[libata] fix PIO completion race

Make sure we that completion is the final action we take; prior to this
change, another CPU may have changed ap->pio_task_state before we tested
it a final time.

Spotted by, and original patch by Albert Lee @ IBM.

Also includes a minor optimization:  eliminate a ton of unnecessary
queue_work() calls, simply by jumping to the beginning of the FSM
function ata_pio_task().

1 files changed, 23 insertions, 14 deletions

diff --git a/drivers/scsi/libata-core.c b/drivers/scsi/libata-core.c
index 5cc53cd9323e..d92273cbe0de 100644
--- a/drivers/scsi/libata-core.c
+++ b/drivers/scsi/libata-core.c
@@ -2465,9 +2465,12 @@ static unsigned long ata_pio_poll(struct ata_port *ap)
  *
  *      LOCKING:
  *      None.  (executing in kernel thread context)
+ *
+ *      RETURNS:
+ *      Non-zero if qc completed, zero otherwise.
  */
 
-static void ata_pio_complete (struct ata_port *ap)
+static int ata_pio_complete (struct ata_port *ap)
 {
         struct ata_queued_cmd *qc;
         u8 drv_stat;
@@ -2486,14 +2489,14 @@ static void ata_pio_complete (struct ata_port *ap)
                 if (drv_stat & (ATA_BUSY | ATA_DRQ)) {
                         ap->pio_task_state = PIO_ST_LAST_POLL;
                         ap->pio_task_timeout = jiffies + ATA_TMOUT_PIO;
-                        return;
+                        return 0;
                 }
         }
 
         drv_stat = ata_wait_idle(ap);
         if (!ata_ok(drv_stat)) {
                 ap->pio_task_state = PIO_ST_ERR;
-                return;
+                return 0;
         }
 
         qc = ata_qc_from_tag(ap, ap->active_tag);
@@ -2502,6 +2505,10 @@ static void ata_pio_complete (struct ata_port *ap)
         ap->pio_task_state = PIO_ST_IDLE;
 
         ata_poll_qc_complete(qc, drv_stat);
+
+        /* another command may start at this point */
+
+        return 1;
 }
 
 
@@ -2709,7 +2716,7 @@ static void __atapi_pio_bytes(struct ata_queued_cmd *qc, unsigned int bytes)
 
 next_sg:
         if (unlikely(qc->cursg >= qc->n_elem)) {
-                /* 
+                /*
                  * The end of qc->sg is reached and the device expects
                  * more data to transfer. In order not to overrun qc->sg
                  * and fulfill length specified in the byte count register,
@@ -2721,7 +2728,7 @@ next_sg:
                 unsigned int i;
 
                 if (words) /* warning if bytes > 1 */
-                        printk(KERN_WARNING "ata%u: %u bytes trailing data\n", 
+                        printk(KERN_WARNING "ata%u: %u bytes trailing data\n",
                                ap->id, bytes);
 
                 for (i = 0; i < words; i++)
@@ -2849,9 +2856,7 @@ static void ata_pio_block(struct ata_port *ap)
         if (is_atapi_taskfile(&qc->tf)) {
                 /* no more data to transfer or unsupported ATAPI command */
                 if ((status & ATA_DRQ) == 0) {
-                        ap->pio_task_state = PIO_ST_IDLE;
-
-                        ata_poll_qc_complete(qc, status);
+                        ap->pio_task_state = PIO_ST_LAST;
                         return;
                 }
 
@@ -2887,7 +2892,12 @@ static void ata_pio_error(struct ata_port *ap)
 static void ata_pio_task(void *_data)
 {
         struct ata_port *ap = _data;
-        unsigned long timeout = 0;
+        unsigned long timeout;
+        int qc_completed;
+
+fsm_start:
+        timeout = 0;
+        qc_completed = 0;
 
         switch (ap->pio_task_state) {
         case PIO_ST_IDLE:
@@ -2898,7 +2908,7 @@ static void ata_pio_task(void *_data)
                 break;
 
         case PIO_ST_LAST:
-                ata_pio_complete(ap);
+                qc_completed = ata_pio_complete(ap);
                 break;
 
         case PIO_ST_POLL:
@@ -2913,10 +2923,9 @@ static void ata_pio_task(void *_data)
         }
 
         if (timeout)
-                queue_delayed_work(ata_wq, &ap->pio_task,
-                                   timeout);
-        else
-                queue_work(ata_wq, &ap->pio_task);
+                queue_delayed_work(ata_wq, &ap->pio_task, timeout);
+        else if (!qc_completed)
+                goto fsm_start;
 }
 
 static void atapi_request_sense(struct ata_port *ap, struct ata_device *dev,