[PATCH] exportfs - Remove unnecessary locking from find_exported_dentry()

From: "Jose R. Santos" <jrsantos@austin.ibm.com>

After discussing it with Neil, he fell that the original justification for
taking the kernel_lock on find_exported_dentry() is not longer valid and
should be safe to remove.

This patch fixes an issue while running SpecSFS where under memory
pressure, shrinking dcache cause find_exported_dentry() to allocate
disconnected dentries that later needed to be properly connected.  The
connecting part of the code was done with BKL taken which cause a sharp
drop in performance during iterations and profiles showing 75% time spent
on find_exported_dentry().  After applying the patch, time spent on the
function is reduce to <1%.

I have tested this on an 8-way machine with 56 filesystems for several days
now with no problems using ext2, ext3, xfs and jfs.

fs/exportfs/expfs.c

@@ -135,7 +135,6 @@ find_exported_dentry(struct super_block *sb, void *obj, void *parent,
 	 * the noprogress counter.  If we go through the loop 10 times (2 is
 	 * probably enough) without getting anywhere, we just give up
 	 */
-	lock_kernel();
 	noprogress= 0;
 	while (target_dir->d_flags & DCACHE_DISCONNECTED && noprogress++ < 10) {
 		struct dentry *pd = target_dir;
@@ -232,7 +231,6 @@ find_exported_dentry(struct super_block *sb, void *obj, void *parent,
 		/* something went wrong - oh-well */
 		if (!err)
 			err = -ESTALE;
-		unlock_kernel();
 		goto err_target;
 	}
 	/* if we weren't after a directory, have one more step to go */
@@ -254,7 +252,6 @@ find_exported_dentry(struct super_block *sb, void *obj, void *parent,
 		}
 	}
 	dput(target_dir);
-	unlock_kernel();
 	/* now result is properly connected, it is our best bet */
 	if (acceptable(context, result))
 		return result;