[PATCH] [SCSI] scsi_remove_target: fix softlockup regression on hot remove

[Louis Bouchard]

From: Dan Williams <djbw at fb.com>

John reports:
 BUG: soft lockup - CPU#2 stuck for 23s! [kworker/u:8:2202]
 [..]
 Call Trace:
  [<ffffffff8141782a>] scsi_remove_target+0xda/0x1f0
  [<ffffffff81421de5>] sas_rphy_remove+0x55/0x60
  [<ffffffff81421e01>] sas_rphy_delete+0x11/0x20
  [<ffffffff81421e35>] sas_port_delete+0x25/0x160
  [<ffffffff814549a3>] mptsas_del_end_device+0x183/0x270

...introduced by commit 3b661a9 "[SCSI] fix hot unplug vs async scan race".

Don't restart lookup of more stargets in the multi-target case, just
arrange to traverse the list once, on the assumption that new targets
are always added at the end.  There is no guarantee that the target will
change state in scsi_target_reap() so we can end up spinning if we
restart.

Cc: <stable at vger.kernel.org>
Acked-by: Jack Wang <jack_wang at usish.com>
LKML-Reference: <CAEhu1-6wq1YsNiscGMwP4ud0Q+MrViRzv=kcWCQSBNc8c68N5Q at mail.gmail.com>
Reported-by: John Drescher <drescherjm at gmail.com>
Tested-by: John Drescher <drescherjm at gmail.com>
Signed-off-by: Dan Williams <djbw at fb.com>
Signed-off-by: James Bottomley <JBottomley at Parallels.com>
(cherry picked from commit bc3f02a795d3b4faa99d37390174be2a75d091bd)

BugLink: http://bugs.launchpad.net/bugs/1056746

Signed-off-by: Louis Bouchard <louis.bouchard at canonical.com>
---
 drivers/scsi/scsi_sysfs.c |   30 ++++++++++++++----------------
 1 file changed, 14 insertions(+), 16 deletions(-)

diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c
index bb7c482..08d48a3 100644
--- a/drivers/scsi/scsi_sysfs.c
+++ b/drivers/scsi/scsi_sysfs.c
@@ -1023,33 +1023,31 @@ static void __scsi_remove_target(struct scsi_target *starget)
 void scsi_remove_target(struct device *dev)
 {
 	struct Scsi_Host *shost = dev_to_shost(dev->parent);
-	struct scsi_target *starget, *found;
+	struct scsi_target *starget, *last = NULL;
 	unsigned long flags;
 
- restart:
-	found = NULL;
+	/* remove targets being careful to lookup next entry before
+	 * deleting the last
+	 */
 	spin_lock_irqsave(shost->host_lock, flags);
 	list_for_each_entry(starget, &shost->__targets, siblings) {
 		if (starget->state == STARGET_DEL)
 			continue;
 		if (starget->dev.parent == dev || &starget->dev == dev) {
-			found = starget;
-			found->reap_ref++;
-			break;
+			/* assuming new targets arrive at the end */
+			starget->reap_ref++;
+			spin_unlock_irqrestore(shost->host_lock, flags);
+			if (last)
+				scsi_target_reap(last);
+			last = starget;
+			__scsi_remove_target(starget);
+			spin_lock_irqsave(shost->host_lock, flags);
 		}
 	}
 	spin_unlock_irqrestore(shost->host_lock, flags);
 
-	if (found) {
-		__scsi_remove_target(found);
-		scsi_target_reap(found);
-		/* in the case where @dev has multiple starget children,
-		 * continue removing.
-		 *
-		 * FIXME: does such a case exist?
-		 */
-		goto restart;
-	}
+	if (last)
+		scsi_target_reap(last);
 }
 EXPORT_SYMBOL(scsi_remove_target);
 
-- 
1.7.9.5