Re: [RFC][PATCH 5/15] Introduce union stack

Serge E. Hallyn
2007-04-17T22:09:06Z

Quoting Bharata B Rao (bharata@linux.vnet.ibm.com):
> 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 | 11 +
> include/linux/dcache_union.h | 243 +++++++++++++++++++++++++++++++++++++++++++
> 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;
> +}

The locking here needs to be laid out. It looks like union_get() needs
to be called under union_lock(), while union_get2() (horrible name)
grabs that lock itself, and returns with the lock held?

Similarly union_put clearly needs to be called under union_lock(), so
that should be commented here.

> +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
> @@ -325,6 +331,11 @@ static inline struct dentry *dget(struct
> return dentry;
> }
>
> +/*
> + * Reference counting for union mounts
> + */
> +#include <linux/dcache_union.h>
> +
> extern struct dentry * dget_locked(struct dentry *);
>
> /**
> --- /dev/null
> +++ b/include/linux/dcache_union.h
> @@ -0,0 +1,243 @@
> +/*
> + * 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 !
> + *
> + * NOTE: Read the remark for union_trylock() below!
> + */
> +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 dentrys 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.
> + */
> +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);
> + }
> +}
> +
> +/*
> + * Two helpers for namespace.c
> + *
> + * FIXME: clean this up to get it right
> + */
> +static inline struct union_info *union_alloc2(struct dentry * dentry)
> +{
> + struct union_info *uinfo;
> +
> + spin_lock(&dentry->d_lock);
> + if (!dentry->d_union) {
> + dentry->d_union = union_alloc();
> + uinfo = union_get(dentry->d_union);
> + spin_unlock(&dentry->d_lock);
> + } else {
> + uinfo = union_get(dentry->d_union);
> + spin_unlock(&dentry->d_lock);
> + union_lock(dentry);
> + }
> +
> + return uinfo;
> +}
> +
> +static inline struct union_info *union_get2(struct dentry * dentry)
> +{
> + struct union_info *uinfo;
> +
> + union_lock(dentry);
> + uinfo = union_get(dentry->d_union);
> + return uinfo;
> +}
> +
> +static inline void union_release(struct union_info *uinfo)
> +{
> + if (!uinfo)
> + return;
> +
> + mutex_unlock(&uinfo->u_mutex);
> + union_put(uinfo);

Is it safe to do this - releasing the lock before doing the put?

> +}
> +
> +/*
> + * 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_alloc2(x) ({ BUG(); (0); })
> +#define union_get2(x) ({ BUG(); (0); })
> +#define union_release(x) do { BUG(); } while (0)
> +
> +#endif /* CONFIG_UNION_MOUNT */
> +#endif /* __KERNEL__ */
> +#endif /* __LINUX_DCACHE_UNION_H */
> -
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