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Subject[RFC][PATCH 5/14] Introduce union stack
From: Jan Blunck <j.blunck@tu-harburg.de>
Subject: Introduce union stack.

Adds union stack infrastructure to the dentry structure and provides
locking routines to walk the union stack.

Signed-off-by: Jan Blunck <j.blunck@tu-harburg.de>
Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
---
fs/Makefile | 2
fs/dcache.c | 5
fs/union.c | 53 +++++++++
include/linux/dcache.h | 6 +
include/linux/dcache_union.h | 248 +++++++++++++++++++++++++++++++++++++++++++
5 files changed, 314 insertions(+)
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -49,6 +49,8 @@ obj-$(CONFIG_FS_POSIX_ACL) += posix_acl.
obj-$(CONFIG_NFS_COMMON) += nfs_common/
obj-$(CONFIG_GENERIC_ACL) += generic_acl.o

+obj-$(CONFIG_UNION_MOUNT) += union.o
+
obj-$(CONFIG_QUOTA) += dquot.o
obj-$(CONFIG_QFMT_V1) += quota_v1.o
obj-$(CONFIG_QFMT_V2) += quota_v2.o
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -936,6 +936,11 @@ struct dentry *d_alloc(struct dentry * p
#ifdef CONFIG_PROFILING
dentry->d_cookie = NULL;
#endif
+#ifdef CONFIG_UNION_MOUNT
+ dentry->d_overlaid = NULL;
+ dentry->d_topmost = NULL;
+ dentry->d_union = NULL;
+#endif
INIT_HLIST_NODE(&dentry->d_hash);
INIT_LIST_HEAD(&dentry->d_lru);
INIT_LIST_HEAD(&dentry->d_subdirs);
--- /dev/null
+++ b/fs/union.c
@@ -0,0 +1,53 @@
+/*
+ * VFS based union mount for Linux
+ *
+ * Copyright © 2004-2007 IBM Corporation
+ * Author(s): Jan Blunck (j.blunck@tu-harburg.de)
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ */
+
+#include <linux/fs.h>
+
+struct union_info * union_alloc(void)
+{
+ struct union_info *info;
+
+ info = kmalloc(sizeof(*info), GFP_ATOMIC);
+ if (!info)
+ return NULL;
+
+ mutex_init(&info->u_mutex);
+ mutex_lock(&info->u_mutex);
+ atomic_set(&info->u_count, 1);
+ UM_DEBUG_LOCK("allocate union %p\n", info);
+ return info;
+}
+
+struct union_info * union_get(struct union_info *info)
+{
+ BUG_ON(!info);
+ BUG_ON(!atomic_read(&info->u_count));
+ atomic_inc(&info->u_count);
+ UM_DEBUG_LOCK("get union %p (count=%d)\n", info,
+ atomic_read(&info->u_count));
+ return info;
+}
+
+void union_put(struct union_info *info)
+{
+ BUG_ON(!info);
+ UM_DEBUG_LOCK("put union %p (count=%d)\n", info,
+ atomic_read(&info->u_count));
+ atomic_dec(&info->u_count);
+
+ if (!atomic_read(&info->u_count)) {
+ UM_DEBUG_LOCK("free union %p\n", info);
+ kfree(info);
+ }
+
+ return;
+}
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -93,6 +93,12 @@ struct dentry {
struct dentry *d_parent; /* parent directory */
struct qstr d_name;

+#ifdef CONFIG_UNION_MOUNT
+ struct dentry *d_overlaid; /* overlaid directory */
+ struct dentry *d_topmost; /* topmost directory */
+ struct union_info *d_union; /* union directory info */
+#endif
+
struct list_head d_lru; /* LRU list */
/*
* d_child and d_rcu can share memory
--- /dev/null
+++ b/include/linux/dcache_union.h
@@ -0,0 +1,248 @@
+/*
+ * VFS based union mount for Linux
+ *
+ * Copyright © 2004-2007 IBM Corporation
+ * Author(s): Jan Blunck (j.blunck@tu-harburg.de)
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ */
+#ifndef __LINUX_DCACHE_UNION_H
+#define __LINUX_DCACHE_UNION_H
+#ifdef __KERNEL__
+
+#include <linux/union_debug.h>
+#include <linux/fs_struct.h>
+#include <asm/atomic.h>
+#include <asm/semaphore.h>
+
+#ifdef CONFIG_UNION_MOUNT
+
+/*
+ * This is the union info object, that describes general information about this
+ * union directory
+ *
+ * u_mutex protects the union stack against modification. You can reach it
+ * through the d_union field in struct dentry. Hold it when you are walking
+ * or modifing the union stack !
+ */
+struct union_info {
+ atomic_t u_count;
+ struct mutex u_mutex;
+};
+
+/* allocate/de-allocate */
+extern struct union_info *union_alloc(void);
+extern struct union_info *union_get(struct union_info *);
+extern void union_put(struct union_info *);
+
+/*
+ * These are the functions for locking a dentry's union. When one
+ * want to acquire a denties union lock, use:
+ *
+ * - union_lock() when you can sleep,
+ * - union_lock_spinlock() when you are holding a spinlock (that
+ * you CAN savely give up and reacquire again)
+ * - union_lock_readlock() when you are holding a readlock (that
+ * you CAN savely give up and reacquire again)
+ *
+ * Otherwise get the union lock early before you enter your
+ * "no sleeping here" code.
+ *
+ * NOTES: union_info structure is reference counted using u_count member.
+ * union_get() and union_put() which get and put references on union_info
+ * should be done under union_info's u_mutex. Since the last union_put() frees
+ * the union_info structure itself it can't obviously be done under u_mutex.
+ * union_release() should be used in such cases (Eg. dput(), umount()) where
+ * union_info is disassociated from the dentries, and it becomes safe
+ * to free the union_info.
+ */
+static inline void __union_lock(struct union_info *uinfo)
+{
+ BUG_ON(!atomic_read(&uinfo->u_count));
+ mutex_lock(&uinfo->u_mutex);
+}
+
+static inline void union_lock(struct dentry *dentry)
+{
+ if (unlikely(dentry && dentry->d_union)) {
+ struct union_info *ui = dentry->d_union;
+
+ UM_DEBUG_LOCK("\"%s\" locking %p (count=%d)\n",
+ dentry->d_name.name, ui,
+ atomic_read(&ui->u_count));
+ __union_lock(dentry->d_union);
+ }
+}
+
+static inline void __union_unlock(struct union_info *uinfo)
+{
+ BUG_ON(!atomic_read(&uinfo->u_count));
+ mutex_unlock(&uinfo->u_mutex);
+}
+
+static inline void union_unlock(struct dentry *dentry)
+{
+ if (unlikely(dentry && dentry->d_union)) {
+ struct union_info *ui = dentry->d_union;
+
+ UM_DEBUG_LOCK("\"%s\" unlocking %p (count=%d)\n",
+ dentry->d_name.name, ui,
+ atomic_read(&ui->u_count));
+ __union_unlock(dentry->d_union);
+ }
+}
+
+static inline void union_alloc_dentry(struct dentry *dentry)
+{
+ spin_lock(&dentry->d_lock);
+ if (!dentry->d_union) {
+ dentry->d_union = union_alloc();
+ spin_unlock(&dentry->d_lock);
+ } else {
+ spin_unlock(&dentry->d_lock);
+ union_lock(dentry);
+ }
+}
+
+static inline struct union_info *union_lock_and_get(struct dentry *dentry)
+{
+ union_lock(dentry);
+ return union_get(dentry->d_union);
+}
+
+/* Shouldn't be called with last reference to union_info */
+static inline void union_put_and_unlock(struct union_info *uinfo)
+{
+ union_put(uinfo);
+ __union_unlock(&uinfo->u_mutex);
+}
+
+/*
+ * Called when we know for sure that there is no reference
+ * to this union_info from any dentry and it can be safely
+ * destroyed.
+ */
+static inline void union_release(struct union_info *uinfo)
+{
+ if (!uinfo)
+ return;
+
+ mutex_unlock(&uinfo->u_mutex);
+ union_put(uinfo);
+}
+
+/*
+ * Immediately return ZERO if the lock is contended, NON-ZERO if it's acquired.
+ */
+static inline int union_trylock(struct dentry *dentry)
+{
+ int locked = 1;
+
+ if (unlikely(dentry && dentry->d_union)) {
+ UM_DEBUG_LOCK("\"%s\" try locking %p (count=%d)\n",
+ dentry->d_name.name, dentry->d_union,
+ atomic_read(&dentry->d_union->u_count));
+ BUG_ON(!atomic_read(&dentry->d_union->u_count));
+ locked = mutex_trylock(&dentry->d_union->u_mutex);
+ UM_DEBUG_LOCK("\"%s\" trylock %p %s\n", dentry->d_name.name,
+ dentry->d_union,
+ locked ? "succeeded" : "failed");
+ }
+ return (locked ? 1 : 0);
+}
+
+/*
+ * The following functions are locking helpers to guarantee the locking order
+ * in some situations.
+ */
+
+static inline void union_lock_spinlock(struct dentry *dentry, spinlock_t *lock)
+{
+ while (!union_trylock(dentry)) {
+ spin_unlock(lock);
+ cpu_relax();
+ spin_lock(lock);
+ }
+}
+
+static inline void union_lock_readlock(struct dentry *dentry, rwlock_t *lock)
+{
+ while (!union_trylock(dentry)) {
+ read_unlock(lock);
+ cpu_relax();
+ read_lock(lock);
+ }
+}
+
+/*
+ * This is a *I can't get no sleep* helper which is called when we try
+ * to access the struct fs_struct *fs field of a struct task_struct.
+ *
+ * Yes, this is possibly starving but we have to change root, altroot
+ * or pwd in the frequency of this while loop. Don't think that this
+ * happens really often ;)
+ *
+ * This is called while holding the rwlock_t fs->lock
+ *
+ * TODO: Unlocking side of union_lock_fs() needs 3 union_unlock()s.
+ * May be introduce union_unlock_fs().
+ *
+ * FIXME: This routine is used when the caller wants to dget one or
+ * more of fs->[root, altroot, pwd]. When the caller doesn't want to
+ * dget _all_ of these, it is strictly not necessary to get union_locks
+ * on all of these. Check.
+ */
+static inline void union_lock_fs(struct fs_struct *fs)
+{
+ int locked;
+
+ while (fs) {
+ locked = union_trylock(fs->root);
+ if (!locked)
+ goto loop1;
+ locked = union_trylock(fs->altroot);
+ if (!locked)
+ goto loop2;
+ locked = union_trylock(fs->pwd);
+ if (!locked)
+ goto loop3;
+ break;
+ loop3:
+ union_unlock(fs->altroot);
+ loop2:
+ union_unlock(fs->root);
+ loop1:
+ read_unlock(&fs->lock);
+ UM_DEBUG_LOCK("Failed to get all semaphores in fs_struct!\n");
+ cpu_relax();
+ read_lock(&fs->lock);
+ continue;
+ }
+ BUG_ON(!fs);
+ return;
+}
+
+#define IS_UNION(dentry) ((dentry)->d_overlaid || (dentry)->d_topmost || \
+ (dentry)->d_overlaid)
+
+#else /* CONFIG_UNION_MOUNT */
+
+#define union_lock(dentry) do { /* empty */ } while (0)
+#define union_trylock(dentry) ({ (1); })
+#define union_unlock(dentry) do { /* empty */ } while (0)
+#define union_lock_spinlock(dentry, lock) do { /* empty */ } while (0)
+#define union_lock_readlock(dentry, lock) do { /* empty */ } while (0)
+#define union_lock_fs(fs) do { /* empty */ } while (0)
+#define IS_UNION(dentry) ({ (0); })
+#define union_alloc_dentry(x) ({ BUG(); (0); })
+#define union_lock_and_get(dentry) ({ (NULL); })
+#define union_unlock_and_put(dentry) do { /* empty */ } while (0)
+#define union_release(x) do { BUG(); } while (0)
+
+#endif /* CONFIG_UNION_MOUNT */
+#endif /* __KERNEL__ */
+#endif /* __LINUX_DCACHE_UNION_H */
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