[PATCH v7 1/9] Fix race between starved list processing and device removal

[Bart Van Assche <bvanassche@acm.org>]

Avoid that the sdev reference count can drop to zero before
a queue is run by scsi_run_queue().

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reported-and-tested-by: Chanho Min <chanho.min@lge.com>
Reference: http://lkml.org/lkml/2012/8/2/96
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: <stable@vger.kernel.org>
---
 drivers/scsi/scsi_lib.c   |   16 +++++++++++-----
 drivers/scsi/scsi_sysfs.c |   14 +++++++++++++-
 2 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index f1bf5af..5c67339 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -452,11 +452,17 @@ static void scsi_run_queue(struct request_queue *q)
 			continue;
 		}
 
-		spin_unlock(shost->host_lock);
-		spin_lock(sdev->request_queue->queue_lock);
-		__blk_run_queue(sdev->request_queue);
-		spin_unlock(sdev->request_queue->queue_lock);
-		spin_lock(shost->host_lock);
+		/*
+		 * Obtain a reference before unlocking the host_lock such that
+		 * the sdev can't disappear before blk_run_queue() is invoked.
+		 */
+		get_device(&sdev->sdev_gendev);
+		spin_unlock_irqrestore(shost->host_lock, flags);
+
+		blk_run_queue(sdev->request_queue);
+		put_device(&sdev->sdev_gendev);
+
+		spin_lock_irqsave(shost->host_lock, flags);
 	}
 	/* put any unprocessed entries back */
 	list_splice(&starved_list, &shost->starved_list);
diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c
index ce5224c..2ff7ba5 100644
--- a/drivers/scsi/scsi_sysfs.c
+++ b/drivers/scsi/scsi_sysfs.c
@@ -348,7 +348,6 @@ static void scsi_device_dev_release_usercontext(struct work_struct *work)
 	starget->reap_ref++;
 	list_del(&sdev->siblings);
 	list_del(&sdev->same_target_siblings);
-	list_del(&sdev->starved_entry);
 	spin_unlock_irqrestore(sdev->host->host_lock, flags);
 
 	cancel_work_sync(&sdev->event_work);
@@ -956,6 +955,8 @@ int scsi_sysfs_add_sdev(struct scsi_device *sdev)
 void __scsi_remove_device(struct scsi_device *sdev)
 {
 	struct device *dev = &sdev->sdev_gendev;
+	struct Scsi_Host *shost = sdev->host;
+	unsigned long flags;
 
 	if (sdev->is_visible) {
 		if (scsi_device_set_state(sdev, SDEV_CANCEL) != 0)
@@ -977,6 +978,17 @@ void __scsi_remove_device(struct scsi_device *sdev)
 	blk_cleanup_queue(sdev->request_queue);
 	cancel_work_sync(&sdev->requeue_work);
 
+	/*
+	 * Remove a SCSI device from the starved list after blk_cleanup_queue()
+	 * finished such that scsi_request_fn() can't add it back to that list.
+	 * Also remove an sdev from the starved list before invoking
+	 * put_device() such that get_device() is guaranteed to succeed for an
+	 * sdev present on the starved list.
+	 */
+	spin_lock_irqsave(shost->host_lock, flags);
+	list_del(&sdev->starved_entry);
+	spin_unlock_irqrestore(shost->host_lock, flags);
+
 	if (sdev->host->hostt->slave_destroy)
 		sdev->host->hostt->slave_destroy(sdev);
 	transport_destroy_device(dev);
-- 
1.7.10.4