[SCSI] fix our current target reap infrastructure

This patch eliminates the reap_ref and replaces it with a proper kref.
On last put of this kref, the target is removed from visibility in
sysfs.  The final call to scsi_target_reap() for the device is done from
__scsi_remove_device() and only if the device was made visible.  This
ensures that the target disappears as soon as the last device is gone
rather than waiting until final release of the device (which is often
too long).

Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org # delay backport by 2 months for field testing
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

1 files changed, 60 insertions, 39 deletions

diff --git a/drivers/scsi/scsi_scan.c b/drivers/scsi/scsi_scan.c
index 307a81137607..5fad646ee6e5 100644
--- a/drivers/scsi/scsi_scan.c
+++ b/drivers/scsi/scsi_scan.c
@@ -371,6 +371,31 @@ static struct scsi_target *__scsi_find_target(struct device *parent,
 }
 
 /**
+ * scsi_target_reap_ref_release - remove target from visibility
+ * @kref: the reap_ref in the target being released
+ *
+ * Called on last put of reap_ref, which is the indication that no device
+ * under this target is visible anymore, so render the target invisible in
+ * sysfs.  Note: we have to be in user context here because the target reaps
+ * should be done in places where the scsi device visibility is being removed.
+ */
+static void scsi_target_reap_ref_release(struct kref *kref)
+{
+	struct scsi_target *starget
+		= container_of(kref, struct scsi_target, reap_ref);
+
+	transport_remove_device(&starget->dev);
+	device_del(&starget->dev);
+	starget->state = STARGET_DEL;
+	scsi_target_destroy(starget);
+}
+
+static void scsi_target_reap_ref_put(struct scsi_target *starget)
+{
+	kref_put(&starget->reap_ref, scsi_target_reap_ref_release);
+}
+
+/**
  * scsi_alloc_target - allocate a new or find an existing target
  * @parent:	parent of the target (need not be a scsi host)
  * @channel:	target channel number (zero if no channels)
@@ -392,7 +417,7 @@ static struct scsi_target *scsi_alloc_target(struct device *parent,
 		+ shost->transportt->target_size;
 	struct scsi_target *starget;
 	struct scsi_target *found_target;
-	int error;
+	int error, ref_got;
 
 	starget = kzalloc(size, GFP_KERNEL);
 	if (!starget) {
@@ -401,7 +426,7 @@ static struct scsi_target *scsi_alloc_target(struct device *parent,
 	}
 	dev = &starget->dev;
 	device_initialize(dev);
-	starget->reap_ref = 1;
+	kref_init(&starget->reap_ref);
 	dev->parent = get_device(parent);
 	dev_set_name(dev, "target%d:%d:%d", shost->host_no, channel, id);
 	dev->bus = &scsi_bus_type;
@@ -441,29 +466,36 @@ static struct scsi_target *scsi_alloc_target(struct device *parent,
 	return starget;
 
  found:
-	found_target->reap_ref++;
+	/*
+	 * release routine already fired if kref is zero, so if we can still
+	 * take the reference, the target must be alive.  If we can't, it must
+	 * be dying and we need to wait for a new target
+	 */
+	ref_got = kref_get_unless_zero(&found_target->reap_ref);
+
 	spin_unlock_irqrestore(shost->host_lock, flags);
-	if (found_target->state != STARGET_DEL) {
+	if (ref_got) {
 		put_device(dev);
 		return found_target;
 	}
-	/* Unfortunately, we found a dying target; need to
-	 * wait until it's dead before we can get a new one */
+	/*
+	 * Unfortunately, we found a dying target; need to wait until it's
+	 * dead before we can get a new one.  There is an anomaly here.  We
+	 * *should* call scsi_target_reap() to balance the kref_get() of the
+	 * reap_ref above.  However, since the target being released, it's
+	 * already invisible and the reap_ref is irrelevant.  If we call
+	 * scsi_target_reap() we might spuriously do another device_del() on
+	 * an already invisible target.
+	 */
 	put_device(&found_target->dev);
-	flush_scheduled_work();
+	/*
+	 * length of time is irrelevant here, we just want to yield the CPU
+	 * for a tick to avoid busy waiting for the target to die.
+	 */
+	msleep(1);
 	goto retry;
 }
 
-static void scsi_target_reap_usercontext(struct work_struct *work)
-{
-	struct scsi_target *starget =
-		container_of(work, struct scsi_target, ew.work);
-
-	transport_remove_device(&starget->dev);
-	device_del(&starget->dev);
-	scsi_target_destroy(starget);
-}
-
 /**
  * scsi_target_reap - check to see if target is in use and destroy if not
  * @starget: target to be checked
@@ -474,28 +506,11 @@ static void scsi_target_reap_usercontext(struct work_struct *work)
  */
 void scsi_target_reap(struct scsi_target *starget)
 {
-	struct Scsi_Host *shost = dev_to_shost(starget->dev.parent);
-	unsigned long flags;
-	enum scsi_target_state state;
-	int empty = 0;
-
-	spin_lock_irqsave(shost->host_lock, flags);
-	state = starget->state;
-	if (--starget->reap_ref == 0 && list_empty(&starget->devices)) {
-		empty = 1;
-		starget->state = STARGET_DEL;
-	}
-	spin_unlock_irqrestore(shost->host_lock, flags);
-
-	if (!empty)
-		return;
-
-	BUG_ON(state == STARGET_DEL);
-	if (state == STARGET_CREATED)
+	BUG_ON(starget->state == STARGET_DEL);
+	if (starget->state == STARGET_CREATED)
 		scsi_target_destroy(starget);
 	else
-		execute_in_process_context(scsi_target_reap_usercontext,
-					   &starget->ew);
+		scsi_target_reap_ref_put(starget);
 }
 
 /**
@@ -1532,6 +1547,10 @@ struct scsi_device *__scsi_add_device(struct Scsi_Host *shost, uint channel,
 	}
 	mutex_unlock(&shost->scan_mutex);
 	scsi_autopm_put_target(starget);
+	/*
+	 * paired with scsi_alloc_target().  Target will be destroyed unless
+	 * scsi_probe_and_add_lun made an underlying device visible
+	 */
 	scsi_target_reap(starget);
 	put_device(&starget->dev);
 
@@ -1612,8 +1631,10 @@ static void __scsi_scan_target(struct device *parent, unsigned int channel,
 
  out_reap:
 	scsi_autopm_put_target(starget);
-	/* now determine if the target has any children at all
-	 * and if not, nuke it */
+	/*
+	 * paired with scsi_alloc_target(): determine if the target has
+	 * any children at all and if not, nuke it
+	 */
 	scsi_target_reap(starget);
 
 	put_device(&starget->dev);