[PATCH v2 1/1] scsi: libiscsi: fix NOP race condition

[lduncan]

From: Lee Duncan <lduncan@xxxxxxxx>

iSCSI NOPs are sometimes "lost", mistakenly sent to the
user-land iscsid daemon instead of handled in the kernel,
as they should be, resulting in a message from the daemon like:

> iscsid: Got nop in, but kernel supports nop handling.

This can occur because of the forward- and back-locks
in the kernel iSCSI code, and the fact that an iSCSI NOP
response can be processed before processing of the NOP send
is complete. This can result in "conn->ping_task" being NULL
in iscsi_nop_out_rsp(), when the pointer is actually in
the process of being set.

To work around this, we add a new state to the "ping_task"
pointer. In addition to NULL (not assigned) and a pointer
(assigned), we add the state "being set", which is signaled
with an INVALID pointer (using "-1").

Signed-off-by: Lee Duncan <lduncan@xxxxxxxx>
---
 drivers/scsi/libiscsi.c | 13 ++++++++++---
 include/scsi/libiscsi.h |  3 +++
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/scsi/libiscsi.c b/drivers/scsi/libiscsi.c
index 1e9c3171fa9f..cade108c33b6 100644
--- a/drivers/scsi/libiscsi.c
+++ b/drivers/scsi/libiscsi.c
@@ -738,6 +738,9 @@ __iscsi_conn_send_pdu(struct iscsi_conn *conn, struct iscsi_hdr *hdr,
 						   task->conn->session->age);
 	}
 
+	if (unlikely(READ_ONCE(conn->ping_task) == INVALID_SCSI_TASK))
+		WRITE_ONCE(conn->ping_task, task);
+
 	if (!ihost->workq) {
 		if (iscsi_prep_mgmt_task(conn, task))
 			goto free_task;
@@ -941,8 +944,11 @@ static int iscsi_send_nopout(struct iscsi_conn *conn, struct iscsi_nopin *rhdr)
         struct iscsi_nopout hdr;
 	struct iscsi_task *task;
 
-	if (!rhdr && conn->ping_task)
-		return -EINVAL;
+	if (!rhdr) {
+		if (READ_ONCE(conn->ping_task))
+			return -EINVAL;
+		WRITE_ONCE(conn->ping_task, INVALID_SCSI_TASK);
+	}
 
 	memset(&hdr, 0, sizeof(struct iscsi_nopout));
 	hdr.opcode = ISCSI_OP_NOOP_OUT | ISCSI_OP_IMMEDIATE;
@@ -957,11 +963,12 @@ static int iscsi_send_nopout(struct iscsi_conn *conn, struct iscsi_nopin *rhdr)
 
 	task = __iscsi_conn_send_pdu(conn, (struct iscsi_hdr *)&hdr, NULL, 0);
 	if (!task) {
+		if (!rhdr)
+			WRITE_ONCE(conn->ping_task, NULL);
 		iscsi_conn_printk(KERN_ERR, conn, "Could not send nopout\n");
 		return -EIO;
 	} else if (!rhdr) {
 		/* only track our nops */
-		conn->ping_task = task;
 		conn->last_ping = jiffies;
 	}
 
diff --git a/include/scsi/libiscsi.h b/include/scsi/libiscsi.h
index c25fb86ffae9..b3bbd10eb3f0 100644
--- a/include/scsi/libiscsi.h
+++ b/include/scsi/libiscsi.h
@@ -132,6 +132,9 @@ struct iscsi_task {
 	void			*dd_data;	/* driver/transport data */
 };
 
+/* invalid scsi_task pointer */
+#define	INVALID_SCSI_TASK	(struct iscsi_task *)-1l
+
 static inline int iscsi_task_has_unsol_data(struct iscsi_task *task)
 {
 	return task->unsol_r2t.data_length > task->unsol_r2t.sent;
-- 
2.26.2