fs: fs_struct rwlock to spinlock

fs: fs_struct rwlock to spinlock

struct fs_struct.lock is an rwlock with the read-side used to protect root and
pwd members while taking references to them. Taking a reference to a path
typically requires just 2 atomic ops, so the critical section is very small.
Parallel read-side operations would have cacheline contention on the lock, the
dentry, and the vfsmount cachelines, so the rwlock is unlikely to ever give a
real parallelism increase.

Replace it with a spinlock to avoid one or two atomic operations in typical
path lookup fastpath.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 4 insertions, 4 deletions

diff --git a/drivers/staging/pohmelfs/path_entry.c b/drivers/staging/pohmelfs/path_entry.c
index cdc4dd50d638..8ec83d2dffb7 100644
--- a/drivers/staging/pohmelfs/path_entry.c
+++ b/drivers/staging/pohmelfs/path_entry.c
@@ -44,9 +44,9 @@ int pohmelfs_construct_path_string(struct pohmelfs_inode *pi, void *data, int le
 		return -ENOENT;
 	}
 
-	read_lock(&current->fs->lock);
+	spin_lock(&current->fs->lock);
 	path.mnt = mntget(current->fs->root.mnt);
-	read_unlock(&current->fs->lock);
+	spin_unlock(&current->fs->lock);
 
 	path.dentry = d;
 
@@ -91,9 +91,9 @@ int pohmelfs_path_length(struct pohmelfs_inode *pi)
 		return -ENOENT;
 	}
 
-	read_lock(&current->fs->lock);
+	spin_lock(&current->fs->lock);
 	root = dget(current->fs->root.dentry);
-	read_unlock(&current->fs->lock);
+	spin_unlock(&current->fs->lock);
 
 	spin_lock(&dcache_lock);