[PATCH 1/13] vfs: Lighten r/o rename_lock lockers

[Pavel Emelyanov <xemul@parallels.com>]

There are some places, that need to _lock_ the rename_lock, but they
do it for reading the dentry tree. When doing the write_seqlock they
force the existing "willing to restart" readers do the restart, which
is not actually required.

Introduce two more seqlock helpers, that lock the lock, but do not
update the seq count.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>

---
 fs/dcache.c             |   40 ++++++++++++++++++++--------------------
 include/linux/seqlock.h |    9 +++++++++
 2 files changed, 29 insertions(+), 20 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 611ffe9..c31c149 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1091,18 +1091,18 @@ resume:
 	if (!locked && read_seqretry(&rename_lock, seq))
 		goto rename_retry;
 	if (locked)
-		write_sequnlock(&rename_lock);
+		read_sequnlock(&rename_lock);
 	return 0; /* No mount points found in tree */
 positive:
 	if (!locked && read_seqretry(&rename_lock, seq))
 		goto rename_retry;
 	if (locked)
-		write_sequnlock(&rename_lock);
+		read_sequnlock(&rename_lock);
 	return 1;
 
 rename_retry:
 	locked = 1;
-	write_seqlock(&rename_lock);
+	read_seqlock(&rename_lock);
 	goto again;
 }
 EXPORT_SYMBOL(have_submounts);
@@ -1207,14 +1207,14 @@ out:
 	if (!locked && read_seqretry(&rename_lock, seq))
 		goto rename_retry;
 	if (locked)
-		write_sequnlock(&rename_lock);
+		read_sequnlock(&rename_lock);
 	return found;
 
 rename_retry:
 	if (found)
 		return found;
 	locked = 1;
-	write_seqlock(&rename_lock);
+	read_seqlock(&rename_lock);
 	goto again;
 }
 
@@ -2572,9 +2572,9 @@ char *__d_path(const struct path *path, struct path *root,
 	int error;
 
 	prepend(&res, &buflen, "\0", 1);
-	write_seqlock(&rename_lock);
+	read_seqlock(&rename_lock);
 	error = prepend_path(path, root, &res, &buflen);
-	write_sequnlock(&rename_lock);
+	read_sequnlock(&rename_lock);
 
 	if (error)
 		return ERR_PTR(error);
@@ -2636,12 +2636,12 @@ char *d_path(const struct path *path, char *buf, int buflen)
 		return path->dentry->d_op->d_dname(path->dentry, buf, buflen);
 
 	get_fs_root(current->fs, &root);
-	write_seqlock(&rename_lock);
+	read_seqlock(&rename_lock);
 	tmp = root;
 	error = path_with_deleted(path, &tmp, &res, &buflen);
 	if (error)
 		res = ERR_PTR(error);
-	write_sequnlock(&rename_lock);
+	read_sequnlock(&rename_lock);
 	path_put(&root);
 	return res;
 }
@@ -2667,12 +2667,12 @@ char *d_path_with_unreachable(const struct path *path, char *buf, int buflen)
 		return path->dentry->d_op->d_dname(path->dentry, buf, buflen);
 
 	get_fs_root(current->fs, &root);
-	write_seqlock(&rename_lock);
+	read_seqlock(&rename_lock);
 	tmp = root;
 	error = path_with_deleted(path, &tmp, &res, &buflen);
 	if (!error && !path_equal(&tmp, &root))
 		error = prepend_unreachable(&res, &buflen);
-	write_sequnlock(&rename_lock);
+	read_sequnlock(&rename_lock);
 	path_put(&root);
 	if (error)
 		res =  ERR_PTR(error);
@@ -2739,9 +2739,9 @@ char *dentry_path_raw(struct dentry *dentry, char *buf, int buflen)
 {
 	char *retval;
 
-	write_seqlock(&rename_lock);
+	read_seqlock(&rename_lock);
 	retval = __dentry_path(dentry, buf, buflen);
-	write_sequnlock(&rename_lock);
+	read_sequnlock(&rename_lock);
 
 	return retval;
 }
@@ -2752,7 +2752,7 @@ char *dentry_path(struct dentry *dentry, char *buf, int buflen)
 	char *p = NULL;
 	char *retval;
 
-	write_seqlock(&rename_lock);
+	read_seqlock(&rename_lock);
 	if (d_unlinked(dentry)) {
 		p = buf + buflen;
 		if (prepend(&p, &buflen, "//deleted", 10) != 0)
@@ -2760,7 +2760,7 @@ char *dentry_path(struct dentry *dentry, char *buf, int buflen)
 		buflen++;
 	}
 	retval = __dentry_path(dentry, buf, buflen);
-	write_sequnlock(&rename_lock);
+	read_sequnlock(&rename_lock);
 	if (!IS_ERR(retval) && p)
 		*p = '/';	/* restore '/' overriden with '\0' */
 	return retval;
@@ -2798,7 +2798,7 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
 	get_fs_root_and_pwd(current->fs, &root, &pwd);
 
 	error = -ENOENT;
-	write_seqlock(&rename_lock);
+	read_seqlock(&rename_lock);
 	if (!d_unlinked(pwd.dentry)) {
 		unsigned long len;
 		struct path tmp = root;
@@ -2807,7 +2807,7 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
 
 		prepend(&cwd, &buflen, "\0", 1);
 		error = prepend_path(&pwd, &tmp, &cwd, &buflen);
-		write_sequnlock(&rename_lock);
+		read_sequnlock(&rename_lock);
 
 		if (error)
 			goto out;
@@ -2827,7 +2827,7 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
 				error = -EFAULT;
 		}
 	} else {
-		write_sequnlock(&rename_lock);
+		read_sequnlock(&rename_lock);
 	}
 
 out:
@@ -2971,12 +2971,12 @@ resume:
 	if (!locked && read_seqretry(&rename_lock, seq))
 		goto rename_retry;
 	if (locked)
-		write_sequnlock(&rename_lock);
+		read_sequnlock(&rename_lock);
 	return;
 
 rename_retry:
 	locked = 1;
-	write_seqlock(&rename_lock);
+	read_seqlock(&rename_lock);
 	goto again;
 }
 
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index e98cd2e..808e124 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -110,6 +110,15 @@ static __always_inline int read_seqretry(const seqlock_t *sl, unsigned start)
 	return unlikely(sl->sequence != start);
 }
 
+static __always_inline void read_seqlock(seqlock_t *sl)
+{
+	spin_lock(&sl->lock);
+}
+
+static __always_inline void read_sequnlock(seqlock_t *sl)
+{
+	spin_unlock(&sl->lock);
+}
 
 /*
  * Version using sequence counter only.
-- 
1.5.5.6