scsi: fix the {host,target,device}_blocked counter mess

Seems like these counters are missing any sort of synchronization for
updates, as a over 10 year old comment from me noted.  Fix this by
using atomic counters, and while we're at it also make sure they are
in the same cacheline as the _busy counters and not needlessly stored
to in every I/O completion.

With the new model the _busy counters can temporarily go negative,
so all the readers are updated to check for > 0 values.  Longer
term every successful I/O completion will reset the counters to zero,
so the temporarily negative values will not cause any harm.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Webb Scales <webbnh@hp.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Tested-by: Bart Van Assche <bvanassche@acm.org>
Tested-by: Robert Elliott <elliott@hp.com>

2 files changed, 6 insertions, 8 deletions

diff --git a/include/scsi/scsi_device.h b/include/scsi/scsi_device.h
index 3329901..0f853f2 100644
--- a/include/scsi/scsi_device.h
+++ b/include/scsi/scsi_device.h
@@ -82,6 +82,8 @@ struct scsi_device {
 	struct list_head    same_target_siblings; /* just the devices sharing same target id */
 
 	atomic_t device_busy;		/* commands actually active on LLDD */
+	atomic_t device_blocked;	/* Device returned QUEUE_FULL. */
+
 	spinlock_t list_lock;
 	struct list_head cmd_list;	/* queue of in use SCSI Command structures */
 	struct list_head starved_entry;
@@ -180,8 +182,6 @@ struct scsi_device {
 	struct list_head event_list;	/* asserted events */
 	struct work_struct event_work;
 
-	unsigned int device_blocked;	/* Device returned QUEUE_FULL. */
-
 	unsigned int max_device_blocked; /* what device_blocked counts down from  */
 #define SCSI_DEFAULT_DEVICE_BLOCKED	3
 
@@ -291,12 +291,13 @@ struct scsi_target {
 						 * the same target will also. */
 	/* commands actually active on LLD. */
 	atomic_t		target_busy;
+	atomic_t		target_blocked;
+
 	/*
 	 * LLDs should set this in the slave_alloc host template callout.
 	 * If set to zero then there is not limit.
 	 */
 	unsigned int		can_queue;
-	unsigned int		target_blocked;
 	unsigned int		max_target_blocked;
 #define SCSI_DEFAULT_TARGET_BLOCKED	3
 
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index 51f7911..5e8ebc1 100644
--- a/include/scsi/scsi_host.h
+++ b/include/scsi/scsi_host.h
@@ -583,6 +583,8 @@ struct Scsi_Host {
 	struct blk_queue_tag	*bqt;
 
 	atomic_t host_busy;		   /* commands actually active on low-level */
+	atomic_t host_blocked;
+
 	unsigned int host_failed;	   /* commands that failed.
 					      protected by host_lock */
 	unsigned int host_eh_scheduled;    /* EH scheduled without command */
@@ -682,11 +684,6 @@ struct Scsi_Host {
 	struct workqueue_struct *tmf_work_q;
 
 	/*
-	 * Host has rejected a command because it was busy.
-	 */
-	unsigned int host_blocked;
-
-	/*
 	 * Value host_blocked counts down from
 	 */
 	unsigned int max_host_blocked;