_v2.c
@@ -262,8 +262,8 @@ static void put_pasid_state(struct pasid_state *pasid_state)
 
 static void put_pasid_state_wait(struct pasid_state *pasid_state)
 {
-	refcount_dec(&pasid_state->count);
-	wait_event(pasid_state->wq, !refcount_read(&pasid_state->count));
+	if (!refcount_dec_and_test(&pasid_state->count))
+		wait_event(pasid_state->wq, !refcount_read(&pasid_state->count));
 	free_pasid_state(pasid_state);
 }
 
-- 
2.31.1