From 4bf46a272647d89e780126b52eda04737defd9f4 Mon Sep 17 00:00:00 2001 From: David Howells Date: Thu, 5 Mar 2015 14:09:22 +0000 Subject: [PATCH] VFS: Impose ordering on accesses of d_inode and d_flags Impose ordering on accesses of d_inode and d_flags to avoid the need to do this: if (!dentry->d_inode || d_is_negative(dentry)) { when this: if (d_is_negative(dentry)) { should suffice. This check is especially problematic if a dentry can have its type field set to something other than DENTRY_MISS_TYPE when d_inode is NULL (as in unionmount). What we really need to do is stick a write barrier between setting d_inode and setting d_flags and a read barrier between reading d_flags and reading d_inode. Signed-off-by: David Howells Signed-off-by: Al Viro --- fs/dcache.c | 47 +++++++++++++++++++++++++++++++++++------- include/linux/dcache.h | 21 +++---------------- 2 files changed, 42 insertions(+), 26 deletions(-) diff --git a/fs/dcache.c b/fs/dcache.c index d99736a63e3c..656ce522a218 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -269,6 +269,41 @@ static inline int dname_external(const struct dentry *dentry) return dentry->d_name.name != dentry->d_iname; } +/* + * Make sure other CPUs see the inode attached before the type is set. + */ +static inline void __d_set_inode_and_type(struct dentry *dentry, + struct inode *inode, + unsigned type_flags) +{ + unsigned flags; + + dentry->d_inode = inode; + smp_wmb(); + flags = READ_ONCE(dentry->d_flags); + flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU); + flags |= type_flags; + WRITE_ONCE(dentry->d_flags, flags); +} + +/* + * Ideally, we want to make sure that other CPUs see the flags cleared before + * the inode is detached, but this is really a violation of RCU principles + * since the ordering suggests we should always set inode before flags. + * + * We should instead replace or discard the entire dentry - but that sucks + * performancewise on mass deletion/rename. + */ +static inline void __d_clear_type_and_inode(struct dentry *dentry) +{ + unsigned flags = READ_ONCE(dentry->d_flags); + + flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU); + WRITE_ONCE(dentry->d_flags, flags); + smp_wmb(); + dentry->d_inode = NULL; +} + static void dentry_free(struct dentry *dentry) { WARN_ON(!hlist_unhashed(&dentry->d_u.d_alias)); @@ -311,7 +346,7 @@ static void dentry_iput(struct dentry * dentry) { struct inode *inode = dentry->d_inode; if (inode) { - dentry->d_inode = NULL; + __d_clear_type_and_inode(dentry); hlist_del_init(&dentry->d_u.d_alias); spin_unlock(&dentry->d_lock); spin_unlock(&inode->i_lock); @@ -335,8 +370,7 @@ static void dentry_unlink_inode(struct dentry * dentry) __releases(dentry->d_inode->i_lock) { struct inode *inode = dentry->d_inode; - __d_clear_type(dentry); - dentry->d_inode = NULL; + __d_clear_type_and_inode(dentry); hlist_del_init(&dentry->d_u.d_alias); dentry_rcuwalk_barrier(dentry); spin_unlock(&dentry->d_lock); @@ -1715,11 +1749,9 @@ static void __d_instantiate(struct dentry *dentry, struct inode *inode) unsigned add_flags = d_flags_for_inode(inode); spin_lock(&dentry->d_lock); - dentry->d_flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU); - dentry->d_flags |= add_flags; if (inode) hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry); - dentry->d_inode = inode; + __d_set_inode_and_type(dentry, inode, add_flags); dentry_rcuwalk_barrier(dentry); spin_unlock(&dentry->d_lock); fsnotify_d_instantiate(dentry, inode); @@ -1937,8 +1969,7 @@ static struct dentry *__d_obtain_alias(struct inode *inode, int disconnected) add_flags |= DCACHE_DISCONNECTED; spin_lock(&tmp->d_lock); - tmp->d_inode = inode; - tmp->d_flags |= add_flags; + __d_set_inode_and_type(tmp, inode, add_flags); hlist_add_head(&tmp->d_u.d_alias, &inode->i_dentry); hlist_bl_lock(&tmp->d_sb->s_anon); hlist_bl_add_head(&tmp->d_hash, &tmp->d_sb->s_anon); diff --git a/include/linux/dcache.h b/include/linux/dcache.h index e83768ee38fc..df334cbacc6d 100644 --- a/include/linux/dcache.h +++ b/include/linux/dcache.h @@ -404,26 +404,11 @@ static inline bool d_mountpoint(const struct dentry *dentry) /* * Directory cache entry type accessor functions. */ -static inline void __d_set_type(struct dentry *dentry, unsigned type) -{ - dentry->d_flags = (dentry->d_flags & ~DCACHE_ENTRY_TYPE) | type; -} - -static inline void __d_clear_type(struct dentry *dentry) -{ - __d_set_type(dentry, DCACHE_MISS_TYPE); -} - -static inline void d_set_type(struct dentry *dentry, unsigned type) -{ - spin_lock(&dentry->d_lock); - __d_set_type(dentry, type); - spin_unlock(&dentry->d_lock); -} - static inline unsigned __d_entry_type(const struct dentry *dentry) { - return dentry->d_flags & DCACHE_ENTRY_TYPE; + unsigned type = READ_ONCE(dentry->d_flags); + smp_rmb(); + return type & DCACHE_ENTRY_TYPE; } static inline bool d_is_miss(const struct dentry *dentry) -- 2.20.1