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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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