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fs: fs_struct rwlock to spinlock
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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 <[email protected]>
Signed-off-by: Al Viro <[email protected]>
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Nick Piggin authored and Al Viro committed Aug 18, 2010
1 parent 44672e4 commit 2a4419b
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Showing 5 changed files with 34 additions and 34 deletions.
8 changes: 4 additions & 4 deletions drivers/staging/pohmelfs/path_entry.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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;

Expand Down Expand Up @@ -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);

Expand Down
4 changes: 2 additions & 2 deletions fs/exec.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1117,7 +1117,7 @@ int check_unsafe_exec(struct linux_binprm *bprm)
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)
Expand All @@ -1134,7 +1134,7 @@ int check_unsafe_exec(struct linux_binprm *bprm)
res = 1;
}
}
write_unlock(&p->fs->lock);
spin_unlock(&p->fs->lock);

return res;
}
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32 changes: 16 additions & 16 deletions fs/fs_struct.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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);
}
Expand All @@ -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);
Expand All @@ -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);
Expand All @@ -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);
Expand All @@ -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);
Expand All @@ -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);
}
Expand All @@ -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)
Expand All @@ -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,
};

Expand All @@ -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)
Expand Down
14 changes: 7 additions & 7 deletions include/linux/fs_struct.h
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Original file line number Diff line number Diff line change
Expand Up @@ -5,7 +5,7 @@

struct fs_struct {
int users;
rwlock_t lock;
spinlock_t lock;
int umask;
int in_exec;
struct path root, pwd;
Expand All @@ -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 */
10 changes: 5 additions & 5 deletions kernel/fork.c
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Original file line number Diff line number Diff line change
Expand Up @@ -752,13 +752,13 @@ static int copy_fs(unsigned long clone_flags, struct task_struct *tsk)
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);
Expand Down Expand Up @@ -1676,13 +1676,13 @@ SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)

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) {
Expand Down

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