[PATCH v2 06/16] RDMA/hns: Fix xarray race in hns_roce_create_srq()

[Jason Gunthorpe <jgg@nvidia.com>]

Sashiko points out that once the srq memory is stored into the xarray by
alloc_srqc() it can immediately be looked up by:

	xa_lock(&srq_table->xa);
	srq = xa_load(&srq_table->xa, srqn & (hr_dev->caps.num_srqs - 1));
	if (srq)
		refcount_inc(&srq->refcount);
	xa_unlock(&srq_table->xa);

Which will fail refcount debug because the refcount is 0 and then crash:

	srq->event(srq, event_type);

Because event is NULL.

Use refcount_inc_not_zero() instead to ensure a partially prepared srq is
never retrieved from the event handler and fix the ordering of the
initialization so refcount becomes 1 only after it is fully ready.

Link: https://sashiko.dev/#/patchset/0-v1-e911b76a94d1%2B65d95-rdma_udata_rep_jgg%40nvidia.com?part=3
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
---
 drivers/infiniband/hw/hns/hns_roce_srq.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/infiniband/hw/hns/hns_roce_srq.c b/drivers/infiniband/hw/hns/hns_roce_srq.c
index cb848e8e6bbd76..d6201ddde0292a 100644
--- a/drivers/infiniband/hw/hns/hns_roce_srq.c
+++ b/drivers/infiniband/hw/hns/hns_roce_srq.c
@@ -16,8 +16,8 @@ void hns_roce_srq_event(struct hns_roce_dev *hr_dev, u32 srqn, int event_type)
 
 	xa_lock(&srq_table->xa);
 	srq = xa_load(&srq_table->xa, srqn & (hr_dev->caps.num_srqs - 1));
-	if (srq)
-		refcount_inc(&srq->refcount);
+	if (srq && !refcount_inc_not_zero(&srq->refcount))
+		srq = NULL;
 	xa_unlock(&srq_table->xa);
 
 	if (!srq) {
@@ -481,8 +481,8 @@ int hns_roce_create_srq(struct ib_srq *ib_srq,
 	}
 
 	srq->event = hns_roce_ib_srq_event;
-	refcount_set(&srq->refcount, 1);
 	init_completion(&srq->free);
+	refcount_set(&srq->refcount, 1);
 
 	return 0;
 
-- 
2.43.0