[PATCH v2] cxl/port: Restart port enumeration when a sibling adds the dport first

[Alison Schofield <alison.schofield@intel.com>]

Endpoint probes can race while enumerating a shared switch. If a
sibling probe adds the dport first, the losing probe finds the dport
already present, gets -EBUSY, and fails to enumerate the endpoint.

Treat this race the same as the existing port-created case by
restarting the port walk, allowing it to find the existing dport
and continue enumeration.

This race was discovered while testing a cxl_test mixed-granularity
topology, where twelve endpoints behind shared switches are probed in
parallel during module load.

Fixes: 4f06d81e7c6a ("cxl: Defer dport allocation for switch ports")
Signed-off-by: Alison Schofield <alison.schofield@intel.com>
---

Changes in v2:
- Commit log: note that race is btw different dport add paths for the
  same shared dport (Ming)

 drivers/cxl/core/port.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/cxl/core/port.c b/drivers/cxl/core/port.c
index 1215ee4f4035..65f2d2f1eb00 100644
--- a/drivers/cxl/core/port.c
+++ b/drivers/cxl/core/port.c
@@ -1749,8 +1749,8 @@ static int add_port_attach_ep(struct cxl_memdev *cxlmd,
 					     parent_dport, uport_dev,
 					     dport_dev);
 		if (IS_ERR(dport)) {
-			/* Port already exists, restart iteration */
-			if (PTR_ERR(dport) == -EAGAIN)
+			/* Port or dport already exists, restart iteration */
+			if (PTR_ERR(dport) == -EAGAIN || PTR_ERR(dport) == -EBUSY)
 				return 0;
 			return PTR_ERR(dport);
 		}

base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
-- 
2.37.3