Optimise dir-lookup's second-lock

This should avoid some lock contention.

* libdiskfs/dir-lookup.c (diskfs_S_dir_lookup): Before releasing &np->lock
to get &dnp->lock safely, try to lock &dnp->lock directly.
* libtreefs/dir-lookup.c (_treefs_s_dir_lookup): Likewise.

2 files changed, 12 insertions, 6 deletions

diff --git a/libdiskfs/dir-lookup.c b/libdiskfs/dir-lookup.c
index 1efdf75c..923be033 100644
--- a/libdiskfs/dir-lookup.c
+++ b/libdiskfs/dir-lookup.c
@@ -310,9 +310,12 @@ diskfs_S_dir_lookup (struct protid *dircred,
 		pthread_mutex_lock (&dnp->lock);
 	      else
 		{
-		  pthread_mutex_unlock (&np->lock);
-		  pthread_mutex_lock (&dnp->lock);
-		  pthread_mutex_lock (&np->lock);
+		  if (pthread_mutex_trylock (&dnp->lock))
+		    {
+		      pthread_mutex_unlock (&np->lock);
+		      pthread_mutex_lock (&dnp->lock);
+		      pthread_mutex_lock (&np->lock);
+		    }
 		}
 	    }
 	}
diff --git a/libtreefs/dir-lookup.c b/libtreefs/dir-lookup.c
index ce2acafd..8665059c 100644
--- a/libtreefs/dir-lookup.c
+++ b/libtreefs/dir-lookup.c
@@ -199,9 +199,12 @@ _treefs_s_dir_lookup (struct treefs_handle *h,
 	     in the right order. */
 	  if (strcmp (path, "..") != 0)
 	    {
-	      pthread_mutex_unlock (&node->lock);
-	      pthread_mutex_lock (&dir->lock);
-	      pthread_mutex_lock (&node->lock);
+	      if (pthread_mutex_trylock (&dir->lock))
+	        {
+		  pthread_mutex_unlock (&node->lock);
+		  pthread_mutex_lock (&dir->lock);
+		  pthread_mutex_lock (&node->lock);
+		}
 	    }
 	  else
 	    pthread_mutex_lock (&dir->lock);