scsi: klist: Make it safe to use klists in atomic context

In the scsi_transport_srp implementation it cannot be avoided to
iterate over a klist from atomic context when using the legacy block
layer instead of blk-mq. Hence this patch that makes it safe to use
klists in atomic context. This patch avoids that lockdep reports the
following:

WARNING: SOFTIRQ-safe -> SOFTIRQ-unsafe lock order detected
 Possible interrupt unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&(&k->k_lock)->rlock);
                               local_irq_disable();
                               lock(&(&q->__queue_lock)->rlock);
                               lock(&(&k->k_lock)->rlock);
  <Interrupt>
    lock(&(&q->__queue_lock)->rlock);

stack backtrace:
Workqueue: kblockd blk_timeout_work
Call Trace:
 dump_stack+0xa4/0xf5
 check_usage+0x6e6/0x700
 __lock_acquire+0x185d/0x1b50
 lock_acquire+0xd2/0x260
 _raw_spin_lock+0x32/0x50
 klist_next+0x47/0x190
 device_for_each_child+0x8e/0x100
 srp_timed_out+0xaf/0x1d0 [scsi_transport_srp]
 scsi_times_out+0xd4/0x410 [scsi_mod]
 blk_rq_timed_out+0x36/0x70
 blk_timeout_work+0x1b5/0x220
 process_one_work+0x4fe/0xad0
 worker_thread+0x63/0x5a0
 kthread+0x1c1/0x1e0
 ret_from_fork+0x24/0x30

See also commit c9ddf734 ("scsi: scsi_transport_srp: Fix shost to
rport translation").
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: James Bottomley <jejb@linux.vnet.ibm.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

lib/klist.c

@@ -336,8 +336,9 @@ struct klist_node *klist_prev(struct klist_iter *i)
 	void (*put)(struct klist_node *) = i->i_klist->put;
 	struct klist_node *last = i->i_cur;
 	struct klist_node *prev;
+	unsigned long flags;
 
-	spin_lock(&i->i_klist->k_lock);
+	spin_lock_irqsave(&i->i_klist->k_lock, flags);
 
 	if (last) {
 		prev = to_klist_node(last->n_node.prev);
@@ -356,7 +357,7 @@ struct klist_node *klist_prev(struct klist_iter *i)
 		prev = to_klist_node(prev->n_node.prev);
 	}
 
-	spin_unlock(&i->i_klist->k_lock);
+	spin_unlock_irqrestore(&i->i_klist->k_lock, flags);
 
 	if (put && last)
 		put(last);
@@ -377,8 +378,9 @@ struct klist_node *klist_next(struct klist_iter *i)
 	void (*put)(struct klist_node *) = i->i_klist->put;
 	struct klist_node *last = i->i_cur;
 	struct klist_node *next;
+	unsigned long flags;
 
-	spin_lock(&i->i_klist->k_lock);
+	spin_lock_irqsave(&i->i_klist->k_lock, flags);
 
 	if (last) {
 		next = to_klist_node(last->n_node.next);
@@ -397,7 +399,7 @@ struct klist_node *klist_next(struct klist_iter *i)
 		next = to_klist_node(next->n_node.next);
 	}
 
-	spin_unlock(&i->i_klist->k_lock);
+	spin_unlock_irqrestore(&i->i_klist->k_lock, flags);
 
 	if (put && last)
 		put(last);