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fs: fs_struct use seqlock

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Use a seqlock in the fs_struct to enable us to take an atomic copy of the
complete cwd and root paths. Use this in the RCU lookup path to avoid a
thread-shared spinlock in RCU lookup operations.

Multi-threaded apps may now perform path lookups with scalability matching
multi-process apps. Operations such as stat(2) become very scalable for
multi-threaded workload.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>
This commit is contained in:
Nick Piggin 2011-01-07 17:49:53 +11:00
parent 31e6b01f41
commit c28cc36469
3 changed files with 34 additions and 13 deletions
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10
fs/fs_struct.c
View File

@ -14,9 +14,11 @@ void set_fs_root(struct fs_struct *fs, struct path *path)
struct path old_root;
spin_lock(&fs->lock);
write_seqcount_begin(&fs->seq);
old_root = fs->root;
fs->root = *path;
path_get(path);
write_seqcount_end(&fs->seq);
spin_unlock(&fs->lock);
if (old_root.dentry)
path_put(&old_root);
@ -31,9 +33,11 @@ void set_fs_pwd(struct fs_struct *fs, struct path *path)
struct path old_pwd;
spin_lock(&fs->lock);
write_seqcount_begin(&fs->seq);
old_pwd = fs->pwd;
fs->pwd = *path;
path_get(path);
write_seqcount_end(&fs->seq);
spin_unlock(&fs->lock);
if (old_pwd.dentry)
@ -52,6 +56,7 @@ void chroot_fs_refs(struct path *old_root, struct path *new_root)
fs = p->fs;
if (fs) {
spin_lock(&fs->lock);
write_seqcount_begin(&fs->seq);
if (fs->root.dentry == old_root->dentry
&& fs->root.mnt == old_root->mnt) {
path_get(new_root);
@ -64,6 +69,7 @@ void chroot_fs_refs(struct path *old_root, struct path *new_root)
fs->pwd = *new_root;
count++;
}
write_seqcount_end(&fs->seq);
spin_unlock(&fs->lock);
}
task_unlock(p);
@ -88,8 +94,10 @@ void exit_fs(struct task_struct *tsk)
int kill;
task_lock(tsk);
spin_lock(&fs->lock);
write_seqcount_begin(&fs->seq);
tsk->fs = NULL;
kill = !--fs->users;
write_seqcount_end(&fs->seq);
spin_unlock(&fs->lock);
task_unlock(tsk);
if (kill)
@ -105,6 +113,7 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old)
fs->users = 1;
fs->in_exec = 0;
spin_lock_init(&fs->lock);
seqcount_init(&fs->seq);
fs->umask = old->umask;
get_fs_root_and_pwd(old, &fs->root, &fs->pwd);
}
@ -144,6 +153,7 @@ EXPORT_SYMBOL(current_umask);
struct fs_struct init_fs = {
.users = 1,
.lock = __SPIN_LOCK_UNLOCKED(init_fs.lock),
.seq = SEQCNT_ZERO,
.umask = 0022,
};

34
fs/namei.c
View File

@ -684,9 +684,12 @@ static __always_inline void set_root_rcu(struct nameidata *nd)
{
if (!nd->root.mnt) {
struct fs_struct *fs = current->fs;
spin_lock(&fs->lock);
nd->root = fs->root;
spin_unlock(&fs->lock);
unsigned seq;
do {
seq = read_seqcount_begin(&fs->seq);
nd->root = fs->root;
} while (read_seqcount_retry(&fs->seq, seq));
}
}
@ -1369,26 +1372,31 @@ static int path_init_rcu(int dfd, const char *name, unsigned int flags, struct n
if (*name=='/') {
struct fs_struct *fs = current->fs;
unsigned seq;
br_read_lock(vfsmount_lock);
rcu_read_lock();
spin_lock(&fs->lock);
nd->root = fs->root;
nd->path = nd->root;
nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
spin_unlock(&fs->lock);
do {
seq = read_seqcount_begin(&fs->seq);
nd->root = fs->root;
nd->path = nd->root;
nd->seq = __read_seqcount_begin(&nd->path.dentry->d_seq);
} while (read_seqcount_retry(&fs->seq, seq));
} else if (dfd == AT_FDCWD) {
struct fs_struct *fs = current->fs;
unsigned seq;
br_read_lock(vfsmount_lock);
rcu_read_lock();
spin_lock(&fs->lock);
nd->path = fs->pwd;
nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
spin_unlock(&fs->lock);
do {
seq = read_seqcount_begin(&fs->seq);
nd->path = fs->pwd;
nd->seq = __read_seqcount_begin(&nd->path.dentry->d_seq);
} while (read_seqcount_retry(&fs->seq, seq));
} else {
struct dentry *dentry;
@ -1411,7 +1419,7 @@ static int path_init_rcu(int dfd, const char *name, unsigned int flags, struct n
if (fput_needed)
nd->file = file;
nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
nd->seq = __read_seqcount_begin(&nd->path.dentry->d_seq);
br_read_lock(vfsmount_lock);
rcu_read_lock();
}

3
include/linux/fs_struct.h
View File

@ -2,10 +2,13 @@
#define _LINUX_FS_STRUCT_H
#include <linux/path.h>
#include <linux/spinlock.h>
#include <linux/seqlock.h>
struct fs_struct {
int users;
spinlock_t lock;
seqcount_t seq;
int umask;
int in_exec;
struct path root, pwd;