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.com
Reviewed-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>

diff --git a/drivers/s390/scsi/zfcp_qdio.c b/drivers/s390/scsi/zfcp_qdio.c
index 9fc045ddf66dba37e964d771c410f8eeff2eb976..23ab16d65f2a4acfd8ec404f69c7495f70d096a0 100644 (file)
--- a/drivers/s390/scsi/zfcp_qdio.c
+++ b/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);