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@xxxxxxxxx>
---
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(¤t->fs->lock);
+ spin_lock(¤t->fs->lock);
path.mnt = mntget(current->fs->root.mnt);
- read_unlock(¤t->fs->lock);
+ spin_unlock(¤t->fs->lock);
path.dentry = d;
@@ -91,9 +91,9 @@ int pohmelfs_path_length(struct pohmelfs
return -ENOENT;
}
- read_lock(¤t->fs->lock);
+ spin_lock(¤t->fs->lock);
root = dget(current->fs->root.dentry);
- read_unlock(¤t->fs->lock);
+ spin_unlock(¤t->fs->lock);
spin_lock(&dcache_lock);
--
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