[patch 04/10] fs: fs_struct rwlock to spinlock

[Nick Piggin <npiggin@kernel.dk>]

[-- Attachment #1: fs-rwlock-to-spinlock.patch --]
[-- Type: text/plain, Size: 7565 bytes --]

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>

---
 drivers/staging/pohmelfs/path_entry.c |    8 ++++----
 fs/exec.c                             |    4 ++--
 fs/fs_struct.c                        |   32 ++++++++++++++++----------------
 include/linux/fs_struct.h             |   14 +++++++-------
 kernel/fork.c                         |   10 +++++-----
 5 files changed, 34 insertions(+), 34 deletions(-)

Index: linux-2.6/fs/fs_struct.c
===================================================================
--- linux-2.6.orig/fs/fs_struct.c	2010-08-18 04:04:01.000000000 +1000
+++ linux-2.6/fs/fs_struct.c	2010-08-18 04:04:29.000000000 +1000
@@ -13,11 +13,11 @@ void set_fs_root(struct fs_struct *fs, s
 {
 	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, st
 {
 	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_roo
 		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_roo
 				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
 	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)
Index: linux-2.6/include/linux/fs_struct.h
===================================================================
--- linux-2.6.orig/include/linux/fs_struct.h	2010-08-18 04:04:01.000000000 +1000
+++ linux-2.6/include/linux/fs_struct.h	2010-08-18 04:04:29.000000000 +1000
@@ -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 */
Index: linux-2.6/fs/exec.c
===================================================================
--- linux-2.6.orig/fs/exec.c	2010-08-18 04:04:01.000000000 +1000
+++ linux-2.6/fs/exec.c	2010-08-18 04:04:29.000000000 +1000
@@ -1117,7 +1117,7 @@ int check_unsafe_exec(struct linux_binpr
 	bprm->unsafe = tracehook_unsafe_exec(p);
 
 	n_fs = 1;
-	write_lock(&p->fs->lock);
+	spin_lock(&p->fs->lock);
 	rcu_read_lock();
 	for (t = next_thread(p); t != p; t = next_thread(t)) {
 		if (t->fs == p->fs)
@@ -1134,7 +1134,7 @@ int check_unsafe_exec(struct linux_binpr
 			res = 1;
 		}
 	}
-	write_unlock(&p->fs->lock);
+	spin_unlock(&p->fs->lock);
 
 	return res;
 }
Index: linux-2.6/kernel/fork.c
===================================================================
--- linux-2.6.orig/kernel/fork.c	2010-08-18 04:04:01.000000000 +1000
+++ linux-2.6/kernel/fork.c	2010-08-18 04:04:29.000000000 +1000
@@ -752,13 +752,13 @@ static int copy_fs(unsigned long clone_f
 	struct fs_struct *fs = current->fs;
 	if (clone_flags & CLONE_FS) {
 		/* tsk->fs is already what we want */
-		write_lock(&fs->lock);
+		spin_lock(&fs->lock);
 		if (fs->in_exec) {
-			write_unlock(&fs->lock);
+			spin_unlock(&fs->lock);
 			return -EAGAIN;
 		}
 		fs->users++;
-		write_unlock(&fs->lock);
+		spin_unlock(&fs->lock);
 		return 0;
 	}
 	tsk->fs = copy_fs_struct(fs);
@@ -1676,13 +1676,13 @@ SYSCALL_DEFINE1(unshare, unsigned long,
 
 		if (new_fs) {
 			fs = current->fs;
-			write_lock(&fs->lock);
+			spin_lock(&fs->lock);
 			current->fs = new_fs;
 			if (--fs->users)
 				new_fs = NULL;
 			else
 				new_fs = fs;
-			write_unlock(&fs->lock);
+			spin_unlock(&fs->lock);
 		}
 
 		if (new_mm) {
Index: linux-2.6/drivers/staging/pohmelfs/path_entry.c
===================================================================
--- linux-2.6.orig/drivers/staging/pohmelfs/path_entry.c	2010-08-18 04:04:01.000000000 +1000
+++ linux-2.6/drivers/staging/pohmelfs/path_entry.c	2010-08-18 04:04:29.000000000 +1000
@@ -44,9 +44,9 @@ int pohmelfs_construct_path_string(struc
 		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
 		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);