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Date: Sat, 27 Nov 2010 20:45:04 +1100
From: Nick Piggin <npiggin@...nel.dk>
To: linux-fsdevel@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
Subject: [PATCH 34/46] fs: fs_struct use seqlock
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.
Signed-off-by: Nick Piggin <npiggin@...nel.dk>
---
fs/fs_struct.c | 10 ++++++++++
fs/namei.c | 36 ++++++++++++++++++++++--------------
include/linux/fs_struct.h | 3 +++
3 files changed, 35 insertions(+), 14 deletions(-)
diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index ed45a9c..60b8531 100644
--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -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,
};
diff --git a/fs/namei.c b/fs/namei.c
index 5185752..c48d208 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -640,9 +640,12 @@ 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));
}
}
@@ -1324,26 +1327,31 @@ 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;
@@ -1366,7 +1374,7 @@ 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();
}
@@ -2199,7 +2207,7 @@ creat:
if (error)
goto out_filp;
error = path_walk_rcu(pathname, &nd);
- path_finish_rcu(&nd);
+ path_finish_rcu(&nd); /* XXX: ok to throw out root here?? */
if (unlikely(error == -ECHILD || error == -ESTALE)) {
/* slower, locked walk */
if (error == -ESTALE) {
diff --git a/include/linux/fs_struct.h b/include/linux/fs_struct.h
index a42b5bf..003dc0f 100644
--- a/include/linux/fs_struct.h
+++ b/include/linux/fs_struct.h
@@ -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;
--
1.7.1
--
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