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, 16 insertions, 16 deletions

diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index 1ee40eb9a2c0..ed45a9cf5f3d 100644
--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -13,11 +13,11 @@ void set_fs_root(struct fs_struct *fs, struct path *path)
 {
         struct path old_root;
 
-        write_lock(&fs->lock);
+        spin_lock(&fs->lock);
         old_root = fs->root;
         fs->root = *path;
         path_get(path);
-        write_unlock(&fs->lock);
+        spin_unlock(&fs->lock);
         if (old_root.dentry)
                 path_put(&old_root);
 }
@@ -30,11 +30,11 @@ void set_fs_pwd(struct fs_struct *fs, struct path *path)
 {
         struct path old_pwd;
 
-        write_lock(&fs->lock);
+        spin_lock(&fs->lock);
         old_pwd = fs->pwd;
         fs->pwd = *path;
         path_get(path);
-        write_unlock(&fs->lock);
+        spin_unlock(&fs->lock);
 
         if (old_pwd.dentry)
                 path_put(&old_pwd);
@@ -51,7 +51,7 @@ void chroot_fs_refs(struct path *old_root, struct path *new_root)
                 task_lock(p);
                 fs = p->fs;
                 if (fs) {
-                        write_lock(&fs->lock);
+                        spin_lock(&fs->lock);
                         if (fs->root.dentry == old_root->dentry
                             && fs->root.mnt == old_root->mnt) {
                                 path_get(new_root);
@@ -64,7 +64,7 @@ void chroot_fs_refs(struct path *old_root, struct path *new_root)
                                 fs->pwd = *new_root;
                                 count++;
                         }
-                        write_unlock(&fs->lock);
+                        spin_unlock(&fs->lock);
                 }
                 task_unlock(p);
         } while_each_thread(g, p);
@@ -87,10 +87,10 @@ void exit_fs(struct task_struct *tsk)
         if (fs) {
                 int kill;
                 task_lock(tsk);
-                write_lock(&fs->lock);
+                spin_lock(&fs->lock);
                 tsk->fs = NULL;
                 kill = !--fs->users;
-                write_unlock(&fs->lock);
+                spin_unlock(&fs->lock);
                 task_unlock(tsk);
                 if (kill)
                         free_fs_struct(fs);
@@ -104,7 +104,7 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old)
         if (fs) {
                 fs->users = 1;
                 fs->in_exec = 0;
-                rwlock_init(&fs->lock);
+                spin_lock_init(&fs->lock);
                 fs->umask = old->umask;
                 get_fs_root_and_pwd(old, &fs->root, &fs->pwd);
         }
@@ -121,10 +121,10 @@ int unshare_fs_struct(void)
                 return -ENOMEM;
 
         task_lock(current);
-        write_lock(&fs->lock);
+        spin_lock(&fs->lock);
         kill = !--fs->users;
         current->fs = new_fs;
-        write_unlock(&fs->lock);
+        spin_unlock(&fs->lock);
         task_unlock(current);
 
         if (kill)
@@ -143,7 +143,7 @@ EXPORT_SYMBOL(current_umask);
 /* to be mentioned only in INIT_TASK */
 struct fs_struct init_fs = {
         .users          = 1,
-        .lock           = __RW_LOCK_UNLOCKED(init_fs.lock),
+        .lock           = __SPIN_LOCK_UNLOCKED(init_fs.lock),
         .umask          = 0022,
 };
 
@@ -156,14 +156,14 @@ void daemonize_fs_struct(void)
 
                 task_lock(current);
 
-                write_lock(&init_fs.lock);
+                spin_lock(&init_fs.lock);
                 init_fs.users++;
-                write_unlock(&init_fs.lock);
+                spin_unlock(&init_fs.lock);
 
-                write_lock(&fs->lock);
+                spin_lock(&fs->lock);
                 current->fs = &init_fs;
                 kill = !--fs->users;
-                write_unlock(&fs->lock);
+                spin_unlock(&fs->lock);
 
                 task_unlock(current);
                 if (kill)