[SCSI] avoid useless free_list lock roundtrips

Avoid hitting the host-wide free_list lock unless we need to put a command
back onto the freelist.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

1 files changed, 6 insertions, 5 deletions

diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
index d8afec8317cf..fb86479ffd4a 100644
--- a/drivers/scsi/scsi.c
+++ b/drivers/scsi/scsi.c
@@ -320,13 +320,14 @@ void __scsi_put_command(struct Scsi_Host *shost, struct scsi_cmnd *cmd,
 {
         unsigned long flags;
 
-        /* changing locks here, don't need to restore the irq state */
-        spin_lock_irqsave(&shost->free_list_lock, flags);
         if (unlikely(list_empty(&shost->free_list))) {
-                list_add(&cmd->list, &shost->free_list);
-                cmd = NULL;
+                spin_lock_irqsave(&shost->free_list_lock, flags);
+                if (list_empty(&shost->free_list)) {
+                        list_add(&cmd->list, &shost->free_list);
+                        cmd = NULL;
+                }
+                spin_unlock_irqrestore(&shost->free_list_lock, flags);
         }
-        spin_unlock_irqrestore(&shost->free_list_lock, flags);
 
         if (likely(cmd != NULL))
                 scsi_pool_free_command(shost->cmd_pool, cmd);