Re: [PATCH v23 11/22] vfs: Cache base_acl objects in inodes

Jeff Layton
2016-07-05T15:56:10Z

On Thu, 2016-06-30 at 15:47 +0200, Andreas Gruenbacher wrote:
> POSIX ACLs and richacls are both objects allocated by kmalloc() with a
> reference count which are freed by kfree_rcu().  An inode can either
> cache an access and a default POSIX ACL, or a richacl (richacls do not
> have default acls).  To allow an inode to cache either of the two kinds
> of acls, introduce a new base_acl type and convert i_acl and
> i_default_acl to that type. In most cases, the vfs then doesn't care which
> kind of acl an inode caches (if any).
>
> Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
> Cc: Andreas Dilger <adilger@dilger.ca>
> ---
>  drivers/staging/lustre/lustre/llite/llite_lib.c |  2 +-
>  fs/9p/acl.c                                     |  8 +--
>  fs/f2fs/acl.c                                   |  4 +-
>  fs/inode.c                                      | 32 +++++++++++-
>  fs/jffs2/acl.c                                  |  6 ++-
>  fs/namei.c                                      | 33 ++++++------
>  fs/nfs/nfs3acl.c                                | 14 ++---
>  fs/posix_acl.c                                  | 69 +++++++------------------
>  fs/richacl.c                                    |  4 +-
>  include/linux/fs.h                              | 41 +++++++++++++--
>  include/linux/posix_acl.h                       | 21 ++++----
>  include/linux/richacl.h                         |  9 ++--
>  12 files changed, 139 insertions(+), 104 deletions(-)
>
> diff --git a/drivers/staging/lustre/lustre/llite/llite_lib.c b/drivers/staging/lustre/lustre/llite/llite_lib.c
> index 96c7e9f..7819a7c 100644
> --- a/drivers/staging/lustre/lustre/llite/llite_lib.c
> +++ b/drivers/staging/lustre/lustre/llite/llite_lib.c
> @@ -1087,7 +1087,7 @@ void ll_clear_inode(struct inode *inode)
>   }
>  #ifdef CONFIG_FS_POSIX_ACL
>   else if (lli->lli_posix_acl) {
> - LASSERT(atomic_read(&lli->lli_posix_acl->a_refcount) == 1);
> + LASSERT(base_acl_refcount(&lli->lli_posix_acl->a_base) == 1);
>   LASSERT(!lli->lli_remote_perms);
>   posix_acl_release(lli->lli_posix_acl);
>   lli->lli_posix_acl = NULL;
> diff --git a/fs/9p/acl.c b/fs/9p/acl.c
> index 0576eae..1ce572f 100644
> --- a/fs/9p/acl.c
> +++ b/fs/9p/acl.c
> @@ -87,14 +87,14 @@ int v9fs_get_acl(struct inode *inode, struct p9_fid *fid)
>
>  static struct posix_acl *v9fs_get_cached_acl(struct inode *inode, int type)
>  {
> - struct posix_acl *acl;
> + struct base_acl *base_acl;
>   /*
>    * 9p Always cache the acl value when
>    * instantiating the inode (v9fs_inode_from_fid)
>    */
> - acl = get_cached_acl(inode, type);
> - BUG_ON(is_uncached_acl(acl));
> - return acl;
> + base_acl = get_cached_acl(inode, type);
> + BUG_ON(is_uncached_acl(base_acl));
> + return posix_acl(base_acl);
>  }
>
>  struct posix_acl *v9fs_iop_get_acl(struct inode *inode, int type)
> diff --git a/fs/f2fs/acl.c b/fs/f2fs/acl.c
> index a31c7e8..6079017 100644
> --- a/fs/f2fs/acl.c
> +++ b/fs/f2fs/acl.c
> @@ -267,7 +267,7 @@ static struct posix_acl *f2fs_acl_clone(const struct posix_acl *acl,
>   sizeof(struct posix_acl_entry);
>   clone = kmemdup(acl, size, flags);
>   if (clone)
> - atomic_set(&clone->a_refcount, 1);
> + base_acl_init(&clone->a_base);
>   }
>   return clone;
>  }
> @@ -279,7 +279,7 @@ static int f2fs_acl_create_masq(struct posix_acl *acl, umode_t *mode_p)
>   umode_t mode = *mode_p;
>   int not_equiv = 0;
>
> - /* assert(atomic_read(acl->a_refcount) == 1); */
> + /* assert(base_acl_refcount(&acl->a_base) == 1); */
>
>   FOREACH_ACL_ENTRY(pa, acl, pe) {
>   switch(pa->e_tag) {
> diff --git a/fs/inode.c b/fs/inode.c
> index 4ccbc21..40c03a7 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -240,14 +240,42 @@ void __destroy_inode(struct inode *inode)
>
>  #ifdef CONFIG_FS_POSIX_ACL
>   if (inode->i_acl && !is_uncached_acl(inode->i_acl))
> - posix_acl_release(inode->i_acl);
> + base_acl_put(inode->i_acl);
>   if (inode->i_default_acl && !is_uncached_acl(inode->i_default_acl))
> - posix_acl_release(inode->i_default_acl);
> + base_acl_put(inode->i_default_acl);
>  #endif
>   this_cpu_dec(nr_inodes);
>  }
>  EXPORT_SYMBOL(__destroy_inode);
>
> +#ifdef CONFIG_FS_POSIX_ACL
> +struct base_acl *__get_cached_acl(struct base_acl **p)
> +{
> + struct base_acl *base_acl;
> +
> + for (;;) {
> + rcu_read_lock();
> + base_acl = rcu_dereference(*p);
> + if (!base_acl || is_uncached_acl(base_acl) ||
> +     atomic_inc_not_zero(&base_acl->ba_refcount))
> + break;
> + rcu_read_unlock();
> + cpu_relax();
> + }
> + rcu_read_unlock();
> + return base_acl;
> +}
> +

I know this is basically copied from the existing get_cached_acl
function, but I'm a little uneasy with the above (and also with the
existing code that does the same thing).

The ba_refcount and ba_rcu are unioned. Once you've done the final
base_acl_put on the object, the you'll end up calling kfree_rcu which
is going to clobber the ba_refcount. How is it then safe to rely on it
still being zero for the atomic_inc_not_zero? ISTM that it would be
safer to make those separate fields and not union them.

...or am I missing something that prevents that?

> +void __forget_cached_acl(struct base_acl **p)
> +{
> + struct base_acl *old;
> +
> + old = xchg(p, ACL_NOT_CACHED);
> + if (!is_uncached_acl(old))
> + base_acl_put(old);
> +}
> +#endif
> +
>  static void i_callback(struct rcu_head *head)
>  {
>   struct inode *inode = container_of(head, struct inode, i_rcu);
> diff --git a/fs/jffs2/acl.c b/fs/jffs2/acl.c
> index bc2693d..6c11909 100644
> --- a/fs/jffs2/acl.c
> +++ b/fs/jffs2/acl.c
> @@ -292,13 +292,15 @@ int jffs2_init_acl_post(struct inode *inode)
>   int rc;
>
>   if (inode->i_default_acl) {
> - rc = __jffs2_set_acl(inode, JFFS2_XPREFIX_ACL_DEFAULT, inode->i_default_acl);
> + rc = __jffs2_set_acl(inode, JFFS2_XPREFIX_ACL_DEFAULT,
> +      posix_acl(inode->i_default_acl));
>   if (rc)
>   return rc;
>   }
>
>   if (inode->i_acl) {
> - rc = __jffs2_set_acl(inode, JFFS2_XPREFIX_ACL_ACCESS, inode->i_acl);
> + rc = __jffs2_set_acl(inode, JFFS2_XPREFIX_ACL_ACCESS,
> +      posix_acl(inode->i_acl));
>   if (rc)
>   return rc;
>   }
> diff --git a/fs/namei.c b/fs/namei.c
> index 663933e..7a822d0 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -259,25 +259,28 @@ void putname(struct filename *name)
>  static int check_acl(struct inode *inode, int mask)
>  {
>  #ifdef CONFIG_FS_POSIX_ACL
> - struct posix_acl *acl;
> -
>   if (mask & MAY_NOT_BLOCK) {
> - acl = get_cached_acl_rcu(inode, ACL_TYPE_ACCESS);
> -         if (!acl)
> + struct base_acl *base_acl;
> +
> + base_acl = rcu_dereference(inode->i_acl);
> +         if (!base_acl)
>                   return -EAGAIN;
>   /* no ->get_acl() calls in RCU mode... */
> - if (is_uncached_acl(acl))
> + if (is_uncached_acl(base_acl))
>   return -ECHILD;
> -         return posix_acl_permission(inode, acl, mask & ~MAY_NOT_BLOCK);
> - }
> -
> - acl = get_acl(inode, ACL_TYPE_ACCESS);
> - if (IS_ERR(acl))
> - return PTR_ERR(acl);
> - if (acl) {
> -         int error = posix_acl_permission(inode, acl, mask);
> -         posix_acl_release(acl);
> -         return error;
> +         return posix_acl_permission(inode, posix_acl(base_acl),
> +     mask & ~MAY_NOT_BLOCK);
> + } else {
> + struct posix_acl *acl;
> +
> + acl = get_acl(inode, ACL_TYPE_ACCESS);
> + if (IS_ERR(acl))
> + return PTR_ERR(acl);
> + if (acl) {
> + int error = posix_acl_permission(inode, acl, mask);
> + posix_acl_release(acl);
> + return error;
> + }
>   }
>  #endif
>
> diff --git a/fs/nfs/nfs3acl.c b/fs/nfs/nfs3acl.c
> index 720d92f5..2b70944 100644
> --- a/fs/nfs/nfs3acl.c
> +++ b/fs/nfs/nfs3acl.c
> @@ -16,28 +16,28 @@
>   * caching get_acl results in a race-free way.  See fs/posix_acl.c:get_acl()
>   * for explanations.
>   */
> -static void nfs3_prepare_get_acl(struct posix_acl **p)
> +static void nfs3_prepare_get_acl(struct base_acl **p)
>  {
> - struct posix_acl *sentinel = uncached_acl_sentinel(current);
> + struct base_acl *sentinel = uncached_acl_sentinel(current);
>
>   if (cmpxchg(p, ACL_NOT_CACHED, sentinel) != ACL_NOT_CACHED) {
>   /* Not the first reader or sentinel already in place. */
>   }
>  }
>
> -static void nfs3_complete_get_acl(struct posix_acl **p, struct posix_acl *acl)
> +static void nfs3_complete_get_acl(struct base_acl **p, struct posix_acl *acl)
>  {
> - struct posix_acl *sentinel = uncached_acl_sentinel(current);
> + struct base_acl *sentinel = uncached_acl_sentinel(current);
>
>   /* Only cache the ACL if our sentinel is still in place. */
>   posix_acl_dup(acl);
> - if (cmpxchg(p, sentinel, acl) != sentinel)
> + if (cmpxchg(p, sentinel, &acl->a_base) != sentinel)
>   posix_acl_release(acl);
>  }
>
> -static void nfs3_abort_get_acl(struct posix_acl **p)
> +static void nfs3_abort_get_acl(struct base_acl **p)
>  {
> - struct posix_acl *sentinel = uncached_acl_sentinel(current);
> + struct base_acl *sentinel = uncached_acl_sentinel(current);
>
>   /* Remove our sentinel upon failure. */
>   cmpxchg(p, sentinel, ACL_NOT_CACHED);
> diff --git a/fs/posix_acl.c b/fs/posix_acl.c
> index edc452c..1b685a1 100644
> --- a/fs/posix_acl.c
> +++ b/fs/posix_acl.c
> @@ -21,7 +21,7 @@
>  #include
>  #include
>
> -static struct posix_acl **acl_by_type(struct inode *inode, int type)
> +static inline struct base_acl **acl_by_type(struct inode *inode, int type)
>  {
>   switch (type) {
>   case ACL_TYPE_ACCESS:
> @@ -33,51 +33,23 @@ static struct posix_acl **acl_by_type(struct inode *inode, int type)
>   }
>  }
>
> -struct posix_acl *get_cached_acl(struct inode *inode, int type)
> +struct base_acl *get_cached_acl(struct inode *inode, int type)
>  {
> - struct posix_acl **p = acl_by_type(inode, type);
> - struct posix_acl *acl;
> -
> - for (;;) {
> - rcu_read_lock();
> - acl = rcu_dereference(*p);
> - if (!acl || is_uncached_acl(acl) ||
> -     atomic_inc_not_zero(&acl->a_refcount))
> - break;
> - rcu_read_unlock();
> - cpu_relax();
> - }
> - rcu_read_unlock();
> - return acl;
> + return __get_cached_acl(acl_by_type(inode, type));
>  }
>  EXPORT_SYMBOL(get_cached_acl);
>
> -struct posix_acl *get_cached_acl_rcu(struct inode *inode, int type)
> -{
> - return rcu_dereference(*acl_by_type(inode, type));
> -}
> -EXPORT_SYMBOL(get_cached_acl_rcu);
> -
>  void set_cached_acl(struct inode *inode, int type, struct posix_acl *acl)
>  {
> - struct posix_acl **p = acl_by_type(inode, type);
> - struct posix_acl *old;
> + struct base_acl **p = acl_by_type(inode, type);
> + struct base_acl *old;
>
> - old = xchg(p, posix_acl_dup(acl));
> + old = xchg(p, &posix_acl_dup(acl)->a_base);
>   if (!is_uncached_acl(old))
> - posix_acl_release(old);
> + base_acl_put(old);
>  }
>  EXPORT_SYMBOL(set_cached_acl);
>
> -static void __forget_cached_acl(struct posix_acl **p)
> -{
> - struct posix_acl *old;
> -
> - old = xchg(p, ACL_NOT_CACHED);
> - if (!is_uncached_acl(old))
> - posix_acl_release(old);
> -}
> -
>  void forget_cached_acl(struct inode *inode, int type)
>  {
>   __forget_cached_acl(acl_by_type(inode, type));
> @@ -93,25 +65,24 @@ EXPORT_SYMBOL(forget_all_cached_acls);
>
>  struct posix_acl *get_acl(struct inode *inode, int type)
>  {
> - void *sentinel;
> - struct posix_acl **p;
> + struct base_acl **p = acl_by_type(inode, type);
> + struct base_acl *sentinel, *base_acl;
>   struct posix_acl *acl;
>
> + if (!IS_POSIXACL(inode))
> + return NULL;
> +
>   /*
>    * The sentinel is used to detect when another operation like
>    * set_cached_acl() or forget_cached_acl() races with get_acl().
>    * It is guaranteed that is_uncached_acl(sentinel) is true.
>    */
>
> - acl = get_cached_acl(inode, type);
> - if (!is_uncached_acl(acl))
> - return acl;
> -
> - if (!IS_POSIXACL(inode))
> - return NULL;
> + base_acl = __get_cached_acl(p);
> + if (!is_uncached_acl(base_acl))
> + return posix_acl(base_acl);
>
>   sentinel = uncached_acl_sentinel(current);
> - p = acl_by_type(inode, type);
>
>   /*
>    * If the ACL isn't being read yet, set our sentinel.  Otherwise, the
> @@ -151,7 +122,7 @@ struct posix_acl *get_acl(struct inode *inode, int type)
>    * Cache the result, but only if our sentinel is still in place.
>    */
>   posix_acl_dup(acl);
> - if (unlikely(cmpxchg(p, sentinel, acl) != sentinel))
> + if (unlikely(cmpxchg(p, sentinel, &acl->a_base) != sentinel))
>   posix_acl_release(acl);
>   return acl;
>  }
> @@ -163,7 +134,7 @@ EXPORT_SYMBOL(get_acl);
>  void
>  posix_acl_init(struct posix_acl *acl, int count)
>  {
> - atomic_set(&acl->a_refcount, 1);
> + base_acl_init(&acl->a_base);
>   acl->a_count = count;
>  }
>  EXPORT_SYMBOL(posix_acl_init);
> @@ -196,7 +167,7 @@ posix_acl_clone(const struct posix_acl *acl, gfp_t flags)
>              sizeof(struct posix_acl_entry);
>   clone = kmemdup(acl, size, flags);
>   if (clone)
> - atomic_set(&clone->a_refcount, 1);
> + base_acl_init(&clone->a_base);
>   }
>   return clone;
>  }
> @@ -418,7 +389,7 @@ static int posix_acl_create_masq(struct posix_acl *acl, umode_t *mode_p)
>   umode_t mode = *mode_p;
>   int not_equiv = 0;
>
> - /* assert(atomic_read(acl->a_refcount) == 1); */
> + /* assert(base_acl_refcount(&acl->a_base) == 1); */
>
>   FOREACH_ACL_ENTRY(pa, acl, pe) {
>                  switch(pa->e_tag) {
> @@ -473,7 +444,7 @@ static int __posix_acl_chmod_masq(struct posix_acl *acl, umode_t mode)
>   struct posix_acl_entry *group_obj = NULL, *mask_obj = NULL;
>   struct posix_acl_entry *pa, *pe;
>
> - /* assert(atomic_read(acl->a_refcount) == 1); */
> + /* assert(base_acl_refcount(&acl->a_base) == 1); */
>
>   FOREACH_ACL_ENTRY(pa, acl, pe) {
>   switch(pa->e_tag) {
> diff --git a/fs/richacl.c b/fs/richacl.c
> index cb0ef3f..8971ead 100644
> --- a/fs/richacl.c
> +++ b/fs/richacl.c
> @@ -31,7 +31,7 @@ richacl_alloc(int count, gfp_t gfp)
>   struct richacl *acl = kzalloc(size, gfp);
>
>   if (acl) {
> - atomic_set(&acl->a_refcount, 1);
> + base_acl_init(&acl->a_base);
>   acl->a_count = count;
>   }
>   return acl;
> @@ -50,7 +50,7 @@ richacl_clone(const struct richacl *acl, gfp_t gfp)
>
>   if (dup) {
>   memcpy(dup, acl, size);
> - atomic_set(&dup->a_refcount, 1);
> + base_acl_init(&dup->a_base);
>   }
>   return dup;
>  }
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index bb36561..06a30b0 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -582,17 +582,23 @@ static inline void mapping_allow_writable(struct address_space *mapping)
>  #define i_size_ordered_init(inode) do { } while (0)
>  #endif
>
> +struct base_acl {
> + union {
> + atomic_t ba_refcount;
> + struct rcu_head ba_rcu;
> + };
> +};
>  struct posix_acl;
>  #define ACL_NOT_CACHED ((void *)(-1))
>
> -static inline struct posix_acl *
> +static inline struct base_acl *
>  uncached_acl_sentinel(struct task_struct *task)
>  {
>   return (void *)task + 1;
>  }
>
>  static inline bool
> -is_uncached_acl(struct posix_acl *acl)
> +is_uncached_acl(struct base_acl *acl)
>  {
>   return (long)acl & 1;
>  }
> @@ -613,9 +619,9 @@ struct inode {
>   kgid_t i_gid;
>   unsigned int i_flags;
>
> -#ifdef CONFIG_FS_POSIX_ACL
> - struct posix_acl *i_acl;
> - struct posix_acl *i_default_acl;
> +#if defined(CONFIG_FS_POSIX_ACL)
> + struct base_acl *i_acl;
> + struct base_acl *i_default_acl;
>  #endif
>
>   const struct inode_operations *i_op;
> @@ -3193,4 +3199,29 @@ static inline bool dir_relax_shared(struct inode *inode)
>  extern bool path_noexec(const struct path *path);
>  extern void inode_nohighmem(struct inode *inode);
>
> +static inline void base_acl_get(struct base_acl *acl)
> +{
> + if (acl)
> + atomic_inc(&acl->ba_refcount);
> +}
> +
> +static inline void base_acl_put(struct base_acl *acl)
> +{
> + if (acl && atomic_dec_and_test(&acl->ba_refcount))
> + kfree_rcu(acl, ba_rcu);
> +}
> +
> +static inline void base_acl_init(struct base_acl *acl)
> +{
> + atomic_set(&acl->ba_refcount, 1);
> +}
> +
> +static inline int base_acl_refcount(struct base_acl *acl)
> +{
> + return atomic_read(&acl->ba_refcount);
> +}
> +
> +extern struct base_acl *__get_cached_acl(struct base_acl **);
> +extern void __forget_cached_acl(struct base_acl **);
> +
>  #endif /* _LINUX_FS_H */
> diff --git a/include/linux/posix_acl.h b/include/linux/posix_acl.h
> index 5b5a80c..daf84fa 100644
> --- a/include/linux/posix_acl.h
> +++ b/include/linux/posix_acl.h
> @@ -43,10 +43,7 @@ struct posix_acl_entry {
>  };
>
>  struct posix_acl {
> - union {
> - atomic_t a_refcount;
> - struct rcu_head a_rcu;
> - };
> + struct base_acl a_base;  /* must be first, see posix_acl_release() */
>   unsigned int a_count;
>   struct posix_acl_entry a_entries[0];
>  };
> @@ -61,8 +58,7 @@ struct posix_acl {
>  static inline struct posix_acl *
>  posix_acl_dup(struct posix_acl *acl)
>  {
> - if (acl)
> - atomic_inc(&acl->a_refcount);
> + base_acl_get(&acl->a_base);
>   return acl;
>  }
>
> @@ -72,10 +68,16 @@ posix_acl_dup(struct posix_acl *acl)
>  static inline void
>  posix_acl_release(struct posix_acl *acl)
>  {
> - if (acl && atomic_dec_and_test(&acl->a_refcount))
> - kfree_rcu(acl, a_rcu);
> + BUILD_BUG_ON(offsetof(struct posix_acl, a_base) != 0);
> + base_acl_put(&acl->a_base);
>  }
>
> +static inline struct posix_acl *
> +posix_acl(struct base_acl *base_acl)
> +{
> + BUILD_BUG_ON(offsetof(struct posix_acl, a_base) != 0);
> + return container_of(base_acl, struct posix_acl, a_base);
> +}
>
>  /* posix_acl.c */
>
> @@ -99,8 +101,7 @@ extern int posix_acl_create(struct inode *, umode_t *, struct posix_acl **,
>  extern int simple_set_acl(struct inode *, struct posix_acl *, int);
>  extern int simple_acl_create(struct inode *, struct inode *);
>
> -struct posix_acl *get_cached_acl(struct inode *inode, int type);
> -struct posix_acl *get_cached_acl_rcu(struct inode *inode, int type);
> +struct base_acl *get_cached_acl(struct inode *inode, int type);
>  void set_cached_acl(struct inode *inode, int type, struct posix_acl *acl);
>  void forget_cached_acl(struct inode *inode, int type);
>  void forget_all_cached_acls(struct inode *inode);
> diff --git a/include/linux/richacl.h b/include/linux/richacl.h
> index be9fb65..35a5bcb 100644
> --- a/include/linux/richacl.h
> +++ b/include/linux/richacl.h
> @@ -31,7 +31,7 @@ struct richace {
>  };
>
>  struct richacl {
> - atomic_t a_refcount;
> + struct base_acl a_base;  /* must be first, see richacl_put() */
>   unsigned int a_owner_mask;
>   unsigned int a_group_mask;
>   unsigned int a_other_mask;
> @@ -56,8 +56,7 @@ struct richacl {
>  static inline struct richacl *
>  richacl_get(struct richacl *acl)
>  {
> - if (acl)
> - atomic_inc(&acl->a_refcount);
> + base_acl_get(&acl->a_base);
>   return acl;
>  }
>
> @@ -67,8 +66,8 @@ richacl_get(struct richacl *acl)
>  static inline void
>  richacl_put(struct richacl *acl)
>  {
> - if (acl && atomic_dec_and_test(&acl->a_refcount))
> - kfree(acl);
> + BUILD_BUG_ON(offsetof(struct richacl, a_base) != 0);
> + base_acl_put(&acl->a_base);
>  }
>
>  /**

--
Jeff Layton <jlayton@redhat.com>