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, 7 insertions, 7 deletions

diff --git a/include/linux/fs_struct.h b/include/linux/fs_struct.h
index eca3d520213..a42b5bf02f8 100644
--- a/include/linux/fs_struct.h
+++ b/include/linux/fs_struct.h
@@ -5,7 +5,7 @@
 
 struct fs_struct {
         int users;
-        rwlock_t lock;
+        spinlock_t lock;
         int umask;
         int in_exec;
         struct path root, pwd;
@@ -23,29 +23,29 @@ extern int unshare_fs_struct(void);
 
 static inline void get_fs_root(struct fs_struct *fs, struct path *root)
 {
-        read_lock(&fs->lock);
+        spin_lock(&fs->lock);
         *root = fs->root;
         path_get(root);
-        read_unlock(&fs->lock);
+        spin_unlock(&fs->lock);
 }
 
 static inline void get_fs_pwd(struct fs_struct *fs, struct path *pwd)
 {
-        read_lock(&fs->lock);
+        spin_lock(&fs->lock);
         *pwd = fs->pwd;
         path_get(pwd);
-        read_unlock(&fs->lock);
+        spin_unlock(&fs->lock);
 }
 
 static inline void get_fs_root_and_pwd(struct fs_struct *fs, struct path *root,
                                        struct path *pwd)
 {
-        read_lock(&fs->lock);
+        spin_lock(&fs->lock);
         *root = fs->root;
         path_get(root);
         *pwd = fs->pwd;
         path_get(pwd);
-        read_unlock(&fs->lock);
+        spin_unlock(&fs->lock);
 }
 
 #endif /* _LINUX_FS_STRUCT_H */