scsi: zfcp: Clarify & assert the stat_lock locking in zfcp_qdio_send()

Explain why the plain spin_lock() suffices in current code, even when the
stat_lock is also used by zfcp_qdio_int_req() in tasklet context.

We could also promote the spin_lock() to a spin_lock_irqsave(), but that
would just obfuscate the locking even further.

Link: https://lore.kernel.org/r/b023b1472630f4bf9fce580577d7bb49de89ccbf.1603908167.git.bblock@linux.ibm.comReviewed-by: Benjamin Block <bblock@linux.ibm.com>
Reviewed-by: Steffen Maier <maier@linux.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/s390/scsi/zfcp_qdio.c

@@ -10,6 +10,7 @@
 #define KMSG_COMPONENT "zfcp"
 #define pr_fmt(fmt) KMSG_COMPONENT ": " fmt
 
+#include <linux/lockdep.h>
 #include <linux/slab.h>
 #include <linux/module.h>
 #include "zfcp_ext.h"
@@ -283,6 +284,13 @@ int zfcp_qdio_send(struct zfcp_qdio *qdio, struct zfcp_qdio_req *q_req)
 	int retval;
 	u8 sbal_number = q_req->sbal_number;
 
+	/*
+	 * This should actually be a spin_lock_bh(stat_lock), to protect against
+	 * zfcp_qdio_int_req() in tasklet context.
+	 * But we can't do so (and are safe), as we always get called with IRQs
+	 * disabled by spin_lock_irq[save](req_q_lock).
+	 */
+	lockdep_assert_irqs_disabled();
 	spin_lock(&qdio->stat_lock);
 	zfcp_qdio_account(qdio);
 	spin_unlock(&qdio->stat_lock);